20 June 2012

Reflection in WinRT: use Rx Extensions for dynamically adding event handlers

Suppose you want to add an event handler, but you don’t know up front which event. Admittedly, this is a bit of an edge case, but I have used it in a behavior that starts a storyboard upon an event (I will publish that later). The designer can specify the event using a string value in XAML, so I cannot simply use object.MyEvent += (handler). We need reflection here.

So suppose you have found the event in the object using the GetRuntimeXXX methods like I described earlier today, and want to dynamically add a handler to the event “MyEvent”:

var evt = MyObject.GetType().GetRuntimeEvent("MyEvent");
evt.AddEventHandler(MyObject, 
  new EventHandler((p, q) => {// your code here }));

This will compile, and even run - unless the object is, for instance, a Grid, and the event “Loaded”. That’s apparently a “WinRT event”, whatever that may be, and this will result in a runtime exception:

"Adding or removing event handlers dynamically is not supported on WinRT events."

Googling Binging this message leads to all kind of complicated solutions using the WindowsRuntimeMarshall class (without any samples, alas), but the solution turns out to be very simple: use Rx Extensions Beta for Windows RT. For the really lazy reader: click tools/Library Package Manager/Package Manager Console and enter “Install-Package Rx-Main –Pre” in the Package Manager Console.

Now add

using System.Reactive.Linq;

to the top of the code file and then simply use the following code:

Observable.FromEventPattern<RoutedEventArgs>(MyObject, "MyEvent")
  .Subscribe(se => { // your code here });

and you are done. The difference between ordinary events and “WinRT events” is apparently swallowed up in the bowels of Rx. This library is great to begin with, but if it saves you the trouble of digging in yet another still not very thoroughly documented API, it’s even better. As a bonus, by very nature of Rx the handler is automatically disposed of when your calling object goes out of scope, which is not necessarily the case in my first code. But that did not work anyway ;-)

Reflection in WinRT: DeclaredProperties vs GetRuntimeProperties caveat

I suppose this is part of some FM I did not R, but I would like to point it out in case some other sod wastes just as much time on it as me.

I embarked upon a journey to port SilverlightSerializer and some behaviors to WinRT. All stuff is heavily making use of reflection. Now reflection has been hugely overhauled in WinRT, so I knew I was in for an interesting ride. Recently I was convinced I managed to do it, but it turned out I did not.

There are apparently two ways to use reflection in RT. What I did use for reading all properties of an object, was:

var properties = MyObject.GetType().GetTypeInfo().DeclaredProperties;

But it turns out this returns only the properties declared in the class itself. What it does not return, are the properties of possible parent classes!

What I should have used, apparently, is:

var allproperties = MyObject.GetType().GetRuntimeProperties();

When I feed a Grid into “MyObject”,  “properties” hold 6 properties, while “allproperties” holds 51.

I don’t know why this distinction is made, and I understand even less how come I missed this the first time. On my first meeting as an MVP, Dennis Vroegop, after welcoming me to the club, already warned me that the MVP award did not automatically come with super powers. Boy, was he right.

14 June 2012

NavigationService for WinRT

"If I have been able to see further than others, it is because I have stood on the shoulders of giants."
 Isaac Newton

It was only 1.5 years ago, but it seems already a long time ago that Laurent Bugnion described a “view service” for Windows Phone navigation, commonly know as the NavigationService. I’ve incorporated this code in my #wp7nl library on codeplex and have been using it happily ever since (and a lot of other people I know have been doing so as well), Time goes on, along came Windows 8 and WinRT and Metro Style Apps.

Laurent ported his MVVMLight framework to WinRT as well. But the NavigationService never has been a core part of MVVMLight (therefore I put it in the #wp7nl library) and porting it to WinRT as well proved to be a bit of a hassle. Some of its premises where no longer valid – most notable the way to retrieve the main application frame by accessing Application.Current.RootVisual, which does not work in WinRT. I messed around a little with the code, did not get anywhere, and left it there. So I was glad I saw my fellow Windows Phone Development MVP Matteo Pagani tweet he got a NavigationService to work. He was kind enough to mail me the code. He basically copied the code from Windows Phone and made the apps rootFrame, as created in the App.xaml.cs, publicly available. A neat trick, with as only drawback that you have to copy the code to every app. As things goes, it’s usually easier to see someone else’s idea and improve it, than think it up out of the blue. I toyed around with it and managed to get rid of the need to copy the code, so I could put it in a library.

