And Now ChocolateyCoolWhip

Following up on CoolWhip, ChocolateyCoolWhip is a nuget package that makes creating, packaging, and deploying your code through Chocolatey.org simple.  Like CoolWhip, all you need to do to publish a new release is create release in GitHub.

Logo

Chocolatey?

In case you aren’t familiar, Chocolatey is a package manager for Windows built on top of NuGet and PowerShell. It allows users to download and install thousands of software programs with a single command.  For example, to install nodejs, all you would need to do is open up a command prompt and enter

> choco install nodejs.install

Getting Started with ChocolateyCoolWhip

To get started, install the nuget package into your project.  The first thing you’ll notice is that it creates nuspec and AppVeyor configuration files.  Chocolatey uses the nuspec as metadata for your package, the same way NuGet does.

Once setup, all you need to do to update your package on Chocolatey.org is create a release in GitHub.  ChocolateyCoolWhip will pull the version information directly from the release name and kick off a build in AppVeryor.  You have complete control over versioning your application

Configuration is simple:

  1. Connect AppVeyor to your GitHub repo
  2. Install the NuGet package >Install-Package ChocolateyCoolWhip
  3. Update the generated nuspec file with your project’s information
  4. Add your authorization tokens to the AppVeyor.yml file
  5. Save/commit your changes

Once this is set up, all you’ll need to do in order to release (or update) your package on NuGet is create a GitHub release with the version number.

There is full step by step documentation in the wiki.

Happy Coding.

Introducing CoolWhip

CoolWhip is a NuGet package aimed at making and deploying other NuGet packages easier. With CoolWhip, creating a Release in GitHub automatically pushes that release to NuGet.

GitGet

Once you’ve installed CoolWhip into your project it will create a nuspec file and AppVeyor configuration file.  These will work together to package your project and automatically upload it to NuGet.org.

Once setup, all you need to do to update your package on NuGet.org is create a release in GitHub.  CoolWhip will pull the version information directly from the release name and kick off a build in AppVeryor.  You have complete control over versioning your application.

To configure CoolWhip you need to

  1. Connect AppVeyor to your GitHub repo
  2. Install the NuGet package >Install-Package CoolWhip
  3. Update the generated nuspec file with your project’s information
  4. Add your authorization tokens to the AppVeyor.yml file
  5. Save/commit your changes

Once this is set up, all you’ll need to do in order to release (or update) your package on NuGet is create a GitHub release with the version number.

There is full step by step documentation in the wiki.

Happy Coding.

 

Source on GitHub

Package on NuGet

Treat Warnings as Errors In TeamCity

I’m OCD when it comes to my code.  I don’t like to have any warnings.  I wanted to configure my TeamCity builds to fail if there were any warnings in the projects. My first option is to tick the “Treat warnings as errors” box in the project settings.  This had a few problems

Problem 1.

I’d have to remember to do it for every project in my repo, including all the projects I make in the future.  Even worse, everyone on my team would have to remember to do that too.  That’s not something I could rely on.

Problem 2. 

While warnings are just errors that you’re ignoring, I will admit that it is convenient to be able to ignore some things some times.  I’m fine with debugging and running locally with some errors.  I really only wanted to stop people from committing code back to master with warnings.  I couldn’t care less about bad hygiene the have when working locally.

The Solution

In searching, I found David Gardiner’s blog post in which he creates a power shell script to edit the xml in each csproj file.  It looks simple enough so I removed the TFS bit and added it as a build step in my TeamCity flow. It runs right before compiling my code.

TreatWarningsAsErrors

Get-ChildItem -Recurse -Filter "*.*csproj" | % {
    Write-Host $_.Name
    
    $filename = $_.Fullname
    
    $proj =
( Get-Content $_.Fullname ) $xmlNameSpace = new-object System.Xml.XmlNamespaceManager($proj.NameTable) $xmlNameSpace.AddNamespace("p", "http://schemas.microsoft.com/developer/msbuild/2003") $nodes = $proj.SelectNodes("/p:Project/p:PropertyGroup[@Condition and not (p:TreatWarningsAsErrors)]", $xmlNameSpace) $touched = $false $nodes | ForEach-Object -Process { $e = $proj.CreateElement("TreatWarningsAsErrors", "http://schemas.microsoft.com/developer/msbuild/2003") $e.set_InnerText("true") $_.AppendChild($e) | Out-Null $touched = $true } if ($touched) { Write-Host "Checkout $filename" $proj.Save("$($filename)") | Out-Null } }

Genymotion Android Emulator

I like to use an emulator to debug my android apps.  My emulator of choice is the Genymotion emulator.  It’s much easier to install and use than the one in the android-sdk.

One drawback I have seen with it it, is that sometimes it just refuses to start.  Getting it to start back up isn’t hard, but it isn’t all that unintuitive.

Try Try Again

The first thing to try is to just restart the emulator a few more times.  It might just work the third or fourth time.  You don’t have to restart the entire Genymotion console, just hit the start button and wait for the emulator to succeed or fail.  Try this a few times and you might just get lucky.

Delete Network Settings

If you’ve tried a few times and still don’t have any success you can force Genymotion to reconfigure its network settings.

Summary

  1. Open up Oracle VM Virtual Box manager
  2. Go to Preferences
  3. Go to the Network tab
  4. Delete the VirtualBox Host-Only Ethernet Adapter
  5. Start up the emulator as normal

Details

Open up the Oracle VM Virtual Box manager.  I do this by typing any part of the name that I happen to remember at the time into my start menu.

image

VirtualBox is what powers the emulator, so you will see all of your android images listed.  Go to File –> Preferences

image

Then click on Network on the right.

image

You should see “VirtualBox Host-Only Ethernet Adapter” in the “Host-only Networks”.  Select it and click the delete button on the right (it’s the button in the middle)

image

Now, go back to Genymotion and try to start the emulator again. It will notice that the virtual network adapter is missing and recreate it. You may see a UAC prompt asking if VirtualBox should be allowed to make changes to your computer.  Click yes.

Your emulator should now startup.

Xamarin.Forms: App and Action Bars

To recap, I’m writing a shopping cart app for Windows Phone, Android, and iOS.  The purpose of the app is primarily to let me use Forms.  Each post will build on top of the previous one.

Last time I wrote data to disk.  This week I’m going to add an application level menu.

Reminder, this article is a direct continuation of last week’s article.  The code base is entirely the same.  The posts were split up so that neither would be too long.  For consistency’s sake, I have created separate releases for each week.

Recap and Code

This is the ninth post in the series, you can find the rest here:

  • Day 0:  Getting Started (blog / code)
  • Day 1:  Binding and Navigation (blog / code)
  • Day 2:  Frames, Event Handlers, and Binding Bugs (blog / code)
  • Day 3:  Images in Lists (blog / code)
  • Day 4:  Search and Barcode Scanner (blog / code)
  • Day 5:  Dependency Injection (blog / code)
  • Day 6:  Styling (blog / code)
  • Day 7:  Attached Behaviors (blog / code)
  • Day 8:  Writing to Disk (blog / code)
  • Day 9:  App and Action Bars (blog / code)

The latest version of the code can always be accessed on the GitHub project page.

Logging Out

Now that I’m all logged in, I need to be able to log out. Ideally this would go in a dedicated page with various settings. But honestly, that’d be boring. This app already has plenty of pages.  What it doesn’t have is a system level menu — an Action Bar on Droid or an App Bar on Windows Phone.

Adding the menu is rather straight forward, just some simple XAML and bindings to CategoriesListPage.xaml:

<ContentPage.ToolbarItems>
  <ToolbarItem Name="Log Out" Command="{Binding LogOut}" Order="Primary" Priority="0">
    <ToolbarItem.Icon>
      <OnPlatform x:TypeArguments="FileImageSource"
                  WinPhone="Assets/Logout.png"
                  Android="ic_action_logout.png" />
    </ToolbarItem.Icon>
  </ToolbarItem>
</ContentPage.ToolbarItems>

Conveniently, ToolbarItem has a Command property that you can bind against to handle clicks.   The command binds the same as a button.  The icon is a bit more involved.

