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.

Xamarin.Forms: Binding and Navigation

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 we did little more than get our feet wet.  The plan for today is to create a page with data binding  At this stage of the app, that sounds like a login page to me.

An aside.  It got rather tiring saying “Xamarin.Forms” all the time and I’m not quite comfortable just calling it “Forms” because I keep thinking back to WinForms whenever I do.  So I’m just going to go the abbreviation route and call it XF.  The abbreviation has the benefit of starting with an “X” and those are just cool.

Recap and Code

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

  • Day 0:  Getting Started (blog / code)
  • Day 1:  Binding and Navigation (blog / code)

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

Binding

Before I’m able to do anything fancy, I need to figure out how to do some basic binding.  To test this out, I create a basic view model with one property that returns some constant text.

public class MainViewModel
{
    public string Message {get { return "Hello World";}}
}

I update the view to simply bind to this message now instead of hard coding the text in the XAML.

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ShoppingCart.Views.MainPage">
    <Label Text="{Binding Message}" HorizontalOptions="Center" />
</ContentPage>

For now, I’ll just set the BindingContext in the page’s constructor.

public partial class MainPage : ContentPage
{
    public MainPage(MainViewModel vm)
    {
        this.BindingContext = vm;
        InitializeComponent();
    }
}

All that’s left is to pass in a new view model when creating the page.

new MainPage(new MainViewModel())

Firing up the emulator, this works out of the gate.  Nice.  Time to move on to something a little more difficult.

The Main Page

I don’t want the login page to be the first page of the app.  I think some sort of splash screen or welcome message would make more sense.  For now It will simply just have some text and a button that’ll take you to the login page.  I’ll add the standard Command=”{Binding GoToLoginPageCommand}” now and wire it up next.

<?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.MainPage"
             xmlns:local="clr-namespace:ShoppingCart;assembly=ShoppingCart"
             BindingContext="{x:Static local:App.MainViewModel}">

  <StackLayout
    VerticalOptions="Center">
    <Label Text="Welcome to the main page." HorizontalOptions="Center" />

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

Now that I know I need to bind to a command, I add the MainViewModel.  The command adds some complexity for two reasons.  First it’s a command so I need to either implement ICommand or find an already ready implementation.  There’s nothing that I could find in XF that implements ICommand for me and I don’t want to bring in an outside framework like MvvmLight just yet so I go ahead and add add a simple implementation of ICommand.  It’s so simple that I even named it SimpleCommand.cs.  I’m not really sure if this will stay long term, but it’ll do the job for now.

The second bit of complexity isn’t as easy.  It’s the fact that I don’t know how to do navigation in XF.  So it’s time for some research.  To the LIBRARY!

Figuring Out Navigation

According to the doco, all you need to do is wrap your ContentPage in a Navigation page.  This is as simple as

new NavigationPage(new MainPage())

Then in your page you can access the Navigation property.  After playing with this property a bit I learned that how it works depends on your platform.  On Windows Phone it was non null and worked regardless of whether or not you created a NavigationPage.  Android was less forgiving.  One nice touch was that as soon as you wrap your droid page you get the ActivityBar back button.

Playing around with the Navigation property on the page on both Windows and Android, I found it to be buggy.  I set up a simple pair of pages with one button each.  The first page would merely navigate to the second page.  The second page would just pop the stack and return to the first.  With some experimenting it looked to me that the XF infrastructure worked best if you always used the same exact instance of INavigation.

In my opinion, there’s a problem with navigation living in the page.  Navigation is important business logic and business logic should not live in the view layer.  I don’t want to fight the infrastructure too much but if at all possible, I’m going to want to see what can be done to move navigation to the view model layer.

With this knowledge and my desire to abstract the navigation property out to the view model, I created my own wrapper navigation service.

public class NavigationService : Xamarin.Forms.INavigation
{
    public INavigation Navi { get; internal set; }

    public Task<Page> PopAsync()
    {
        return Navi.PopAsync();
    }

    public Task<Page> PopModalAsync()
    {
        return Navi.PopModalAsync();
    }

    public Task PopToRootAsync()
    {
        return Navi.PopToRootAsync();
    }

    public Task PushAsync(Page page)
    {
        return Navi.PushAsync(page);
    }

    public Task PushModalAsync(Page page)
    {
        return Navi.PushModalAsync(page);
    }
}

Very simply, it directs all calls to an internal implementation.  Again, the static App class is responsible for coordinating the creation of these classes

NavigationService navi = new NavigationService();
MainViewModel = new MainViewModel(navi);

MainPage = new NavigationPage(new MainPage());

navi.Navi = MainPage.Navigation;

Back to the MainViewModel

Now that we’ve solved the problems of commands and navigation, it’s pretty simple to finish off our MainViewModel.

public class MainViewModel
{
    private readonly INavigation _navi;

    public MainViewModel(INavigation navi)
    {
        _navi = navi;
    }

    public ICommand GoToLoginPageCommand
    {
        get { return new SimpleCommand(() => _navi.PushAsync(App.LoginPage)); }
    }
}

I left the creation of the login page in the App.  This way all my page creation can live in one place.

