Notification when rendering a Visual in WPF is complete

问题

Is there a way to get notified (e.g., by an event) when a Visual has been (re-)rendered?

Basically, I would like to take a screen shot whenever a Visual's appearance has changed. I assume the rendering thread is taking care of this in the background.

I am trying to avoid grabbing a screen shot periodically since I am only interested in changes (e.g., hovering the cursor over a button would change its appearance).

Any hint is highly appreciated!

回答1:

The best way I've found is to add an empty panel to the layout root attach an event like on size changed and work from there.

回答2:

This MSDN thread provides quite a bit of information on the subject. Basically it doesn't really work that way and you'll find the exact reasons there.

回答3:

This is not possible in WPF. Your best bet would be to take a snapshot in memory frequently and compare it with the previous. If you detect a diffrence, persist the snapshot. You could use the CompositionTarget.Rendering event for this. Just make sure that you don't take a snapshot in each event (since it is called as freuqently as the graphics card swaps its buffer).

回答4:

I had a similar problem where I had a GridControl (devexpress WPF) that I needed to perform an action on whenever it was redrawn. The issue was I needed to perform the action AFTER it had finishes populating the grid and drawing all elements.

This solution is a hack however in practice it works 100% of the time with no apparent downsides. It works by simply starting a timer after the visible status has changed and firing an event afterwards.

public ctor()
{
    Grid.IsVisibleChanged += TableOnIsVisibleChanged;
}


const int _msItTakesToDrawGrid = 5;
private Timer _timer;
private void TableOnIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
{
    if ((bool)e.NewValue != true || (bool)e.OldValue != false || _timer != null)
        return;


    _timer = new Timer { Interval = _msItTakesToDrawGrid };
    _timer.Elapsed += DoStuff;
    _timer.AutoReset = false;
    _timer.Start();
}

private void DoStuff(object sender, ElapsedEventArgs e)
{
    _timer.Stop();
    _timer= null;
    Dispatcher?.Invoke(stuff that needs to be done on the UI thread...);
}

来源：https://stackoverflow.com/questions/3684211/notification-when-rendering-a-visual-in-wpf-is-complete

标签

wpf

events

rendering