Why does calling my CABasicAnimation not work in viewDidAppear?

Question

In viewDidAppear I call the following code:

MyView *myView = [[MyView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
myView.backgroundColor = [UIColor clearColor];

[self.view addSubview:myView];

[myView setProgress:0.3 animated:YES];

That last line calls this method:

- (void)setProgress:(CGFloat)progress animated:(BOOL)animated {
    self.innerPie.hidden = NO;

    self.innerPie.strokeEnd = progress;

    CABasicAnimation *pathAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
    pathAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear];
    pathAnimation.duration = 3.0;
    pathAnimation.fromValue = [NSNumber numberWithFloat:self.progress];
    pathAnimation.toValue = [NSNumber numberWithFloat:progress];
    [self.innerPie addAnimation:pathAnimation forKey:@"strokeEndAnimation"];

    self.progress = progress;
}

Yet the animation never starts when I load the simulator. Why? Has the view not already appeared, as indicated?

Oddly enough, if I use dispatch_after with a 0 second delay (instant) it will show.

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    [myView setProgress:0.8 animated:YES];
});

What am I doing wrong?

Answer 1

Your solution (delayed performance) is excellent, so there's really no problem. The only question is why your code doesn't work without the delayed performance. I suspect it has to do with these lines:

[self.view addSubview:myView];
[myView setProgress:0.3 animated:YES];

You have ordered the MyView to be added to the interface, but apparently it hasn't really been added yet, at least not in such a way that the interface has settled down and we are ready to add an animation to the layer render tree. Your delayed performance accomplishes exactly what's needed: we wait one cycle of the run loop, and now the interface (including the added MyView) is ready to rock and roll.