The Login Page

The login page and view model model are pretty simple.  Fields for username and password, and a button to submit.  Upon “successfully” logging in we’ll just return back to the  the main page.  If there’s a failure then we display a message box.

Like navigation, displaying a message box is tightly coupled to the page.  I wound up wrapping it up in my Navigation class in a similar fashion as navigation.  It’s not really the best spot for it, but it cut down on the number of classes and interfaces i was creating.  Also, it was a fast addition.  Given the size of the project I don’t really have a problem with it now, but I foresee it being moved out at some point.

Here’s what the navigation interface and implementation look like now:

public interface INavigationService : INavigation
{
    Task<bool> DisplayAlert(string title, string message, string accept, string cancel = null);
}

public class NavigationService : INavigationService
{
    public INavigation Navi { get; internal set; }
    public Page myPage { get; set; }

    public Task<Page> PopAsync()
    {
        return Navi.PopAsync();
    }

    public Task<Page> PopModalAsync()
    {
        return Navi.PopModalAsync();
    }

    public Task PopToRootAsync()
    {
        return Navi.PopToRootAsync();
    }

    public Task PushAsync(Page page)
    {
        return Navi.PushAsync(page);
    }

    public Task PushModalAsync(Page page)
    {
        return Navi.PushModalAsync(page);
    }

    public Task<bool> DisplayAlert(string title, string message, string accept, string cancel = null)
    {
        return myPage.DisplayAlert(title, message, accept, cancel);
    }
}

Wiring up ViewModels

A pattern I have always liked from the MvvmLight toolkit is setting up the binding to the view model directly in the view’s XAML.  I had to tweak the default MvvmLight template a bit to get this to work, but in the end it wasn’t pretty simple.  In the view, create a static binding to a property exposed by the App class.  This could be any class, but I went with the App class since it is already being used as a static resource for all of the views anyway.

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

</ContentPage>

This lets us simplify App.cs just a little bit, now that we don’t have to pass in the view models to the views.

public static class App
{
    public static MainViewModel MainViewModel { get; set; }
    public static LoginViewModel LoginViewModel { get; set; }

    static App()
    {
        ILoginService login = new LoginService();
        NavigationService navi = new NavigationService();

        MainViewModel = new MainViewModel(navi);
        LoginViewModel  = new LoginViewModel(login, navi);

        MainPage = new NavigationPage(new MainPage());
        LoginPage = new NavigationPage(new LoginPage());

        navi.Navi = MainPage.Navigation;
        navi.myPage = MainPage;
    }

    public static Page LoginPage { get; private set; }
    public static Page MainPage { get; private set; }
}

Summary

This is pretty much it for this week.  I’ve added binding, commands, and navigation as well as wired up the view models.  Next week I’ll flesh out the products pages and try to actually add the shopping cart.  Until then, happy coding.

Xamarin.Forms: Setup

This will be the first in a series where I create a shopping cart application using Xamarin.Forms  While I have experience using Xamarin and MVVMCross to create applications, I have never used Forms before.  This series is intended as a journal of my learning experience where I will point out stumbling blocks and tricks I pick up along the way.

Setting out, I plan to build a cross-platform shopping cart application using XAML using the MVVM design pattern.  I’ll focus my tests on Windows Phone because it’s much easier and faster to test but I will also verify that the app works on Android whenever it makes sense.  Sadly I don’t have the equipment necessary to test on iPhone.

The source code for this article can be downloaded from GitShoppingCart-Day0.zip

Quickly, What is Xamarin.Forms?

Traditionally Xamarin has let you write a WindowsPhone, iOS, and Android apps in C#, letting you share all app logic.  You would only need to rewrite your views for each platform, using the device specific controls and paradigms.

With Forms is Xamarin has added a cross platform UI framework.  It allows you to define your UI with device agnostic names for common controls.  For instance, in Forms you’d use an Entry object instead of the WindowsPhone TextBox or the Android EditText.  When run, the app still renders 100% native controls.  So even though they share a common UI definition, buttons on  WindowsPhone look like WindowsPhones butons; buttons on Android look like Android buttons; Buttons on iPhone look like iPhone buttons.