And here it is, an improved version of a NavigationService for WinRT, based upon Matteo’s code based upon Laurent’s code ;-)

First of all, the NavigationService’s interface:

using System;
using Windows.UI.Xaml.Navigation;

namespace Win8nl.Services
{
  public interface INavigationService
  {
    event NavigatingCancelEventHandler Navigating;
    void Navigate(Type type);
    void Navigate(Type type, object parameter);
    void Navigate(string type);
    void Navigate(string type, object parameter);
    void GoBack();
  }
}

This quite looks like the old interface, with some notable exceptions. First of all, the NavigateTo method is renamed Navigate, to mimic the method name used in WinRT itself. And it has not one but four overloads. The first two are pretty logical – WinRT does not use an uri for navigation, but an actual Type object, making strongly-typed navigation possible. And it supports an overload for a parameter object, as well. The two other methods, with the string type object… well see for yourself below, in the implementation of the service:

using System;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Navigation;

namespace Win8nl.Services
{
  public class NavigationService : INavigationService
  {
  public NavigationService(Frame mainFrame)
  {
    _mainFrame = mainFrame;
  }

  private Frame _mainFrame;
  
  public event NavigatingCancelEventHandler Navigating;

  public void Navigate(Type type)
  {
    _mainFrame.Navigate(type);
  }

  public void Navigate(Type type, object parameter)
  {
    _mainFrame.Navigate(type, parameter);
  }

  public void Navigate(string type, object parameter)
  {
    _mainFrame.Navigate(Type.GetType(type), parameter);
  }

  public void Navigate(string type)
  {
    _mainFrame.Navigate(Type.GetType(type));
  }

  public void GoBack()
  {
    if (_mainFrame.CanGoBack)
    {
      _mainFrame.GoBack();
    }
  }  
}

So the string type methods allow you to specify the page to navigate by string. Before you all think I really lost my marbles this time, re-introducing weakly typed navigation just as the Windows team introduced strongly typed: I specifically added this as to allow navigation to be initiated from a ViewModel contained in an assembly not containing the XAML code. This also enables you to test code as well. This is the way I built my solutions for Windows Phone, and I intend to go on this way in Windows 8. ;-)

Anyway, there’s a thing missing in this code: the EnsureMainFrame that looked up the root page, for which Matteo used a public property. I completely removed it, and added a constructor accepting that root page. By creating that constructor I deleted the default constructor, so registering the NavigationService at he SimpleIoc container shipped with MVVMLight in the App.xaml.cs, like this:

SimpleIoc.Default.Register<INavigationService, Wp7nl.Utilities.NavigationService>();

as we did in Windows Phone code, won’t work anymore. Fortunately, SimpleIoc also supports factory methods to create an instance. So now you go to the App.xaml.cs of your Windows 8 app, find the OnLaunched method and just behind this line:

var rootFrame = new Frame();

you add this piece of code:

SimpleIoc.Default.Register<INavigationService>(() => 
                                   {return new NavigationService(rootFrame);});

Now if your XAML code and ViewModels are all in one assembly, you can call the NavigationService to navigate to the main page this:

SimpleIoc.Default.GetInstance<INavigationService>().Navigate(typeof(MainPage));

or if you are a bit stubborn like me and would like to separate ViewModels and views, you can use the string overload to specify the full qualified name and for instance use it in a ViewModel command like this

public ICommand TestNavigatieCommand
{
   get
   {
     return new RelayCommand(() => 
       SimpleIoc.Default.GetInstance<INavigationService>().Navigate("MyApp.MainPage,MyApp"));
   }
}

And there you have, a fully functional NavigationService for Windows 8. Code can be downloaded here. I will soon incorporate it in the provisional port of the #wp7nl library called win8nl.

Thanks to Laurent and Matteo for being my trailblazers ;-)

Behavior to make Bing Maps view area bindable

The Bing Maps control in Windows Phone 7 is pretty versatile but does not always play along very well with data binding. You can bind ZoomLevel and MapCenter, which is all very well, but if you want to know what area is actually shown within the map – for instance, for making a GIS application that loads only visible data, and not stuff that’s outside the view area anyway, things get a lit complicated.