Now the question is: where can I get good icons for my app?  This leads me to a quick aside…

Windows Phone Icons

The easiest way to get icons for a Windows Phone or Metro-style Windows apps is to use Metro Studio by Syncfusion.  First off, it’s free.  Second off, it’s full of great icons.  Just search for whatever you want, and then  customize it for your uses.  You can even create icons out of text in whatever font you like.  Finally, a use for Wingdings.

Metro-Studio

Custom Text

Wingdings

Android Icons

Metro Studio packs a lot of punch, but doesn’t offer any help for Android apps (kinda makes sense given the name).  For Android assets, check out the Android Asset Studio.  There’s a lot of good stuff in here, but for this project I went straight to the Action Bar and Tab Icons section.  They provide a library of images (although significantly smaller than Metro Studio), the ability to upload an image of your choice, or to enter an arbitrary string (and we still get Wingdings!).  The best part is that it generates a zip with assets for various screen resolutions.  Just unzip and drop the folders into the Resources directory.

Image Asset Locations

<ToolbarItem.Icon>
  <OnPlatform x:TypeArguments="FileImageSource"
              WinPhone="Assets/Logout.png"
              Android="ic_action_logout.png" />
</ToolbarItem.Icon>

Because I’m specifying the location of the icon as a string, I need to make sure that the images are placed in the correct locations in the corresponding project folders.  The XF documentation explains pretty clearly where each goes.

For my WinPhone project, I place the Logout.png in the “Assets” folder.

Assets Folder

Android is a little bit more complicated since the platform allows you to serve different sized images for devices with different resolutions.  Each file is put in the corresponding drawable folder under “Resources”.  As I said earlier, the Android Asset Studio does all this for you.  You can just drop the folders it generates directly into the Resources folder.

image

Doing Async Work at Startup

Previously, when the app started up it went directly to the login screen.  Now it has to first check to see if the user is logged in.  If they are then the app goes to the categories page, otherwise it goes to the login page.  This is easy, except for the fact that the check happens on the background thread and takes some time since it is going to disk.  Not a lot of time, but enough that we aren’t guaranteed to get the result back before the app is ready to start up.  In order to accommodate this, I’m turning the “WelcomePage” into a splash page.  It will display a progress dialog for a little bit and then navigate to the next page once it knows which page that is.  Let’s start with the view:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ShoppingCart.Views.WelcomePage"
             xmlns:local="clr-namespace:ShoppingCart;assembly=ShoppingCart"
             BindingContext="{x:Static local:App.WelcomeViewModel}"
             BackgroundColor="White">

  <StackLayout VerticalOptions="Center"
               IsVisible="{Binding IsLoaded, Mode=OneWayToSource}">
    <Frame>
      <Label Text="Welcome to The Store"
             Font="Bold, Large"
             HorizontalOptions="Center"
             TextColor="{x:Static local:App.AccentColor}" />
    </Frame>

    <ActivityIndicator IsRunning="{Binding IsBusy.IsNotCompleted}"
                       HorizontalOptions="FillAndExpand"
                       VerticalOptions="FillAndExpand"
                       Color="{x:Static local:App.AccentColor}" />
  </StackLayout>
</ContentPage>

The ActivityIndicator starting on line 18 is pretty straight forward.  I bind whether or not it is running to the NotifyTaskCompletion property on the view model (more on that later), and the color to the static representation of the accent color in the same way that I do for the label.  You should remember this from the styling article from a couple of weeks ago.

A little more interesting is the one way to source binding on the visibility of the stack layout.  This is here to tell the view model that the view has been displayed.  Since this is the first page in the XF app it is wrapped in the NavigationPage, which initializes the NavigationService.  If we try to navigate away from this view before it is shown, we have no navigation service, and so navigation won’t work.  Classic Catch-22.

public class WelcomeViewModel : BaseViewModel
{
    private readonly IAppNavigation _navi;
    private SemaphoreSlim _slim;

    public WelcomeViewModel(IAppNavigation navi)
    {
        _navi = navi;
        _slim = new SemaphoreSlim(0, 1);
        IsBusy = new NotifyTaskCompletion<int>(GoToFirstPage());
    }

    public NotifyTaskCompletion<int> IsBusy { get; private set; }

    public bool IsLoaded
    {
        get { return GetValue<bool>(); }
        set
        {
            SetValue(value);
            if (value)
            {
                _slim.Release();
            }
        }
    }

    private async Task<int> GoToFirstPage()
    {
        await _slim.WaitAsync();
        await _navi.SecondPage();
        return 0;
    }
}

The first thing the view model does is initialize a SemaphoreSlim in the unsignaled state, i.e., calling Wait() will block.  It then creates a NotifyTaskCompletion of type int.  Creating this kicks off the async call to GoToFirstPage.  The return value doesn’t matter in this case, the object is just being used to run a background task and update the UI with the status.

When IsLoaded is set to true by the binding to the visibility of the StackLayout in the view, it signals the semaphore, allowing the WaitAsync in GoToFirstPage to complete.  GoToFirstPage then tells the navigation service to show the second page in the app.  The navigation service handles the logic of determining which page to show as well as actually navigating to the page.  For the sake of demonstration, I’ve added a delay in the video below.

Here’s what the app looks like when the user needs to log in

WinPhone Login  Droid Login

And here’s what the returning a user sees

WinPhone Startup Droid Startup

Happy Coding

Xamarin.Forms: Writing to Disk With Akavache

To recap, I’m writing a shopping cart app for Windows Phone, Android, and iOS.  The purpose of the app is primarily to let me use Forms.  Each post will build on top of the previous one.

Last time I added behaviors to my XF xaml.  This week I’m going to save settings to disk using Akavache.

Once I started writing the post to go along with this week’s post, I quickly became aware that the it was longer than what really makes sense for a single post.  So I’m splitting the article in two.  This week will be logging in, next week will be logging out.  Both weeks will use the same code.  For consistency’s sake, I’ll be creating a new release for next week, even though it will point to the same code.

Recap and Code

This is the eight post in the series, you can find the rest here:

  • Day 0:  Getting Started (blog / code)
  • Day 1:  Binding and Navigation (blog / code)
  • Day 2:  Frames, Event Handlers, and Binding Bugs (blog / code)
  • Day 3:  Images in Lists (blog / code)
  • Day 4:  Search and Barcode Scanner (blog / code)
  • Day 5:  Dependency Injection (blog / code)
  • Day 6:  Styling (blog / code)
  • Day 7:  Attached Behaviors (blog / code)
  • Day 8:  Akavache (blog / code)

The latest version of the code can always be accessed on the GitHub project page.

Installing Akavache Cheat Sheet

At the risk of spoiling the narrative below, here’s a very brief outline of the steps to install and use Akavache in a Xamarin Forms app.  For links and more details, continue reading.

  1. Install Akavache package in common PCL project (Install-Package akavache)
  2. Install Akavache package in platform specific projects
    1. Droid
    2. Win Phone
    3. iOS*
  3. WinPhone: Update Splat package (Update-Package splat)
  4. WinPhone:  Change build configuration to x86
  • The iOS step is assumed to be necessary and sufficient to make Akavache work, but I am unable to verify it.

Logging In

From the beginning one of the things that really bothered me about this app is that I have to log in each time I use it.  And that’s including the fact that I don’t have to type in a particular username/password.  Any text will do.  I guess I’m just lazy.

To persist logging in, I’m just going to write a little bit of information to disk when I’m logging in for the first time.  Next time I open the app, I’ll just check for that value, if it exists I’ll go straight to the main landing page.