Getting Started

With all the preliminaries out of the way, let’s create our first app.  Fire up Visual Studio (you can use Xamarin Studio if you like, but VS is my home). Go to File-> New-> Project and pick Blank App (Xamarin.Forms Portable).  You can use the filter in the top right to find the project template more easily.  I’m calling it ShoppingCart.

image

This will take a second to generate 4 projects for you:

  • ShoppingCart
  • ShoppingCart.Android
  • ShoppingCart.iOS
  • ShoppingCart.WindowsPhone

image

ShoppingCart is your main project.  It is referenced by the other three.  At this point you can set any of the device projects (ShoppingCart.Android, ShoppingCart.iOS, ShoppingCart.WindowsPhone) as the startup object and deploy the app to your device or emulator.

ShoppingCart only has one code file in it, App.cs. All this does is create an instance of a Page and return it.

public class App
{
    public static Page GetMainPage()
    {
        return new ContentPage
        {
            Content = new Label
            {
                Text = "Hello, Forms !",
                VerticalOptions = LayoutOptions.CenterAndExpand,
                HorizontalOptions = LayoutOptions.CenterAndExpand,
            },
        };
    }
}

So it seems the Page class is the base of every view, analogous to a WindowsPhone PhoneApplicationPage, iOS UIViewController, or Android  Activity.  The device specific startup logic grabs the Forms Page object and sets it as it’s startup object.  Pretty straight forward.

Starting out with XAML

All I want to accomplish today is to change the definition of that view from C# to XAML.  In the ShoppingCart project, I’ll create a new folder called “Views” an add a Forms.Xaml Page named “MainPage”

image

image

This is a new item template that you get with the Forms projects.  Similar to the PhoneApplicationPage, it’s a XAML file with a code behind.  You can see that it inherits from ContentPage, the same type that App.GetMainPage creates.

At this point, I’m starting to notice that Visual Studio is starting to slow down. Switching between tabs and making edits to the .xaml file is sluggish.  Now, this may just be my machine.  It’s fast, but only has 4GB or RAM.  It’s also been acting up in other ways lately and I’ve been meaning to reformat.  Since I don’t seem to be getting intellisense in the xaml file, I’ll just switch to an alternative text editor instead.  I’ve already set up VS to open files in vim when I hit Ctrl-Alt-V so this isn’t really a big annoyance for me.

This view is supposed to be a simple copy of the one defined in App.cs, so I’ll delete the default Label they give me from the template and replace it with one with some static text.  Although, it is nice to note that the sample uses a simple binding for the text property.  I wonder where we set what we bind against, but I’ll worry about that later.

<?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.MainPage">
  <Label Text = "Hello, XAML Forms!"
         VerticalOptions = "CenterAndExpand"
         HorizontalOptions = "CenterAndExpand" />
</ContentPage>

OK, my first bit of Form xaml done and it wasn’t too painful.  Now to wire it up.  Going back to App.cs, instead of creating a new ContentPage, I new up an instance of MainPage and return that.

public static Page GetMainPage()
{
    return new MainPage();
}

Firing up the emulator, it looks like it works.  I changed the text in the label ever so slightly to make it easy to ensure that I know that I’m seeing my changes.

So that’s enough for me to call it a victory and go reward myself with some XBox One.

The source code for this article can be downloaded from GitShoppingCart-Day0.zip.  Note how it is stunningly close to the default template.  Not much changed.

Building iOS views in code

The Problem

I started doing iOS development in Xamarin in C# (one look at Objective-C was enough for me).  Writing C# for iOS in Xamarin Studio is where I spend most of my days.  Occasionally I switch over to Windows to lather in the Visual Studio goodness, however, Xamarin Studio is mostly enjoyable.  When it comes to creating UIs in iOS, the standard approach is to fire up Xcode, build your UI in the WYSIWYG editor, wire up your outlets, save your changes, and hope Xamarin Studio picks up your changes.  It’s a fairly arduous process, but it works.

My coworkers don’t have access to an instance of Xcode (there’s only one MacBook in my team).  This means, they can’t make any UI changes.  On top of that, I don’t like building UIs in the Xib interface builder.  I find it rather confusing and overkill for my needs.  I’m also the type of person who would rather build my UI out in Photoshop, and translate it directly to code.  I guess that’s the web developer in me.

The Solution

With iOS 6 came Auto Layout – the ability to create relationships between UI elements using constraints.  Once you add all your elements to the parent element, you define the relationships between each of the objects.  The platform will then draw the UI for you, honoring all of your constraints (if you specified them correctly).

Let’s say, I want to build the following view.  Here’s what the code looks like (come back when you’re finished throwing up):

OldConstraintsParentChild

That just sucks.