Not so if you uses this little behavior. It builds upon the SafeBehavior that is already in my wp7nl library on codeplex. It sports two GeoCoordinate depencency propertise, NorthEast and SouthWest

using System.Device.Location;
using System.Windows;
using Microsoft.Phone.Controls.Maps;

namespace Wp7nl.Behaviors
{
  public class ViewportAreaBehavior
  {
    /// <summary>
    /// A behavior to make southwest and northeast bindable
    /// </summary>
    public class ViewportwatcherBehavior : SafeBehavior<Map>
    {
      #region NorthWest
      public const string NorthWestPropertyName = "NorthWest";

      public GeoCoordinate NorthWest
      {
        get { return (GeoCoordinate)GetValue(NorthWestProperty); }
        set { SetValue(NorthWestProperty, value); }
      }

      public static readonly DependencyProperty NorthWestProperty = 
	DependencyProperty.Register(
          NorthWestPropertyName,
          typeof(GeoCoordinate),
          typeof(ViewportwatcherBehavior),
          new PropertyMetadata(null));

      #endregion

      #region SouthEast
      public const string SouthEastPropertyName = "SouthEast";

      public GeoCoordinate SouthEast
      {
        get { return (GeoCoordinate)GetValue(SouthEastProperty); }
        set { SetValue(SouthEastProperty, value); }
      }

      public static readonly DependencyProperty SouthEastProperty = 
	DependencyProperty.Register(
          SouthEastPropertyName,
          typeof(GeoCoordinate),
          typeof(ViewportwatcherBehavior),
          new PropertyMetadata(null));

      #endregion
    }
  }
}

All pretty standard stuff. Next comes the setup/cleanup stuff,

protected override void OnSetup()
{
  AssociatedObject.ViewChangeEnd += AssociatedObjectViewChangeEnd;
  CalcRectangle();
}

protected override void OnCleanup()
{
  AssociatedObject.ViewChangeEnd -= AssociatedObjectViewChangeEnd;
}

in which you can see I intercept the ViewChangeEnd event. I don’t do anything special there: I just call the method CalcRectangle, just as on startup.

void AssociatedObjectViewChangeEnd(object sender, MapEventArgs e)
{
  CalcRectangle();
}

The final missing piece of the puzzle is then of course CalcRectangle itself:

private void CalcRectangle()
{
  NorthWest = AssociatedObject.ViewportPointToLocation(new Point(0, 0));
  SouthEast = AssociatedObject.ViewportPointToLocation(
    new Point(AssociatedObject.ViewportSize.Width, 
              AssociatedObject.ViewportSize.Height));
}

So I basically take the point left top (0,0) and and use ViewportPointToLocation to convert it to a coordinate, and do the same think with the point that left bottom. And that’s all.

Now you can bind to the NorthWest and SouthEast property of the behavior and you get the view area in your view model. Be aware this works one way. It gets the view area to the ViewModel, not the other way around.

Contrary to my custom there’s no solution with the code in it, mostly because it’s already in the wp7nl library since yesterday. You can download the code from the library itself here, if you want.

06 June 2012

Detecting your Windows 8 app is running in the background

In Windows Phone life was so simple: when the user put your app in the background, as of version 7.5 the OS effectively froze all threads, timers and whatnot completely automatically. And it was automatically restarted too, if you used fast application switching. That kind of freeloading appeals to my natural programmer’s laziness so I put that to good use. That is, by doing nothing and letting the OS taking care of things.

Enter Windows 8: that has a even more sophisticated multi tasking system that allows things to actually keep running in the background. Excellent for most scenarios, but not if you are writing a game. I am currently porting Catch’em Birds to Windows 8 and observed that although the game music (thankfully) shuts up when the game is put in the background, the birds keep on flying and the timer keeps on running.

Now in App.Xaml.cs there is an OnSuspending method, but that is only called when the application is suspended – i.e., pushed out of memory. This is comparable to the point where a Windows Phone application would tombstone itself. You need to trap another event: VisibilityChanged of the current window.

In the OnLaunchedmethod of App.Xaml.cs, just above the Window.Current.Activate you can add

Window.Current.VisibilityChanged += Current_VisibilityChanged;
That gives a Windows.UI.Core.VisibilityChangedEventArgs which has a boolean Visible property that you can use like this:
void Current_VisibilityChanged(object sender, 
                               Windows.UI.Core.VisibilityChangedEventArgs e)
{
  System.Diagnostics.Debug.WriteLine("Visibile: " + e.Visible);
}

I am going to use this to send this event trough the MVVMLight messenger and adapt my game so that is reacts to this event – like in ‘stop the birds, or at least the timer. The OS being more capable actually requires me to do a wee bit more work. No more freeloading ;-)