I don’t want to tie my implementation down to an unknown framework, so the first thing I need to do is create an interface to abstract away persisting data:

public interface ICache
{
    Task<T> GetObject<T>(string key);
    Task InsertObject<T>(string key, T value);
    Task RemoveObject(string key);
}

This interface should handle all of m CRUD operations.  Right now the only person that needs to use it is my LoginService.

public async Task<User> LoginAsync(string username, string password)
{
    User user = Login(username, password);

    await _cache.InsertObject("LOGIN", user);

    return user;
}

public async Task<bool> IsLoggedIn()
{
    User user = await _cache.GetObject<User>("LOGIN");
    return user != null;
}

LoginAsync performs the standard log in check, verifying that username and password are valid credentials (in reality it just checks that they are non-null) and returns a user object if they are valid.  I then save that in the cache using “LOGIN” as the key.  Next, IsLoggedIn in checks to see if there is a non-null value stored in the cache.

Now that we know how the cache is going to be used, let’s implement it.

Akavache

I’ll be using Akavache as my data store.  It’s probably a bit overkill for the very simple use case I have but I have a few reasons why I want to use it.  First, I really don’t want to think about where exactly I’m storing this data.  It could be in a database, it could be on disk, my app doesn’t really care.  It’s just a set of keys and values to me.  While Akavache does use a SQLite backend, I don’t have to know any of the details.  I’m also looking for something that is async/await friendly.  Akavache supports many platforms, I’d love to be able to write this once for all platforms.  And finally, I recently heard about it and kind of wanted to play with it.

Here’s a summary of what I just stated above as my reasons for using Akavache

  • Abstract away storage details
  • async API
  • Cross platform
  • It’s new to me

Akavache on Droid

Akavache is a nuget package, so installation is easy.  I’m installing the package into the core project (ShoppingCart.csproj).  For it to work it also needs to be installed on the Droid project as well.  In my Services folder I’ll add my Akavache implementation of ICache.  This class will be used by all of the platforms.

using Akavache;
using ShoppingCart.Services;
using System.Collections.Generic;
using System.Reactive.Linq;
using System.Threading.Tasks;

namespace ShoppingCart.Services
{
    public class Cache : ICache
    {
        public Cache()
        {
            BlobCache.ApplicationName = "ShoppingCart";
        }

        public async Task RemoveObject(string key)
        {
            await BlobCache.LocalMachine.Invalidate(key);
        }

        public async Task<T> GetObject<T>(string key)
        {
            try
            {
                return await BlobCache.LocalMachine.GetObject<T>(key);
            }
            catch (KeyNotFoundException)
            {
                return default(T);
            }
        }

        public async Task InsertObject<T>(string key, T value)
        {
            await BlobCache.LocalMachine.InsertObject(key, value);
        }
    }
}

First thing to note here is that I’m importing the System.Reactive.Linq namespace.  This enables us to await the Akavache calls.  Next thing to note is that I’m setting the ApplicationName property in the constructor.  This is needed to initialize storage.  The rest of the implementation is pretty straight forward.  You can even see that I got most of my method names from the Akavache API.

The RemoveObject implementation invalidates the object in Akavache.  GetObject tries to read the object from the cache, catching any KeyNotFoundExceptions.

Next week I’ll go over how all of this gets plugged into the UI.

Akavache on Win Phone

Akavache is a bit more fiddly on Win Phone.

To start off, install the Akavache nuget package directly in the Win Phone project.  Remember, it’s already installed in my PCL project where it is being used, it needs to be installed here as well, even though none of the code reference it directly.  At this point, I get my first error:

System.TypeInitializationException was unhandled by user code
  HResult=-2146233036
  Message=The type initializer for 'Akavache.BlobCache' threw an exception.
  Source=Akavache
  TypeName=Akavache.BlobCache
  StackTrace:
       at Akavache.BlobCache.set_ApplicationName(String value)
       at ShoppingCart.WinPhone.Services.WinPhoneCache..ctor()
       at lambda_method(Closure , Object[] )
       at Autofac.Core.Activators.Reflection.ConstructorParameterBinding.Instantiate()
  InnerException: System.IO.FileLoadException
       HResult=-2146234304
       Message=Could not load file or assembly 'Splat, Version=1.4.1.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
       Source=Akavache
       StackTrace:
            at Akavache.BlobCache..cctor()
       InnerException:

There is an assembly that can’t be found.  Splat is a another package produced by Paul Betts (the producer of Akavache).  The problem here is not that the file isn’t there, but that it’s an older version.  All I need to do is update the nuget package for Splat and I’m good.

Updating Splat

With this resolved, the next exception I get is that correct version of SQLitePcl.Raw is missing.

System.TypeInitializationException was unhandled by user code
  HResult=-2146233036
  Message=The type initializer for 'Akavache.Sqlite3.Internal.SQLiteConnection' threw an exception.
  Source=Akavache.Sqlite3
  TypeName=Akavache.Sqlite3.Internal.SQLiteConnection
  StackTrace:
       at Akavache.Sqlite3.Internal.SQLiteConnection..ctor(String databasePath, Boolean storeDateTimeAsTicks)
       at Akavache.Sqlite3.SQLitePersistentBlobCache..ctor(String databaseFile, IScheduler scheduler)
       at Akavache.Sqlite3.Registrations.<>c__DisplayClass6.<Register>b__2()
       at System.Lazy`1.CreateValue()
    --- End of stack trace from previous location where exception was thrown ---
       at System.Lazy`1.get_Value()
       at Akavache.Sqlite3.Registrations.<>c__DisplayClass6.<Register>b__3()
       at Splat.ModernDependencyResolver.GetService(Type serviceType, String contract)
       at Splat.DependencyResolverMixins.GetService[T](IDependencyResolver This, String contract)
       at Akavache.BlobCache.get_UserAccount()
       at ShoppingCart.Services.LoginService.get_Users()
       at ShoppingCart.Services.LoginService.<LoginAsync>d__4.MoveNext()
    --- End of stack trace from previous location where exception was thrown ---
       at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
       at ShoppingCart.ViewModels.LoginViewModel.<Login>d__0.MoveNext()
  InnerException: System.IO.FileNotFoundException
       HResult=-2147024894
       Message=Could not load file or assembly 'SQLitePCL.raw, Version=0.5.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.
       Source=Akavache.Sqlite3
       FileName=SQLitePCL.raw, Version=0.5.0.0, Culture=neutral, PublicKeyToken=null
       StackTrace:
            at Akavache.Sqlite3.Internal.SQLiteConnection..cctor()
       InnerException:

Again this is a nuget package, but this time updating does not fix the problem.  I reached out to Paul about the issue and he told me that I have to change the build configuration of the Win Phone project to specify x86 as my platform.

Open Configuraton Manager

Configuraton Manager

Now running the app will work.

Thanks

A big thanks to Paul Betts both for writing Akavache and for providing the last mile of help when I needed it.

Happy Coding

Xamarin.Forms: Attached Behaviors

To recap, I’m writing a shopping cart app for Windows Phone, Android, and iOS.  The purpose of the app is primarily to let me use Forms.  Each post will build on top of the previous one.

Last time I styled the app so it looked slick.  This week I am going to revisit a problem I had in my Day 2 post, namely the lack of an EventToCommand behavior.  A developer named Corrado created a Behaviors library specifically for Xamarin.Forms.  This library comes with an EventToCommand behavior out of the box, and lets you create your own.

Recap and Code

This is the seventh post in the series, you can find the rest here:

  • Day 0:  Getting Started (blog / code)
  • Day 1:  Binding and Navigation (blog / code)
  • Day 2:  Frames, Event Handlers, and Binding Bugs (blog / code)
  • Day 3:  Images in Lists (blog / code)
  • Day 4:  Search and Barcode Scanner (blog / code)
  • Day 5:  Dependency Injection (blog / code)
  • Day 6:  Styling (blog / code)
  • Day 7:  Attached Behaviors (blog / code)

The latest version of the code can always be accessed on the GitHub project page.