Another solution

XibFree is a really nice solution.  It does what it says – let’s you build UIs without a XIB.  It was the first solution I played around with.  I’m not going to go into too many details about it (ironically, the documentation is what I find most lacking).  It’s a lot less wordy and mostly uses the Android XML language.  If you’re doing cross platform development AND you’re coming from Android, the learning curve is not as steep.

I guess my biggest issue with XibFree was animating, showing, hiding, etc.  For the most part, it worked fine.  As my interface got more and more complicated, it became difficult to debug some of these issues.

My favorite solution

If you aren’t familiar with @slodge or MvvmCross, get ready to have a good time.

In a mere couple hours, Stuart Lodge changed me life.  To see how, read hist post (http://slodge.blogspot.com/2013/07/playing-with-constraints.html).  In short, he’s build a library which allows you to use a declarative way to define your views using the native NSLayoutConstraints.  Score!

It’s so easy to use, let’s make the example a little bit more complicated.

ConstraintsParentChild

   

And another example.

ConstraintsSameLeftBelow

I know you’re excited.  Now go download the library and develop away.  It’s available on GitHub – https://github.com/MvvmCross/MvvmCross.

What can you do

Like XibFree, the documentation is a little lacking.  However, you have the source.  So, let’s see what we can do with this library.

  • child.AtTopOf(parent[, margin]) – attaches the top of the child’s frame to the top of parent’s frame
  • child.AtLeftOf(parent[, margin]) – attaches the left side of the child’s frame to the left side of the parent’s frame
  • child.AtRightOf(parent[, margin]) – attaches the right side of the child’s frame to the right side of the parent’s frame
  • child.AtBottomOf(parent[, margin]) – attaches the bottom of child’s frame to the bottom of the parent’s frame
  • view1.WithSameLeft(view2) – attaches the left value of view1 (view1.Frame.X) to the left X value of view2
  • view1.WithSameRight(view2) – attaches the position of the right side of view1 (view1.Frame.Right) to the same right value as view2
  • view1.WithSameTop(view2) – attaches the top Y value of view1’s frame (view1.Frame.Y) to the top Y value of view2
  • view1.WithSameBottom(view2) – attaches the bottom of view1’s frame (view1.Frame.Bottom) to the bottom as view2
  • view1.WithSameCenterX(view2) – attaches the horizontal center of view1 to the horizontal center of view2
  • view1.WithSameCenterY(view2) – attaches the vertical center of view1 to the vertical center of view2
  • view1.WithSameWidth(view2) – attaches the width of view1 to the width of view2
  • view1.WithRelativeWidth(view2[, scale]) – attaches the width of view1 to the width of view2, with an optional scale factor
  • view1.WithSameHeight(view2) – attaches the height of view1 to the height of view2
  • view1.WithRelativeHeight(view2[, scale]) – attaches the height of view1 to the height of view2, with an optional scale factor
  • view1.Below(view2[, margin]) – attaches the top value of view1 to the bottom value of view frame
  • view1.Above(view2[, margin]) – attaches the bottom value view1 to the top value of view2
  • view1.ToRightOf(view2[, margin]) – attaches the left value of view1 to the right value of view2
  • view1.ToLeftOf(view2[, margin]) – attaches the right value of view1 to the left value of view2
  • view1.FullWidthOf(view2[, margin]) – attaches the left and right values of view1 to view2
  • view1.FullHeightOf(view2[, margin]) – attaches the top and bottom of view1 to view2

Headers and Footers on an MvxListView

Full source for this post can be found on GitHub:  HeaderedGridView

Last week I posted about adding a header to a GridView.  Today I’ll be discussing how to add a header (and footer!) to a MvxListView.  The MvxListView is a data-binding friendly ListView from MvvmCross.  Adding a header here is a little different because we need an instance of HeaderViewListAdapter that implements IMvxAdapter .  Other than that, the code should be very familiar.

Requirements

Just so it is clear what we are building, here are my requirements for this control:

  1. Ability to add a header and footer to the MvxListView in xml markup
  2. The header is to be laid out inline with the rest of the items, at the top
  3. The footer is to be laid out inline with the rest of the items, at the bottom
  4. The header and footer need to scroll with the rest of the content, i.e., they are not always visible
  5. The header and footer must be able to use MvvmCross bindings

Some quick screenshots from the sample project will help us understand what this should look like.

top.png

The list is scrolled to the top with purple header visible

middle.png

Shows the list at some point in the middle when neither header nor footer is visible

bottom.png

The list scrolled all the way to the bottom with the green footer visible

Code

Full source for this post can be found on GitHub:  HeaderedGridView

The MvxListView assumes that the adapter it has implements IMvxAdapter.  We can create a simple wrapper class that inherits from HeaderViewListAdapter and implements that interface.  The constructor will accept the list of headers and footers like the base class in addition to accepting an IMvxAdapter to wrap.  All of the interface methods for IMvxAdapter can then be passed down to the internal adapter.

