Strange event crosstalk: Mouse & Keyboard, WPF & Rx?

I've written a really tiny WPF + Rx program in VS 2010, and it has some unexpected behavior -- seemingly picking up alt-key events in the mouse-event observables. I'd be grateful for advice on any of

  1. what's specifically wrong
  2. ideas or tools on how to diagnose it further on my own [e.g., are there some tools that instruments the code to track and log all events that get fired so I can see more clearly what's happening?]
  3. how to understand the behavior -- is it "by design"? Am I misusing WPF or Rx or both?
  4. how to get the behavior I'd rather have, described below

First, what it does correctly is fire mouse move observers starting from a mouse down and ending with a mouse up. The particular test observer I have just puts the proper mouseDown + mouseUp coordinates in the TitleBar of the Window (see code + XAML below).

The strangeness starts if I press and release the alt key while the mouse is down. That seems immediately to stop the mouseMove event observers. Now, if I release the mouseButton, the mouseMove event observers start up again, and continue until I either generate a new mouseDown or press and release alt again. Further more, if I go through this sequence, namely

mouseDown mouseMove alt-key down alt-key up mouseUp alt-key down alt-key up

and then, the normal

mouseDown mouseMove

the print in the Title bar has some odd flashing, telling me that there are other, extraneous events getting tracked and pumped through my observer lambda.

I haven't been able to make this fail with keys other than alt, deepening the mystery.

The desired behavior could be

  1. either checking for alt key state explicitly in the mouse-move handlers
  2. or separate event streams for the keyDown and keyUp events, and I'll figure out the interleaving in my code

But, for sure, having this behavior and, worse, having no clue how to proceed to understand it, is really bad. I'll be grateful for any ideas!

(NOTE: add references to System.coreex, System.Reactive, and System.Interactive)

using System; using System.Collections.Generic; using System.Linq; using System.Windows; using System.Windows.Input; namespace DownAndMoveTest { public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); (from mDown in this.GetLeftMouseDownObservable(). Do(e => this.Title = "DOWN: " + e.EventArgs.GetPosition(this).ToString()) let mDownP = mDown.EventArgs.GetPosition(this) from mMove in this.GetMouseMoveObservable().TakeUntil(this.GetLeftMouseUpObservable()) select new { D = mDownP, M = mMove.EventArgs.GetPosition(this) }).Subscribe(ps => { this.Title = String.Format( "Window MouseDown({0:G}, {1:G}), MouseMove({2:G}, {3:G})", ps.D.X, ps.D.Y, ps.M.X, ps.M.Y); }); } } public static partial class UIElementExtensions { public static IObservable<IEvent<MouseButtonEventArgs>> GetLeftMouseDownObservable(this UIElement uiElement) { return Observable.FromEvent<MouseButtonEventHandler, MouseButtonEventArgs>( h => new MouseButtonEventHandler(h), h => uiElement.MouseLeftButtonDown += h, h => uiElement.MouseLeftButtonDown -= h); } public static IObservable<IEvent<MouseEventArgs>> GetMouseMoveObservable(this UIElement uiElement) { return Observable.FromEvent<MouseEventHandler, MouseEventArgs>( h => new MouseEventHandler(h), h => uiElement.MouseMove += h, h => uiElement.MouseMove -= h); } public static IObservable<IEvent<MouseButtonEventArgs>> GetLeftMouseUpObservable(this UIElement uiElement) { return Observable.FromEvent<MouseButtonEventHandler, MouseButtonEventArgs>( h => new MouseButtonEventHandler(h), h => uiElement.MouseLeftButtonUp += h, h => uiElement.MouseLeftButtonUp -= h); } } }

and its XAML

<Window x:Class="DownAndMoveTest.MainWindow" xmlns="" xmlns:x="" Title="MainWindow" Height="350" Width="525"> <Grid> </Grid> </Window>

-------------Problems Reply------------

Does your application have a main menu? Pressing the alt key focuses the menu bar, and pressing alt again returns focus to whatever had focus before. This would explain the functionality you are seeing. (It does not look like an Rx issue)

Category:wpf Views:0 Time:2010-11-26

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