Getting Behaviors

First off, you can check out Corrado’s own blog post about this library.  You can also take a look at his code on GitHub, or just grab the library from nuget.

The first thing I did was install the nuget package in my core project (ShoppingCart).  I did two things wrong here.  First, there are two nuget packages to choose from:  Xamarin.Behaviors and Xamarin.Forms.Behaviors.  Unintuitively, the correct one to choose is Xamarin.Behaviors.  The next mistake I made was that I installed it in just the core project.  When I ran up the solution, I saw this error immediately:

System.IO.FileNotFoundException was unhandled by user code
Message=Could not load file or assembly 'Xamarin.Behaviors, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.

I realized that the platform projects also need to reference the package.  Easy enough.

TL;DR

To install behaviors install the nuget package in your shared project as well as all platform projects:

PM> Install-Package Xamarin.Forms.Behaviors

Using Behaviors

My first use case for behaviors is to remove the ugly event to command code I have in my code behind.  Here’s the xaml that I want to get rid of:

<ListView ItemsSource="{Binding Categories.Result}" 
    IsGroupingEnabled="false" 
    ItemSelected="OnItemSelected">
  <ListView.ItemTemplate>
    <DataTemplate>
      <ViewCell>
        <Label Text="{Binding .}" />
      </ViewCell>
    </DataTemplate>
</ListView>

Specifically, I don’t want the ItemSelected property set to the OnItemSelcted method in the code behind file:

private void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
{
    var param = e.SelectedItem as string;
    var command = ((CategoriesListViewModel)BindingContext).NavigateToCategory;

    if (command.CanExecute(param))
    {
        command.Execute(param);
    }
}

This method casts the context to the view model, grabs the command, casts the SelectedItem into a string to act as the parameter, checks to see if it can call execute, and then calls execute.

First things first, I delete the OnItemSelected method.  Gone.  No more.  Next, I add an EventToCommand behavior in my xaml:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ShoppingCart.Views.CategoriesListPage"
             xmlns:b="clr-namespace:Xamarin.Behaviors;assembly=Xamarin.Behaviors"
             xmlns:local="clr-namespace:ShoppingCart;assembly=ShoppingCart"
             BindingContext="{x:Static local:App.CategoriesListViewModel}"
             BackgroundColor="White">
  <ListView ItemsSource="{Binding Categories.Result}">
    <ListView.ItemTemplate>
      <DataTemplate>
        <TextCell Text="{Binding Name}"
                  Detail="{Binding Count}">
          <b:Interaction.Behaviors>
            <b:BehaviorCollection>
              <b:EventToCommand EventName="Tapped"
                                Command="{Binding NavigateToCategory}"
                                CommandParameter="{Binding Category}" />
            </b:BehaviorCollection>
          </b:Interaction.Behaviors>
        </TextCell>
      </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

There’s a little more going on here than just the behavior, so I’ll explain that first.  First off, on line 4, I add the reference to the Behaviors namespace.  I also change the DataTemplate from the generic ViewCell to the TextCell.  This is mostly just to simplify my layout and because I only recently learned about the TextCell after reading a recent blog on the Xamarin Newsletter.  The TextCell lets you create a row in a ListView with a main text field, and a description underneath.  I also just realized that the ViewCell and TextCell both already have Command and CommandParameter properties that I could have bound to directly. Evidently I don’t need behaviors for this at all.  I’m still going to use behaviors, just so I can play with them a bit.  But, if you want to see how to do this without behaviors, check out my list view in the ProductsListPage.

So, now that I have my TextCell, I can use the Interaction.Behaviors attached property and add an EventToCommand behavior.  The EventToCommand maps an event on the UI control to an ICommand on the view model.  In this case, when the Tapped event of the TextCell is raised, the NavigateToCategory command will be executed.  But which NavigateToCategory command?  Originally this command existed on the CategoriesListViewModel, but that was when we were in the code behind and our BindingContext was the CategoriesListViewModel.  By the time our EventToCommand is created, we are in the DataTemplate and only have access to the individual members of Categories.Results which was originally a list of strings.  If we were using WPF, we would have been able to bind to our parent’s context using RelativeSource binding and access the command.  RelativeSource binding is not an option in XF.  The easiest way around this for me is to change my categories list from strings to CategoryViewModels.  Here’s my new view model:

public class CategoryViewModel : BaseViewModel
{
    private readonly Category _category;

    public CategoryViewModel(Category category, ICommand navigateCommand)
    {
        _category = category;
        Name = _category.Name;

        NavigateToCategory = navigateCommand;
    }

    public Category Category { get { return _category; } }

    public string Count { get; private set; }

    public string Name { get; private set; }

    public ICommand NavigateToCategory { get; private set; }
}

The  CategoriesListViewModel creates these instances, and just passes the navigate command in.  The implementation of the command itself isn’t changed. Truth be told, passing the command in like this is a bit of a hack.  It would be cleaner to use the Message Center.  That’s a bit out of the scope for this article, perhaps I’ll clean this up next week.

Another thing I’m doing that’s not strictly necessary, is passing in a CommandParameter.  I’m just using it here just to show how it can be done.  Currently, you can’t pass in the EventArgs as the parameter.  There are times when that is useful, so hopefully it’s added some time in the future before I really need it.

What Did I Do Wrong?

Typo in the EventName

At one point in my testing, I had a typo in my EventToCommand where I was trying to bind to a nonexistent event.

<b:EventToCommand EventName="OnTapped"
                  Command="{Binding NavigateCommand}"
                  CommandParameter="{Binding Category}" />

“OnTapped” doesn’t exist.  The correct event name is “Tapped”.  This is the error you’ll see if/when you make that mistake:

System.FormatException: Index (zero based) must be greater than or equal to zero and less than the size of the argument list

The exception is confusing until you look at the EventToCommand code and see that there is a small bug in it when it is trying to throw what would be a much more helpful exception.

Typo in the Command

I also had some trouble with typos where I misspelled the name of the command.  This was worse.  It just silently doesn’t work.  Typos are bad.

Creating Behaviors

Xamarin.Forms.Behaviors comes with two behaviors out of the box:  EventToCommand which we discussed earlier, and TextChangedBehavior.  Even better though, it gives you all the building blocks you need to create behaviors of your own.  Suppose you want to have your Entry (text box) animate when you click in it.  Something like this:

WP_Animation      Droid_Animation

Here’s the behavior that handles this:

using System;
using Xamarin.Behaviors;
using Xamarin.Forms;

namespace ShoppingCart.Behaviors
{
    public class AnimateSizeBehavior : Behavior<View>
    {
        public static readonly BindableProperty EasingFunctionProperty = BindableProperty.Create<AnimateSizeBehavior, string>(
            p => p.EasingFunctionName,
            "SinIn",
            propertyChanged: OnEasingFunctionChanged);

        public static readonly BindableProperty ScaleProperty = BindableProperty.Create<AnimateSizeBehavior, double>(
            p => p.Scale,
            1.25);

        private Easing _easingFunction;

        public string EasingFunctionName
        {
            get { return (string)GetValue(EasingFunctionProperty); }
            set { SetValue(EasingFunctionProperty, value); }
        }

        public double Scale
        {
            get { return (double)GetValue(ScaleProperty); }
            set { SetValue(ScaleProperty, value); }
        }

        protected override void OnAttach()
        {
            this.AssociatedObject.Focused += OnItemFocused;
        }

        protected override void OnDetach()
        {
            this.AssociatedObject.Focused -= OnItemFocused;
        }

        private static Easing GetEasing(string easingName)
        {
            switch (easingName)
            {
                case "BounceIn": return Easing.BounceIn;
                case "BounceOut": return Easing.BounceOut;
                case "CubicInOut": return Easing.CubicInOut;
                case "CubicOut": return Easing.CubicOut;
                case "Linear": return Easing.Linear;
                case "SinIn": return Easing.SinIn;
                case "SinInOut": return Easing.SinInOut;
                case "SinOut": return Easing.SinOut;
                case "SpringIn": return Easing.SpringIn;
                case "SpringOut": return Easing.SpringOut;
                default: throw new ArgumentException(easingName + " is not valid");
            }
        }

        private static void OnEasingFunctionChanged(BindableObject bindable, string oldvalue, string newvalue)
        {
            (bindable as AnimateSizeBehavior).EasingFunctionName = newvalue;
            (bindable as AnimateSizeBehavior)._easingFunction = GetEasing(newvalue);
        }

        private async void OnItemFocused(object sender, FocusEventArgs e)
        {
            await this.AssociatedObject.ScaleTo(Scale, 250, _easingFunction);
            await this.AssociatedObject.ScaleTo(1.00, 250, _easingFunction);
        }
    }
}