In the snippet of HeaderMxAdapter above we have two constructors.  The first can be used if you do not want to add a footer.  It simply calls into the second passing an empty list of footers to add to the adapter.  The only other thing the constructor does is to squirrel away the adapter in a private field for later use.

The implementation for DropDownItemTemplateId is simple and indicative of the rest of the methods in the class.  It merely calls into that squirreled away adapter, calling its implementation of DropDownItemTemplateId.  The rest of the methods are exactly the same and have been omitted for brevity.

Next step is to add a pair of attributes to use for our header and footer.  This is exactly the same as the headered grid view and is done in attrs.xml.

Some boiler-plate code in MvxListViewWithHeader will read these values so we know what views to use for the header and footer.  Like last time, I’m using some helper classes to do the parsing.  They are included in the project.

Next we need to create the views for the header and footer .

First we check if the id is the default header id, if it is then we don’t have a view defined in the xml, and we just return null.  After that we get the current binding context and use it to inflate our view.  This is the magic that lets us bind within our header and footer.

Once we have the view, we need to wrap it in a list of FixedViewInfo which is what the HeaderMxAdapter expects in its constructor.

Nothing earth shattering here.  If the view is null, return an empty list, otherwise construct the FixedViewInfo and add it to the list.  Returning an empty list allows us to specify only the header, only the footer, or neither without causing any errors.

A couple of helper methods make help to make it clear and concrete what GetFixedViewInfos does.

All of this is called from the constructor which creates the MvxAdapter and wraps it in our new HeaderMvxAdapter.  It then set the adapter property in the base class.

The constructor is also responsible for reading in the item template id which the MvxListView uses to display the individual items in the list.

Using the Control

You would use the MvxListViewWithHeader the same way as a MvxListView.  The only difference is that you can now optionally specify a header or footer attribute which would point to another layout.  The following is taken from the FirstView in the sample.

The header and footer layout in the sample are very simple.  They are just a TextView with the text property bound to the view model.  Here is the header layout, but the footer is almost identical:

Future Improvements

Like the headered grid view, this implementation only allows a single header. If we had need for multiple headers we could copy the  onFinishedInflate method from ListView.  It collects all the child layouts for the list view and adds them to the list of headers.

Happy Coding

Headered Grid View in MvvmCross. Now with bindings!

Full source code for this article can be downloaded on GitHub:  HeaderedGridView.

For the easy import feature of Yarly, I needed to create a view that would easily allow a user to select an existing photo to import into the app or to take new photo. A grid view of photos from the camera roll but with the first item as a button that would load the camera would serve nicely for this use case. See the screen shot below of an early version of the UI:

Yarly Screen Shots 2014-04-20_194747_242000

This reminded me of the AddHeaderView method available on ListView. With that as a guide, I decided to add similar functionality to the GridView.  Since I’m using MvvmCross, I also want to make sure that I can use bindings in the header control. While I’m at it, having to explicitly call AddHeaderView in my activity code is useful, but I’d much rather to be able to do this in markup. 

Requirements

To sum up the above requirements:

  1. Ability to add a header to a GridView in xml markup
  2. The header is laid out inline with the rest of the items in the GridView
  3. The header scrolls with the rest of the content, i.e., it is not always visible
  4. The header must be able to use MvvmCross bindings

Code

A full sample project can be downloaded from my GitHub repo:  HeaderedGridView.

In attrs.xml, add an attribute to use for our header. 

<!-- attrs.xml --> <declare-styleable name="GridView">

   <attr name="header" format="reference" /> </declare-styleable>

In the HeaderedGridView, all we need to do now is check for the presence of this attribute, and if it exists, inflate the view.  Reading the header id is straight forward processing of the IAttributeSet.  I use some helper classes to iterate and dispose of the attributes in a more C# way.  The details aren’t very important, but the relevant classes are included in the sample solution.  The MvvmCross class MvxAndroidBindingContextHelpers can return the current binding context which can be used to inflate and bind the header at the same time.

// HeaderedGridView.cs private void ProcessAttrs(Context c, IAttributeSet attrs) {

     _headerId = DEFAULT_HEADER_ID;

     using (var attributes = c.ObtainDisposableStyledAttributes(attrs, Resource.Styleable.GridView))

     {

         foreach (var a in attributes)

         {

             switch (a)

             {

                 case Resource.Styleable.GridView_header:

                     _headerId = attributes.GetResourceId(a, DEFAULT_HEADER_ID);

                     break;

             }

 

         }

     } } private void LoadHeader() {

     if (_headerId == DEFAULT_HEADER_ID) return;

     IMvxAndroidBindingContext bindingContext = MvxAndroidBindingContextHelpers.Current();

     _header = bindingContext.BindingInflate(_headerId, null); }

Now that we have a header, we can wrap our current adapter in the HeaderViewListAdapter.  As the name implies, this is the exact same adapter used by the ListView.  It handles knowing when and where to show the header.  In my sample code, I have the grid create the adapter directly, but this can work just as well if an adapter is passed in from outside.

