Friday, December 16, 2011

Hello World: Hooking events

I promised before I'd try to get this code up somewhere. I've uploaded this entire example to github here. Please read the notes in the Readme about deployment, it requires signing the package with a certificate you've added to your trusted root stores.

The app I discussed in my previous post is fairly boring: all it does is present some text to the screen. I wanted it to do a little bit more to experiment with how events are attached to controls in WinRT. How about I make the text change color when the mouse hovers over it? Actually I shouldn't say mouse, I mean Pointer. In Xaml apps, mouse, keyboard, and touch events are all unified into a Pointer events so they all operate in similar ways. Anyway, it is a simple addition that makes the app more interesting.

WinRTs notion of events is a little different from VCL, and I think it'll be a welcome change. Events are handled with multicast delegates, similar to in C#. With any given event for any control, you could add multiple delegates to listen for that event. Each event has an add_* and a remove_* method to install or uninstall an event listener. For my Text, I'll be adding PointerEntered and PointerExited events. Both of these require a Windows.UI.Xaml.Input.PointerEventHandler, so I'll create a little delphi wrapper for that class.

type
  TPointerEventHandler = class(TInspectableObject, PointerEventHandler)
  private
    FProc: TProc ;
  public
    constructor Create(Proc: TProc );
    // PointerEventHandler methods
    procedure Invoke(sender: IInspectable; e: IPointerEventArgs); safecall;
  end;

{ TPointerEventHandler }

constructor TPointerEventHandler.Create(
  Proc: TProc );
begin
  FProc := Proc;
end;

procedure TPointerEventHandler.Invoke(sender: IInspectable; e: IPointerEventArgs);
begin
  FProc(sender,  e);
end;
This creates an instance of a handler, and stores of a reference to a procedure that should handle the event. That means the code for my event handler can still live in TDerivedApp, even though it's handed off to WinRT by this helper class. I've added an OnPointerEntered and OnPointerExited methods to my application class. In my OnLaunched event is where I actually hook the event handler up to the code:
procedure TDerivedApp.OnLaunched(args: ILaunchActivatedEventArgs);
var
  insp: IInspectable;
  element: Windows_UI_Xaml_IUIElement;
begin
  // Get the IWindowStatics
  OleCheck(RoGetActivationFactory(TWindowsString(SWindow), IWindowStatics, insp));
  WinStatic := insp as IWindowStatics;

  // Get an IXamlReaderStatics
  OleCheck(RoGetActivationFactory(TWindowsString(SXamlReader), IXamlReaderStatics, insp));
  ReaderStatic := insp as IXamlReaderStatics;

  WinStatic.Current.Content := ReaderStatic.Load(TWindowsString(content)) as IUIElement;
  FInner.OnLaunched(args);

  // New code here: added event handlers for pointer enter and exit
  insp := (WinStatic.Current.Content as IFrameworkElement).FindName(TWindowsString('Text'));
  element := (insp as IUIElement);

  element.add_PointerEntered(TPointerEventHandler.Create(self.OnPointerEntered));
  element.add_PointerExited(TPointerEventHandler.Create(self.OnPointerExited));

  WinStatic.Current.Activate;
end;

procedure TDerivedApp.OnPointerEntered(sender: IInspectable; e: IPointerEventArgs);
var
  brush: ISolidColorBrush;
  insp: IInspectable;
  color: IInspectable;
begin
  // Create a red brush
  OleCheck(RoGetActivationFactory(TWindowsString(SSolidColorBrush), ISolidColorBrushFactory, insp));
  OleCheck(RoGetActivationFactory(TWindowsString(SColors), IColorsStatics, color));
  brush := (insp as ISolidColorBrushFactory).CreateInstanceWithColor((color as IColorsStatics).Red);
  // Assign it to the text block
  (sender as ITextBlock).Foreground := (brush as IBrush);
end;
I've omitted the code for OnPointerExited here; it's nearly identical to OnPointerEntered except it uses a white brush. Again, some of the code here, like creating a brush, is quite verbose and ugly; eventually class wrappers will make this look much nicer, similar to the C# and C++ projections that would alow this to look something more like:
  sender.Foreground := Windows.UI.Xaml.Media.SolidColorBrush.Create(
    Windows.UI.Xaml.Media.Colors.Red);

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