This is a big file, but not that much is really going on.  AnimateSizeBehavior inherits from Behavior<View>.  This means that we can apply it to any type of control.  It also means that the AssociatedObject property will be of type View.  The key methods to look at are the OnAttach and OnDetach.

protected override void OnAttach()
{
    this.AssociatedObject.Focused += OnItemFocused;
}

protected override void OnDetach()
{
    this.AssociatedObject.Focused -= OnItemFocused;
}

OnAttach is called when the behavior is added to the the control and OnDetach is called when it is removed from the control.  This is where I registered to receive the Focused event.  Now it’s a simple matter that whenever the control gains focus my animation code in OnItemFocused will be called.

private async void OnItemFocused(object sender, FocusEventArgs e)
{
    await this.AssociatedObject.ScaleTo(Scale, 250, _easingFunction);
    await this.AssociatedObject.ScaleTo(1.00, 250, _easingFunction);
}

The animation is very straight forward.  I use the ScaleTo method on View to scale the control up, then I call ScaleTo a second time to return it to its original size.  If I didn’t want to provide any flexibility with my behavior, I could stop there.  The rest of the code is just there to let me pass in parameters and configure how to perform the scale.  Let’s look at Scale.

public static readonly BindableProperty ScaleProperty = BindableProperty.Create<AnimateSizeBehavior, double>(
    p => p.Scale,
    1.25,
    propertyChanged: OnScaleChanged);

public double Scale
{
    get { return (double)GetValue(ScaleProperty); }
    set { SetValue(ScaleProperty, value); }
}

First I set up a static BindableProperty called ScaleProperty.  This is what lets me bind to properties on the behavior.  The first parameter ties it to the double Scale instance property.  The second parameter sets the default value to 1.25.

The EasingFunction property is a little more complicated.  It requires validation when it is set.  This is accomplished by setting the propertyChanged parameter in the factory method.

public static readonly BindableProperty EasingFunctionProperty = BindableProperty.Create<AnimateSizeBehavior, string>(
    p => p.EasingFunctionName,
    "SinIn",
    propertyChanged: OnEasingFunctionChanged);

private Easing _easingFunction;

public string EasingFunctionName
{
    get { return (string)GetValue(EasingFunctionProperty); }
    set { SetValue(EasingFunctionProperty, value); }
}

private static void OnEasingFunctionChanged(BindableObject bindable, string oldvalue, string newvalue)
{
    (bindable as AnimateSizeBehavior).EasingFunctionName = newvalue;
    (bindable as AnimateSizeBehavior)._easingFunction = GetEasing(newvalue);
}

The propertyChagned parameter is set to the static OnEasingFunctionChanged method.  The instance of the behavior is passed in as the bindable parameter, along with the old and new values being set.  In my example, we ignore the old value and just set the new value to the string property of EasingFunctionName.  We also parse the new value to determine what type of easing function to use in the GetEasing method.  If the easing function supplied is not an expected value, an exception is thrown.  This happens not when we try to run the animation, but as soon as we set the value.

Now all I need to do is add the behavior to my text boxes.  I’ll do this for the login page because there are two text boxes on the page so we can see how to tweak it.

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:bLocal="clr-namespace:ShoppingCart.Behaviors;assembly=ShoppingCart"
             xmlns:b="clr-namespace:Xamarin.Behaviors;assembly=Xamarin.Behaviors"
             x:Class="ShoppingCart.Views.LoginPage" >
  <StackLayout VerticalOptions="FillAndExpand" Padding="50">

    <Entry Text ="{Binding Username}" Placeholder ="User name goes here" >
      <b:Interaction.Behaviors>
        <b:BehaviorCollection>
          <bLocal:AnimateSizeBehavior />
        </b:BehaviorCollection>
      </b:Interaction.Behaviors>
    </Entry>

    <Entry Text ="{Binding Password}"
      Placeholder ="Password goes here"
      HorizontalOptions="FillAndExpand">
      <b:Interaction.Behaviors>
        <b:BehaviorCollection>
          <bLocal:AnimateSizeBehavior EasingFunction="BounceIn"
                                      Scale="1.50" />
        </b:BehaviorCollection>
      </b:Interaction.Behaviors>
    </Entry>

  </StackLayout>
</ContentPage>

I stripped a lot out of the xaml here for clarity (like the submit button).  The username textbox has the default behavior set.  The password box changes the scale size to 1.5 and selects a different easing function.  It would be possible to bind to those values as well, but it didn’t make sense in my already pointless example.

And that’s all there is to it.  A quick thanks to lobrien for a useful sample on how to do animations in XF.  This sped up my coding quite a bit.  And of course a thanks to Corrado for the Xamarin.Forms.Behaviors library.

Happy Coding

Xamarin.Forms: Styling

To recap, I’m writing a shopping cart app for Windows Phone, Android, and iOS.  The purpose of the app is primarily to let me use Forms.  Each post will build on top of the previous one.

Last time I added an dependency injection framework.  This week I want to make the app look a little less blah so I’ll be adding themes and styling.

Recap and Code

This is the seventh post in the series, you can find the rest here:

  • Day 0:  Getting Started (blog / code)
  • Day 1:  Binding and Navigation (blog / code)
  • Day 2:  Frames, Event Handlers, and Binding Bugs (blog / code)
  • Day 3:  Images in Lists (blog / code)
  • Day 4:  Search and Barcode Scanner (blog / code)
  • Day 5:  Dependency Injection (blog / code)
  • Day 6:  Styling (blog / code)

The latest version of the code can always be accessed on the GitHub project page.

Styling Options in XF

The guidance in Xamarin’s documentation is to create custom renders for each control you want to style for each platform.  This seemed like an unintuitive and complicated way to go about solving the problem so I avoided trying it as long as possible.

@TheRealJoeRall suggested an interesting idea to try: use the native theming options from each platform.  On WindowsPhone add styles with a TargetType but no key to the App.xaml.  On Android create a theme in xml and apply it to the top level activity.  On iOS use the UIAppearance class.  Like the content render solution this approach requires writing a different theme for each platform, but it uses the native approach meaning that there will be more documentation and tooling around it.  Now, let’s just hope it works.

Quick Reminder about iOS

Before I go any further, I want to put out a reminder that since I don’t have access to iOS these articles won’t cover it.  Which means that I was unable to determine if the UIAppearance class would be able to help in styling a XF application.

Styling Windows Phone

There’s lots of documentation on how to style and theme a native Windows Phone app.  Most of it written by people who know a lot more about the topic than I do.  I’m going to keep mine to the basics.

The first thing to know is that any styles you create in the App.xaml file will be accessible from all of your content pages by default.  But how do you create a style in the first place?

<Application>
  <Application.Resources>
    <ResourceDictionary>
      <Style TargetType="TextBlock" >
        <Setter Property="Foreground" Value="Black"/>
      </Style>
    </ResourceDictionary>
  </Application.Resources>
</Application>

Above is a very simple style that sets the Foreground of all TextBlocks to black.  Again, this style can get much more complex, but I’ll leave it here for now.