// HeaderedGridView.cs

private IListAdapter GetAdapter() {

     var headerInfo = GetHeaders();

     ICursor cursor = ImageAdapter.CreateCursor(Context);

     IListAdapter adapter = new ImageAdapter(Context, cursor);

 

     if (headerInfo != null)

     {

         adapter = new HeaderViewListAdapter(headerInfo, null, adapter);

     }

     return adapter; }

So I don’t have to hard code sizes and so the header matches the rest of the items in the grid, I set the height and width once the grid is being laid out.  In the OnMeasure method we check if the header isn’t null, and if the ColumnWidth doesn’t match the previous column width we saw.  Caching the width and testing this prevents us from setting the layout parameters when we don’t have to; OnMeasure is called multiple times.

// HeaderedGridView.cs protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec) {

     base.OnMeasure(widthMeasureSpec, heightMeasureSpec);

 

     if (_header != null && base.ColumnWidth != _cachedColumnWidth)

     {

         _cachedColumnWidth = base.ColumnWidth;

         _header.LayoutParameters = new ViewGroup.LayoutParams(_cachedColumnWidth, _cachedColumnWidth);

     } }

Now all we need to do is include our grid in a layout.   It’s almost exactly the same as the regular GridView, except we can now optionally specify a header.  This is from the FirstView layout in the sample.

<!--FirstView.axml--> <HeaderedGridView

   android:layout_width="fill_parent"

   android:layout_height="fill_parent"

   android:layout_weight="1"

   android:numColumns="3"

   android:verticalSpacing="10dp"

   android:horizontalSpacing="10dp"

   android:stretchMode="columnWidth"

   android:gravity="center"

   android:fastScrollEnabled="true"

   android:background="#000000"

   local:header="@layout/gridheader" />

The gridheader layout is just a simple layout with a single image button button.  Note that we are binding the click of the button to the ClickCommand in our view model.

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android=http://schemas.android.com/apk/res/android

     xmlns:local=http://schemas.android.com/apk/res-auto

     android:orientation="vertical"

     android:layout_width="fill_parent"

     android:layout_height="fill_parent"

     android:padding="5dp">

     <ImageButton

         android:layout_width="fill_parent"

         android:layout_height="fill_parent"

         android:background="#90C53E"

         android:src="@drawable/camera"

         local:MvxBind="Click ClickCommand" /> </LinearLayout>

Firing up the sample solution and we should see something similar to:

Yarly Screen Shots 2014-04-21_184439_262000 

Future Improvements

My implementation restricts me to only being able to add one header.  While one header is currently good enough for me, if I wanted to expand it add more, I could emulate ListView a bit more.  Specifically the onFinishedInflate method which adds child layouts as a list of headers.  Of course, I could also add an explicit AddHeaderView method that could be called multiple times.

I’ve ignored footers entirely.  Again, this is just because I don’t currently have any need for a footer.  They could be added easily enough following the same pattern as headers.

Happy Coding.

Setting up Xamarin Studio and MonoDroid

I’ve spent the past few hours working through a fresh install of Xamarin Studio.  While most of it is really easy, there are some major gotchas.  The best thing about the process is that it is one installer for everything including dependencies.  The download page misleadingly asks you what level license you want when.  Ignore that.  It has nothing to do with the installer or the license you actually wind up using.  If you’re just trying it out there’s a 30 day trial license that gets you access to all of the features.

The installer pulls down all of the Android development tools as well:  the emulator, the SDK manager, java samples.  Everything.  After the installer finishes you are ready to write your first android app.  Almost.

Hello world in Xamarin Studio

First thing I did was fire up Xamarin Studio (their IDE) and create a new Android Application.  Out of the box this is a “Hello World” app with a single button on a page.  Couldn’t get much simpler.  It didn’t compile for me.  I got:

‘Error MSB4018: The “Aapt” task failed unexpectedly’ on new Android Application. No code added.

