Why addAnimation:forKey not working in viewDidLoaded

Question

I have the following method called by viewDidLoad. This method is to create a CALayer to show a image. This layer has a mask whose path is a UIBezierPath created by my private method. I want the mask to rotate infinitely, and then I add a CABasicAnimation object to the mask.

- (void) createPathLmask
{
    // mask layer
    self.pathLayer                 = [CALayer layer];
    self.pathLayer.bounds          = CGRectMake(0.0, 0.0, 120, 120);

    CGPoint position = self.view.layer.position;
    position.y += 140;
    self.pathLayer.position        = position;
    self.pathLayer.backgroundColor = [UIColor redColor].CGColor;

    UIImage *backimage = [UIImage imageNamed:@"image2"];
    self.pathLayer.contents = (__bridge id)backimage.CGImage;
    self.pathLayer.contentsGravity = kCAGravityResizeAspectFill;

    // mask
    CAShapeLayer *mask = [CAShapeLayer layer];
    mask.bounds          = CGRectMake(0.0, 0.0, 120, 120);
    mask.position        = CGPointMake(60.0f, 60.0f);
    mask.contentsGravity = kCAGravityResizeAspectFill;
    mask.path            = [self createBezierPathInRect:mask.bounds].CGPath;

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        // rotate the mask repeatedly
        CABasicAnimation *animation = [CABasicAnimation animation];
        animation.keyPath = @"transform.rotation";
        animation.duration = 4.0f;
        animation.byValue  = @(M_PI * 2);
        animation.repeatCount = HUGE_VALF;
        [mask addAnimation:animation forKey:@"rotation_repeatedly"];
    });

    self.pathLayer.mask = mask;

    [self.view.layer addSublayer:self.pathLayer   ];
}

I find that the rotation animation can only work when I put the addAnimation:forKey into the dispatch_after block with 1 second delay. If those codes are moved out of the block, the mask will not rotate.

So there must be something not ready when animation is added to the layer in viewDidLoaded. I am wondering what is not ready yet? Is there any document or explanation about the suitable chance to add the animation?

matt matt · Accepted Answer · 2016-07-22T01:59:50

So there must be something not ready when animation is added to the layer in viewDidLoaded

Correct. This is way too early for animation. Keep in mind that the view at this point merely exists and that's all; it isn't even part of the interface. You cannot animate a view that isn't part of the view hierarchy. There is nothing, at this point, to animate.

The view first becomes part of the interface between the first call to viewWillAppear and the first call to viewDidAppear. That is what "appear" means (as opposed to what "loaded") means.

Why addAnimation:forKey not working in viewDidLoaded

2 Answers