You could make this an explicit style by giving it a key, x:key=”BlackTextBlock” for example.  This way it would only apply to TextBlocks that specifically reference the style.  But since all of our pages are currently defined in the common Xamarin Forms layer, we don’t have TextBlocks at all, and can’t reference explicit styles.  So while explicit styles are great when writing native Windows Phone pages, they are not very interesting in XF.

Again, I could rant on this a lot longer, but I’m not the best source.  My style for WP is quite long but not much more than a basic example, so I won’t bother with the snippet here.  If you’re interested, you can view my full App.xaml on GitHub.

Limitations With Using Windows Phone Styling

My goal with the styling was to create a white background.  I could not figure out a way to set the background color from within the Windows Phone project.  I tried several techniques that did not work.

First, I tried explicitly setting the color in the MainPage.xaml (the following code is stripped down for size and clarity):

<phone:PhoneApplicationPage
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    Background="White" />

This had no effect so I tried setting it programmatically in MainPage.xaml.cs.  I tried setting both the MainPage background as well as the navigation page returned from the common layer.  Neither worked, either separately or together.  I’ll show them all at once.

public MainPage()
{
    this.Background = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Colors.White);

    var startupPage = ShoppingCart.App.StartupPage;
    startupPage.BackgroundColor = Xamarin.Forms.Color.White;

    Content = startupPage.ConvertPageToUIElement(this);
}

Digging down into the child page of the navigation page did work, but that had two drawbacks.  First it only worked for that one page, so I’d have to change the color of all the pages.  Second, it pushed me up into the common layer.  Since I was in the common layer anyway it would just be easier to set the background page color directly in the XAML definitions of the pages.  This is ultimately what I did.  Here’s an example from the Welcome page.

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ShoppingCart.Views.WelcomePage"
             BackgroundColor="White" >
   <!-- Layout Removed -->
</ContentPage>

Note that I could have used Xamarin Form’s OnPlatform mechanism to selectively set the property only for Windows Phone.  I elected not to since I did want the background to be white on all platforms anyway, even if I did not need to set the property explicitly for Android to work.

Styling Android

Android also has a very simple way to set the overall style for all Activities.  They also have much better documentation than I could mimic.  The key take away here is to define your android theme in an xml file in the values folder of your project.  Then reference it in your AndroidManifest.xml file.  The quickest way I’ve found to create a theme is to use the online Holo Colors Generator.

Holo COlor Generator Screen Shot

You just enter the color you want to use in your theme and tick off the checkboxes of the controls you want this theme to apply to.  It generates a zip that you can extract directly into your resources directory.  Just include all of the files and set your theme in the AndroidManifest like this where AppTheme is the the name of the theme in the themes_apptheme.xml file:

<application android:theme="@style/AppTheme" />

That’s it for styling droid.  It’s pretty straight forward and took me about 15 minutes.  The color I picked was a nice light green:  #afcca6.

Problems with This Approach

One major limitation with this approach is that it treats all things equally  All text boxes will look the same.  If for example you wanted the text box at the top of the product page to be a color based on the theme, you can’t easily achieve that.  You’d have to fallback to manually setting the various properties correctly on each page.

Solution 1:  Theme Class

For this specific case, I created an interface IThemer with a default implementation that could be overriden for specific platforms.  For now, the only property exposed is AccentColor, but it could be expanded to provide other style related properties.

public interface IThemer
{
    Color AccentColor { get; }
}

public class DefaultThemer : IThemer
{
    public Color AccentColor { get { return Color.Accent; } }
}
public class AppSetup
{
    protected virtual void RegisterDepenencies(ContainerBuilder cb)
    {
        // ... Code removed for clarity
        cb.RegisterType<DefaultThemer>().As<IThemer>().SingleInstance();
    }
}

Originally, the DefaultThemer I created just returned a randomly generated dark color and the Windows Phone implementation overrode it with the AccentColor pulled from the app’s resource dictionary.  Then while playing around I found the Xamarin.Forms.Color.Accent property.  On Windows Phone this returns the phone’s accent color.  On Droid it returns black.  This is great in that it gets me half way there, but I want to use my nice light green color I defined in my droid styles:  #afcca6.  So in the droid project I created an implementation of IThemer and registered it with my dependency injection container from last week:

public class DroidThemer : IThemer
{
    public DroidThemer()
    {
        var resources = Forms.Context.Resources;
        int colorId = Resource.Color.apptheme_color;
        var color = resources.GetColor(colorId);

        AccentColor = Color.FromRgba(color.R, color.G, color.B, color.A);
    }

    public Color AccentColor { get; private set; }
}
public class DroidSetup : AppSetup
{
    protected override void RegisterDepenencies(ContainerBuilder cb)
    {
        base.RegisterDepenencies(cb);

        cb.RegisterType<DroidLogger>().As<ILogger>().SingleInstance();
        cb.RegisterType<DroidScanner>().As<IScanner>().SingleInstance();
        cb.RegisterType<DroidThemer>().As<IThemer>().SingleInstance();
    }
}

The code grabs the color as it is defined in my app’s theme and sets the AccentColor property.  Now if the color is ever changed in the theme, it will propagate to my special theming class.  Again, I simply override the existing registration for the IThemer with the DroidThemer.  AutoFac is smart enough to remove the prior registration made in AppSetup.

Now the question becomes, where do I put this class?  I could make the BaseViewModel take an instance of it so that all of the views can easily know where to access it.  I don’t like this for a number of reasons.  First and foremost this is not view model information.  It has nothing to do with the state of the app or data, it’s purely view.  Secondly, adding constructor parameters on a base class is a headache to mange long term, especially when you wind up having a lot of inheriting classes.  You need to touch each child class whenever those parameters change.  This is definitely not the solution for me.

My second idea was to expose the AccentColor property on the App class.  This really is a system wide app setting so the App class does make a certain amount of sense.  Besides, all of my views are already aware of it since that’s where they are getting their view models.  Also, it’s really quick to add it in this one place.  It’s not an ideal solution since I’m trying to NOT have App turn into a dumping ground for properties (hence my clean up last week) but it’s so quick and easy now, that I’ll go with it.

public static class App
{
    private static IContainer _container;

    public static void Init(AppSetup appSetup)
    {
        _container = appSetup.CreateContainer();
        AccentColor = _container.Resolve<IThemer>().AccentColor;
    }

    public static Color AccentColor { get; private set; }
}

The init method pulls out the IThemer implementation and grabs the AccentColor from it.  After this I bind to the property from within the WelcomePage.

<Label Text="Welcome to The Store"
       Font="Bold, Large" 
       HorizontalOptions="Center" 
       TextColor="{x:Static local:App.AccentColor}" />

I only used this solution in one place:  WelcomePage.  While it is good for keeping all of the style definitions in one place I really only had one style that I wanted to use:  accent color.  As it turned out Xamarin already did most of the heavy lifting for me.  Isn’t that always the case?

Solution 2:  Color.Accent and OnPlatform

After finding out that I could just use the Color.Accent static color (I really did find that very late in the game), I realized that there was a much simpler way to solve the problem.  On platforms that support the Color.Accent, use it, on other platforms use my alternate color.  This is easy to accomplish with the OnPlatform class.  Here’s an example from my ProductsPage:

<Label Text="{Binding Product.Name}"
       HorizontalOptions="Center"
       Font="Bold,Large">
  <Label.TextColor>
    <OnPlatform x:TypeArguments="Color"
                iOS="Accent"
                WinPhone="Accent"
                Android="#afcca6" />
  </Label.TextColor>
</Label>

On WinPhone and iOS, the Accent color is used.  On Droid, I fallback to my custom light green color.

Pics or It Didn’t Happen

I was mostly concerned with the how-to aspect of this write up and avoided screen shots throughout the post.  But, now that the post is over, here’s what the app looks like after my styling efforts.