The solution here was fairly simple.  Go to the Android SDK Manager (it should be in your start menu/screen) and install all of the updates.  Just click the install button on the bottom right.

image

Once that’s done you can go back and run your app.  It will prompt you to launch an emulator and deploy the app automatically.  One thing to note here is that the emulator can take a while (minutes) to start up.  Even worse, once it is fully started it stays on the ANDROID loading screen waiting for you to hit a button.  There’s no way for you to know it’s ready for you unless you go back and poke it every now and then.

image

Debugging in Visual Studio

While Xamarin Studio seems to be a decent IDE and you can debug an application from there (breakpoints, watch screens, variable evaluation on mouse hover, threads list, etc.) I am definitely more comfortable with Visual Studio.  If you don’t have a trial (or real) license yet, you will be prompted for it when you open up a Xamarin project in Visual Studio, either by opening the solution file you created in Xamarin Studio or by creating a new Android project directly in Visual Studio.  That’s where it stops being easy though.  The first thing you might notice is that when you try to debug the application you get the following error:

The application could not be started. Ensure that the application has been installed to the target device and has a launchable activity (MainLauncher = true).

Additionally, check Build->Configuration Manager to ensure this project is set to Deploy for this configuration.

The error message is right, the application wasn’t installed on the “target device” (the emulator).  If you created the project in Xamarin Studio you need to explicitly tell VS that you want to deploy the app the phone before debugging.  Go to the Configuration Manager and ensure that Deploy is checked off for your project.

image

image

If you created the project in Visual Studio, you shouldn’t have this problem.  it seems to only happen when you create the project in Xamarin Studios first.

Now that the application is deployed to the phone there is still one more hurdle before you can debug from Visual Studio.  Whenever I tried to attach to a running virtual machine the debugger would deploy the app and chug along for a little while.  Eventually it would quietly die and disconnect.  The application wouldn’t load in either the debugger or emulator.  I didn’t see anything in the log file that helped point to the problem.   Eventually I got lucky and determined that I could connect if the virtual machine was running an Intel Atom (x86) processor then everything worked.  My assumption is that Visual Studio doesn’t know how to compile for ARM.  If you look at the project properties for an Android app in Visual Studio, there is only x86 and x64:

image

Xamarin’s site has some pretty easy to follow instructions on how to configure an x86 emulator.  The main task is to install Intel’s HAXM software.  What they skip over is that you can’t install this if you have Microsoft Hyper-V installed.  Hyper-V is Microsoft’s hardware virtualization stack that is used by the Windows Phone Emulator.  Ben Armstrong has some really good instructions on how to create a profile in Windows that will disable Hyper-V at startup.  They boil down to two commands to run on the command line and a restart:

bcdedit /copy {current} /d "No hypervisor"
bcdedit /set {GUID From the previous command} hypervisorlaunchtype off

For the GUID in the second command, copy the output of the first command including the brackets.  It should look something like this:  {7d067ad2-16ce-11e2-a059-9b573bf76ddc}.  Then just restart the computer and select “No hypervisor” when prompted at the boot screen.  From here you can continue through the instructions on how to configure an x86 emulator.

There’s a trade off here.  You won’t be able to switch between debugging a Windows Phone application and an Android application without restarting your computer, but on the other hand, because it uses hardware level virtualization, the x86 emulator is significantly faster than the ARM versions.

UPDATE:

After running these two commands a couple of times, I got lazy and wrote a powershell script to do everything in one go:

# This script makes a copy of the current boot record and disables Hyper-V
# The next time the computer is restarted you can elect to run without Hyper-V
 
$output  = invoke-expression 'bcdedit /copy "{current}" /d "Hyper-V Disabled"'
$output -match '{.*}'
$guid = $matches[0]
$hyperVCommand = 'bcdedit /set "' + $guid + '" hypervisorlaunchtype off'
invoke-expression $hyperVCommand

Summary

Here’s a quick run down of the steps covered above

  1. Download and install Xamarin Studio
  2. Install all the updates in the Android SDK Manager
  3. Make sure Visual Studio knows to deploy the app before trying to debug
  4. Disable Hyper-V
  5. Configure an x86 emulator

INotifyPropertyChanged – Hardcode This

Background

One thing I always hated about INotifyPropertyChanged is that when it was first introduced, the only way to actually notify a property is by specifying a hard coded value. I call this pattern, The Verbose. It looked something like this:

private int _Integer;
public int Integer
{
    get { return _Integer; }
    set
    {
        if (value != _Integer)
        {
            _Integer = value;
            Notify(@"Integer"); 
        }
    }
}

(Note: Notify is just a helper method to call PropertyChanged(this, new PropertyChangedEventArgs(propertyName));)

The thing that bothered me with this is that pesky Notify(@”Integer”). If you ever refactor that property, you’ll no longer be notified when Integer has been changed unless you go in and manually change the string. There has to be a better way.

