反应性扩展和MouseEventArgs源

Question

I am using RX in my WPF applications to track mouse movement.

When subscribing directly to the mouse move event I get a different source in my MouseEventArgs compared to when I use the sample method of RX.

For the sake of explanation here is a simple example:

I have a window containing a grid and a button:

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication4"
        Title="MainWindow" Height="350" Width="525">
    <Grid x:Name="grid">
        <Button></Button>
    </Grid>
</Window>

I subscribe to the mouse move event with RX:

void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    Observable.FromEventPattern<MouseEventArgs>(grid, "MouseMove").Subscribe(mouseMoveRx);
    Observable.FromEventPattern<MouseEventArgs>(grid, "MouseMove").Sample(TimeSpan.FromSeconds(1)).Subscribe(mouseMoveRxSample);
}

private void mouseMoveRx(System.Reactive.EventPattern<MouseEventArgs> obj)
{
    var element = obj.EventArgs.Source as UIElement; 
    //element is System.Windows.Controls.Button
}

private void mouseMoveRxSample(System.Reactive.EventPattern<MouseEventArgs> obj)
{
    var element = obj.EventArgs.Source as UIElement;
    //element is Microsoft.Windows.Themes.ButtonChrome
}

The first handler is having System.Windows.Controls.Button as source whereas the second handler is having Microsoft.Windows.Themes.ButtonChrome as source.

What is the reason for the different source?

Answer 1

I have no experience with WPF so take this answer with a grain of salt...

But, based on your experiment with Do , it appears that what is happening is that after the event fires and Sample caches the value, the event object is mutated presumably by WPF.

The Remarks section of MouseEventArgs documentation says this:

The attached events and the base element routed events share their event data, and the bubbling and tunneling versions of the routed events also share event data. This can affect the handled characteristics of the event as it travels the event route. For details, see Input Overview.

This seems to suggest that indeed WPF mutates the event object as it bubbles it up the element hierarchy. Any settable property on the object is subject to this behavior. In this case it appears that Source is the only thing you need to worry about.

To handle this, you effectively need to cache the Source value before you apply any Rx operator that introduces asynchronicity because you can only trust the Source property for as long as your WPF event handler is running. The simplest way is to use a Select clause to capture Source :

Observable.FromEventPattern<MouseEventArgs>(grid, "MouseMove")
    .Select(e => Tuple.Create(e.EventArgs.Source as UIElement, e.EventArgs))
    .Sample(TimeSpan.FromSeconds(1))
    .Subscribe(mouseMoveRxSample);

// ...

private void mouseMoveRxSample(Tuple<UIElement, MouseEventArgs> obj)
{
    var element = obj.Item1;
    //element is System.Windows.Controls.Button
}