Welcome Screen

Both_Welcome

Categories Screen

Both_Categories

It’s starting to look just a little more polished.

Happy Coding.

Xamarin.Forms: Search and Barcode Scanner

To recap, I’m writing a shopping cart app for Windows Phone, Android, and iOS.  The purpose of the app is primarily to let me use Forms.  Each post will build on top of the previous one.

Last time I worked I added asynchronously loaded images to a list view.  Today I plan on adding a product search that uses a barcode scanner using the ZXing component.

Recap and Code

This is the fifth post in the series, you can find the rest here:

  1. Day 0:  Getting Started (blog / code)
  2. Day 1:  Binding and Navigation (blog / code)
  3. Day 2:  Frames, Event Handlers, and Binding Bugs (blog / code)
  4. Day 3:  Images in Lists (blog / code)
  5. Day 4:  Search and Barcode Scanner (blog / code)

The latest version of the code can always be accessed on the GitHub project page.

Search

Before I implement search with a barcode, it’ll be easier if I add a search box.  For now, I’ll just add it on the categories page.  This should be simple, I’ll throw up a text box and a button.

<StackLayout VerticalOptions="FillAndExpand" Orientation="Horizontal">
  <Entry Text="{Binding SearchTerm}"
         Placeholder="Search"
         HorizontalOptions="FillAndExpand" />
  <Button Text ="Search"
    Command="{Binding SearchCommand}" />
</StackLayout>

Next I wire them into the view model.

private readonly RelayCommand _searchCommand;

public CategoriesListViewModel(IProductService service, INavigationService navi)
{
    _searchCommand = new RelayCommand(async () =>
    {
        var items = (await _service.Search(SearchTerm))
                    .OrderByDescending(i => i.Rating)
                    .ToList();

        if (items != null && items.Any())
        {
            Page page = items.Count == 1
                 ? page = App.GetProductPage(items.First())
                 : page = App.GetProductsListPage(items, SearchTerm);

            await _navi.PushAsync(page);
            SearchTerm = string.Empty;
        }
        else
        {
            await _navi.DisplayAlert("Error", "No results for search " + SearchTerm);
        }
    },
    () => !string.IsNullOrWhiteSpace(SearchTerm));
}

public ICommand SearchCommand { get { return _searchCommand; } }

public string SearchTerm
{
    get { return GetValue<string>(); }
    set
    {
        SetValue(value);
        _searchCommand.RaiseCanExecuteChanged();
    }
}

If our search has exactly one result, then we go directly to that product page; if there are multiple results, we show them in a list.

The ProductService already has a stub for search, but was only half implemented.

public async Task<List<Product>> Search(string searchString)
{
    var items = await _itemsAsync;

    if (string.IsNullOrWhiteSpace(searchString)) return items;

    searchString = searchString.ToLower();
    var filterd = items.Where(i => i.Name.ToLower().Contains(searchString))
                       .ToList();

    return filterd;
}

Adding a Component

The first step in using a component is to add it to your project.  Right click on the “Components” folder in your Droid or iOS project and select “Ge More Components…”

image

I searched for a barcode scanner and got a list of packages.  ZXing sounds familiar and supports all platforms so I go with that.

image

Window’s phone is a little simpler in that I can just download the package from NuGet:

Install-Package ZXing.Net.Mobile

Using a Component

Since each platform references its own implementation of the barcode scanner software, I now need to create a device agnostic interface in the common project.

public interface IScanner
{
    Task<ScanResult> Scan();
}

public class ScanResult
{
    public string Text { get; set; }
}

I then implement these interfaces for each of the platforms.

Android first:

public class DroidScanner : IScanner
{
    public async Task<ScanResult> Scan()
    {
        var scanner = new MobileBarcodeScanner(Forms.Context)
        {
            UseCustomOverlay = false,
            BottomText = "Scanning will happen automatically",
            TopText = "Hold your camera about \n6 inches away from the barcode",
        };

        var result = await scanner.Scan();

        return new ScanResult
        {
            Text = result.Text,
        };
    }
}

The MobileBarcodeScanner takes the current instance of the Android context.  Because this is a common requirement in Android API’s Xamarin exposes the current context in the static property Forms.Context.

WindowsPhone is essentially the same, just a different constructor:

public class WinPhoneScanner : IScanner
{
    public async Task<ScanResult> Scan()
    {
        var scanner = new MobileBarcodeScanner(MainPage.DispatcherSingleton)
        {
            UseCustomOverlay = false,
            BottomText = "Scanning will happen automatically",
            TopText = "Hold your camera about \n6 inches away from the barcode",
        };

        var result = await scanner.Scan();

        return new ScanResult
        {
            Text = result.Text,
        };
    }
}

Next I register the device specific implementations with the XF DependencyService.  In my Droid project I create a file called ServiceRegistration and add the following lines

using ShoppingCart.Droid.Services;

[assembly: Xamarin.Forms.Dependency(typeof(DroidScanner))]

A similar file is created for WP:

using ShoppingCart.WinPhone.Services;

[assembly: Xamarin.Forms.Dependency(typeof(WinPhoneScanner))]

Now to create an instance of IScanner I resolve it from the DependencyService:

IScanner scanner = DependencyService.Get<IScanner>();

Now all I have to do is wire up a new button to to launch the scanner and use the results.

ScanCommand = new RelayCommand(async () =>
{
    var result = await _scanner.Scan();

    SearchTerm = result.Text;
    Search();
});

The scan command just calls into the search implementation I wrote above.

Adding Permissions

And finally, I just need to add in the permission to access the camera, otherwise the barcode scanner component won’t be able to start up at all.  On Windows phone open up the WMAppManifest, switch to the Capabilities tab, and make sure that ID_CAP_ISV_CAMERA is checked.

imageOr just edit the file by hand and add the permission by hand.

<Capability Name="ID_CAP_ISV_CAMERA" />

Android is similar. Right click the project, select Properties, go to the Android Manifest tab, and select CAMERA in the Required permissions section.

image

Again, you can do this manually by editing the AndroidManifest.xml file directly and adding the CAMERA permission.

<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="auto">
  <uses-sdk />
  <application></application>
  <uses-permission android:name="android.permission.CAMERA" />
</manifest>

Happy coding

Xamarin.Forms: Frames, Event Handlers, and Binding Bugs

To recap, I’m writing a shopping cart app for Windows Phone, Android, and iOS.  The purpose of the app is primarily to let me use Forms.  Each post will build on top of the previous one.

Last time I got some basic binding setup and navigation working.  Today I plan on cleaning up the views a bit so that they look “nice”.  Well nicer.  After that I’d like to show a list of products and let the user drill in and look at details.

Recap and Code

This is the third post in the series, you can find the rest here:

  • Day 0:  Getting Started (blog / code)
  • Day 1:  Binding and Navigation (blog / code)
  • Day 2:  Frames, Event Handlers, and Binding Bugs (blog / code)

The latest version of the code can always be accessed on the GitHub project page.

Cleaning up Login and Main Page

Last time I was focused on functionality.  I was happy to just get the pages to do what I want. Now i want to take a little time and play with the layout a bit.  First on the docket is “MainPage”.  It really isn’t the main page, so I’ll rename that to “WelcomePage”.  This includes the the view model as well.

With that done I want to add some space around all of the text box and button.  Problem is: there’s no “margin” property on any of the controls.  After a little digging, it seems that the only way to add spacing is to wrap each control in its own ContentView and set the Padding property on that.  A slightly simpler approach is to use a Frame instead.  It inherits directly from ContentView and has a default padding of 20.  Despite the fact that this only saves me from setting one property, the fact that it’s in a Frame helps me remember why I’m wrapping the control in the first place.  Let’s wait a few weeks and see if I continue using Frames.