After browsing some of the code from this project http://winrtclipboard.codeplex.com/, I found a really neat solution. They kept a Dictionary<string, object> that held all of the property values. This removed the need for the backing property (in this case _Integer). Then they used an Expression Class to pass a lambda in and get the property name. As a result, I dubbed this approach, The Expression and its definition looked like this:

public int Integer
{
    get { return GetValue(() => Integer); }
    set { SetValue(value, () => Integer); }
}

Now you can refactor all you want without ever breaking your notifications because the property name is coming from the Expression you pass into GetValue / SetValue. The one caveat to this implementation is performance. On every call to GetValue or SetValue you’re using reflection to get the member name. So depending on your particular scenario this can either be perfectly fine or a total deal breaker.

For a time, this was all we had . . . and then .NET 4.5 came along and we now have CallerMemberNameAttribute. It’s part of the new Complier Services that ship with VS 2012. The documentation indicates that INotifyPropertyChanged is a intended use for this attribute and they’re right. So if we leverage the Dictionary approach to holding our backing values, we can write properties that look like this:

public int Integer
{
    get { return GetValue<int>(); }
    set { SetValue(value); }
}

I dubbed this approach The Caller. Now that looks very clean. GetValue actually takes an optional parameter with the CallerMemberNameAttribute attached to it. So when we call it without any parameters, the compiler adds the parameter for us and in this case sets it to “Integer”. As it turns out this is faster then the Expression Class method, but using the Dictionary and constantly doing look ups still gives us a performance hit. With the use of a backing object and a helper method though, we can write clean code that performs well and refactors without issue. The result is The Caller Fast and a property definition that looks like this:

private int _Integer;
public int Integer
{
    get { return _Integer; }
    set { SetValue(ref _Integer, value); }
}

SetValue in this case simply compares _Integer & value. If they’re different, _Integer is set to value and Notify is called. Thanks to to CallerMemberNameAttribute we don’t have to pass in what property name just changed.

Performance

After discovering this wonderful attribute, I decided to create a little project that vetted the performance of several of these approaches. I’ve created a little solution that you can download to get all of the source code.

The test consisted of creating several objects, each implemented INotifyPropertyChanged in its own way. The objects are:

  • The Verbose – The initial method I showed. Classic INotifyPropertyChanged.
  • The Expression – The method from WinRTClipboard.CodePlex.com.
  • The Caller – Used CallerMemberAttribute with the dictionary from The Expression.
  • The Caller Fast – A modified version from MSDN to keep our property definitions looking nice and clean.

Each implementation implemented an interface that has 1 property, Integer. I created an array of 100,000 random integers. I then looped over this array for each object and set and get the value of Integer. I wrapped the entire loop in a stopwatch and wrote the results to the console. The results are as follows:

00:00:00.0040000 – The Verbose
00:00:00.0070000 – The Caller Fast
00:00:00.0280000 – The Caller
00:00:00.3070000 – The Expression

As you can see, The Verbose is the fastest. This makes sense as it’s the simplest implementation. The Caller Fast though is a very close second (Remember this a difference of 3 milliseconds for 100000 iterations). When you use a dictionary however, you loose an order of magnitude. The Expression, which uses both reflection and a dictionary to store values is 2 orders of magnitude slower then The Verbose.

Summary

In conclusion, if you can use .NET 4.5 you should really take advantage of CallerMemberNameAttribute. It performs very well and you can refactor your code without worrying about updating literals littered in your code. If you can’t upgrade to .NET 4.5, the Expression method is a very clean solution but shouldn’t be used in situations where you’ll be doing a lot of updates to your properties (i.e. animations).

I hope you found this useful and would love to hear your feedback, so leave us a comment.

Cheers!

//– Joe

Just a little closer

As much as I love it, the resolution on my Acer Aspire S7 ultrabook can make it difficult to work in Visual Studio.  The code is just a little too small.  While the built in zoom feature is great, you need to zoom in on every document you open, which can be a little distracting, especially since I tend to not zoom in to the same zoom level each time.

What’s a developer to do?

Enter TroutZoom by Sam Trost.  Zooming in on one file resets the zoom level for all other open files as well as for all files you open afterward.  I’ve been using it for several weeks now to get a little closer to my code.  It’s a breeze to go from my laptop zoomed in at 161% to docking and zooming everything back out to 100%.

A specified communication resource (port) is already in use by another application.

Trying to debug a new Windows Phone 8 application today I came across this problem.  Whenever I hit F5, the emulator would start up like normal but Visual Studio would prompt me with a message box saying:

A specified communication resource (port) is already in use by another application.

Then the debugger would fail to start.  After trying all of the obvious options and restarting everything in sight, I stumbled across a bit of advice which worked for me.  Right click on your project and select Deploy.  This will copy all necessary code to the emulator.  After that you are free to hit F5 like normal.

image