Reset UIProgressView and begin animating immediate

问题:

Imagine I have a UIProgressView that is showing the progress of some task (in this case triggered by a timer at a set interval).

Under normal circumstances this works fine. It starts off at 0, so in my initial call to animate the progress view, everything works as expected.

However, when I need to reset the progress, things start breaking.

Imagine I have the following sequence of events:

1. The user initiated a refresh, and so the progress view begins animating with a timer.
2. After several seconds, the progress view has reached 70%.
3. Before it can advance further, the user has initiated another refresh.

I would like the progress bar to be reset to 0 unanimated, and then fire off the first step of the animation immediately. When I try to do so, it is as though the first call (reset to 0) is completely ignored, and instead I get an animation from 70% to 10%. (I.e. the progress bar is moving backwards!)

I don't want the progress to ever move backwards during an animation.

Here is my code:

- (void)beginAnimation {
    // omitted: clear the timer
    [self.progressView setProgress:0 animated:NO];
    [self.progressView setProgress:0.1 animated:YES];
    // omitted: fire off a timer to trigger every few seconds and advance the progress
}

回答1:

Essentially, we want to schedule the animation on the next run loop.

In this case, I was able to do so by using dispatch_async():

- (void)beginAnimation {
    // omitted: clear the timer
    [self.progressView setProgress:0 animated:NO];
    dispatch_async(dispatch_get_main_queue(), ^{
        // Animate on the next run loop so the animation starts from 0.
        [self.progressView setProgress:0.1 animated:YES];
    });
    // omitted: fire off a timer to trigger every few seconds and advance the progress
}