The WelcomePage (né MainPage) now looks like this:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ShoppingCart.Views.WelcomePage"
             xmlns:local="clr-namespace:ShoppingCart;assembly=ShoppingCart"
             BindingContext="{x:Static local:App.WelcomeViewModel}">

  <StackLayout
    VerticalOptions="Center">
    <Frame>
      <Label Text="Welcome to The Store" Font="Bold, Large" HorizontalOptions="Center" />
    </Frame>

    <Label Text="Login to start shopping" HorizontalOptions="Center" />

    <Frame>
      <Button Text ="Log In" Command="{Binding GoToLoginPageCommand}" HorizontalOptions="Center" />
    </Frame>
  </StackLayout>
</ContentPage>

I also tweaked the welcome text making it bigger and bold as well as adding a call to action to help the user navigate to their next step.  There’s a fair bit you can do with the Font (size, style) property just by providing a comma separated list of values.

The login page got the same spacing treatment, including a nice fat margin around the entire page just so the text boxes don’t sit flush against the right side.  It’s still a little stark so I’ll throw in a touch of color on the title text, just because I can.

<StackLayout VerticalOptions="FillAndExpand" Padding="50">

<Frame Padding="75">
  <Label Text="Login" 
    Font="Bold,Large"
    TextColor="Yellow"
    HorizontalOptions="Center" />
</Frame>

<Label Text="User name" 
  HorizontalOptions="Start" />
<Entry Text ="{Binding Username}" 
  Placeholder ="User name goes here" 
  HorizontalOptions="FillAndExpand" />

<Label Text="Password" 
  HorizontalOptions="Start" />
<Entry Text ="{Binding Password}" 
  Placeholder ="Password goes here" 
  HorizontalOptions="FillAndExpand" />

<Button Text ="Log In" 
  Command="{Binding LoginCommand}" 
  HorizontalOptions="CenterAndExpand" />

</StackLayout>

Now that looks a little bit nicer.

image

Data

Now that the two pages I have look reasonable, I’ll add another.  In order to show some data, I actually need data.  To keep it simple, I start off by creating a list of hard coded C# data.  I have to admit that I got a bit silly here.  At first I tried to hand craft a back log of data.  That got old really fast.  In fact I only got one item defined before I realized I was wasting a lot of time.  Next I decided to grab a list of products (books) from the web and just tweak the data to my needs.  This too proved onerous.  Then I broke down and went to the web to generate all of my data.  I found a great site that even outputs the data in JSON.  It was the first hit on Google.  To process the JSON i nuget and install Newtonsoft’s Json.NET.

I add a ProductLoader and a ProductService class.  The loader simply stores the literal string of JSON and deserializes it on request.  In the future I want to create another implementation that reads the data from disk or the web.  The ProductService doesn’t care where the data comes from, it provides the view models with an interface to query and filter the data.  Because the underlying data will eventually come from a web request, both of these services asynchronously return Tasks. I use Stephen Cleary’s NotifyTaskCompletion in my view model to consume these services.  For a detailed explanation of what’s going on here, take a look at his Patterns for Asynchronous MVVM Applications series.

The data object itself is pretty simple.

public class Item
{
    public string Category { get; set; }
    public string Description { get; set; }
    public string ImageUrl { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }
    public string ProductCode { get; set; }
    public int Rating { get; set; }
    public List<string> Tags { get; set; }
}

The Category property lets us show a short list to the user once they log in.  Once they pick a category they see all of the items in that category and then drill down into a specific item.  To accommodate this flow, I’ll add three more pages with corresponding view models:

  • CategoriesListPage/ViewModel
  • ProductsListPage/ViewModel
  • ProductPage/ViewModel

With a bigger app I’d lean towards single instances of each of these pages and using a message broker to update one from the other.  I..e, when a category is clicked on in the CategoriesList page I’d send an “Update List of Products” message and then navigate to the ProductsPage.  But since I already have the convenient App.cs handling all of my interactions between pages, I’ll just squash it into there.  Not ideal for a larger app that I’d like to keep decoupled, but fine for the five pages I currently have.

Lists and DataTemplates

The first thing to tackle is to show the list of categories.  This is similar to traditional two step process in Windows XAML.  Step one:  bind the ItemsSource property to the list.  A quick reminder that the Categories property is a NotifyTaskCompletion<List<string>> which means I need to bind against Categories.Result.  Step two:  define a data template to define how each row looks.  Strictly speaking, since my underlying data is a string, I can skip this step.  Since this is my first time defining a data template in XF, I’ll define one anyway just to make sure it works how I expect.  In my example I’m making the background color red just so I can verify that it’s using my DataTemplate.

<ListView ItemsSource="{Binding Categories.Result}" ItemSelected="OnItemSelected">
    <ListView.ItemTemplate>
      <DataTemplate>
        <ViewCell>
          <Label Text="{Binding .}" BackgroundColor = "Red" YAlign="Center" Font="Medium" />
        </ViewCell>
      </DataTemplate>
    </ListView.ItemTemplate>
  </ListView>

Handling Events

You’ll notice in the above definition of my ListView that I’m setting the ItemsSelected event handler.  I’m not binding against the view model here, I’m calling into code behind which then calls into my view model.

public partial class CategoriesListPage
{
    public CategoriesListPage()
    {
        InitializeComponent();
    }

    private void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
    {
        var param = e.SelectedItem as string;
        var command = ((CategoriesListViewModel)BindingContext).NavigateToCategory;

        if (command.CanExecute(param))
        {
            command.Execute(param);
        }
    }
}

This is a lot of boilerplate code for handling a click.  The problem is that the ListView doesn’t expose a command I can bind against when an item is selected.  In a traditional Xaml app I’d use the EventToCommand pattern except that it is built on Blend behaviors which aren’t PCL compatible and therefore not Xamarin compatible.

Another option was to subclass the ListView class and expose the command logic that I need.  I might eventually go this route, but I’ll probably need more than a few list boxes in my app to justify it.

Binding Bug in XF

The command for navigating to a category’s page gets a list of all the items for that category and calls into the static App class to get the page to navigate to.

var page = App.GetProductsListPage(items, categoryName);
await _navi.PushAsync(page);

Originally App.cs just updated the properties on the view model and returned the single instance of the ProductsListPage.

public static Page ProductsListPage { get; private set; }

public static Page GetProductsListPage(List<Item> items, string title)
{
    if (string.IsNullOrWhiteSpace(title)) title = "Products";

    ProductsListViewModel.Products = items;
    ProductsListViewModel.Title = title;
    return ProductsListPage;
}

This relies on the binding to update the view when something has changed and INotifyProperty.PropertyChanged is raised.  The first time this is called it works just fine.  It fails on all subsequent calls.  After a lot of debugging and assuming that I was wrong I found a recent post on Xamarin’s forums explaining that there is a bug where the UI is not updated when a bound value is changed.  Note that this only effects values that are updated in the view model updating on the view; the other way works just fine.  Updating a value in the view (like a text entry) correctly updates the bound value in the view model.  This is why my login page worked just fine.

Xamarin has released a fix for this bug, but as of writing this it is in a pre build of XF.  I tried to use it but kept getting runtime DLL errors.  I tried several times before having to give up on this as an immediate solution.  I will say that this may have just been an issue with user error since it was close to 1 AM at this point.

In my specific case, there was no reason that I had to reuse the same view.  Simply recreating the view every time I wanted to display it was simple enough.

public static Page GetProductsListPage(List<Item> items, string title)
{
    if (string.IsNullOrWhiteSpace(title)) title = "Products";

    ProductsListViewModel.Products = items;
    ProductsListViewModel.Title = title;
}

Summary

Today I was able to clean up some of the views using the Frame control to provide margins.  I had to resort to two workarounds, one for routing events to commands in code behind the other for updating the view when a bound value has changed.

I’m not sure what I want to tackle next week.  Perhaps reading from the file stream?  Maybe fleshing out the products view a bit so I display an image to go with the product?

Until then, happy coding.