如何使动画可以期待？

Question

Sorry about not being able to give a more precise headline, but here is what I want to do. I have a method foo which will initiate some UI animations. When these animations are finished, I want to signal completion of the method. But instead of signaling completion through an event, I would like to make the method awaitable . How can this be done?

void foo()
{  
  // start some animations 
   storyboard.Begin();
   storyboard.Completed += (s, e) => { // signal that foo has completed }
}

calling code should be able to write:

await foo();

My actual foo is more complex than illustration above, in that I have a series of animations, which occur one after another, and foo is supposed to complete when all animations are completed.

Answer 1

Generally things that complete based on an event get 'wrapped' by using a TaskCompletionSource. Unfortunately, the example on MSDN is (IMHO) overly complicated instead of just being a single instance+event. You'd have something like:

public Task Foo()
{
    var tcs = new TaskCompletionSource<bool>();
    storyboard.Begin();
    storyboard.Completed += (s, e) => tcs.SetResult(true);
    return tcs.Task;
}

In this scenario, there's no real need for the TResult, but TaskCompletionSource doesn't appear to have a non-generic version and I'm not sure what the equivalent pattern would be

Answer 2

I recently ran into a situation where the Completed event wasn't firing on time, or sometimes not firing at all, even if I set the event handler prior to calling Begin() (that was the culprit on most other related posts I found). So I was looking for a way to wait for the animation to end, then continue on in the same method. I ended up creating the below static method that takes the storyboard you want to await as the parameter.

public async static Task RunStoryboard(Storyboard Story)
{
    Story.Begin();
    while (Story.GetCurrentState() == ClockState.Active && Story.GetCurrentTime() < Story.Duration)
    {
        await Task.Delay(100);
    }
}

I added the second condition because, during those situations where the Completed event never fired, the ClockState would always appear as active, even after the animation finished. As such, this requires setting the Storyboard's duration explicitly, but it's been working well for me so far.