Can I animate the UIScrollView contentOffset property via its layer?

Question

I want to zoom and scroll a UIScrollView with a CGPathRef. Because of that I assume I have to animate the UIScrollView\'s layer property? But which property would I animate th

Answer 1

Swift 4.2

This example is base on pt2ph8's obj-c answer.

https://stackoverflow.com/a/8741283/6113158

var scrollView = UIScrollView()

func animateScrollView(duration: Double, to newBounds: CGRect) {
    let animation = CABasicAnimation(keyPath: "bounds")
    animation.duration = duration
    animation.fromValue = scrollView.bounds
    animation.toValue = newBounds

    animation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.easeOut)

    scrollView.layer.add(animation, forKey: nil)

    scrollView.bounds = newBounds
}

Answer 2

Moving a CALayer around is done by (preferably) setting it's .position property - or possibly the anchorPoint (c.f. the docs on that: http://developer.apple.com/library/ios/#documentation/GraphicsImaging/Reference/CALayer_class/Introduction/Introduction.html ).

...but I don't think you want to mess with the CALayer if you're working with a UIScrollView. Have you tried applying plain CoreAnimations to your ScrollView?

(the problem is: the UIScrollView is implemented on top of CALayer - so even if you can hack it to work today, it's quite likely to break in future iOS versions. If possible, you want to avoid the CALayer for that particular class)

Answer 3

You have to animate the bounds property. In fact, that's what the contentOffset property uses behind the scenes.

Example:

CGRect bounds = scrollView.bounds;

CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"bounds"];
animation.duration = 1.0;
animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];

animation.fromValue = [NSValue valueWithCGRect:bounds];

bounds.origin.x += 200;

animation.toValue = [NSValue valueWithCGRect:bounds];

[scrollView.layer addAnimation:animation forKey:@"bounds"];

scrollView.bounds = bounds;

If you're curious, the way I used to get this information is the following:

[UIView beginAnimations:nil context:NULL];
[UIView setAnimationDuration:0.5];

[scrollView setContentOffset:CGPointMake(200, 0) animated:NO];

[UIView commitAnimations];

NSLog(@"%@",scrollView);

The NSLog call will output:

<UIScrollView: 0x860ba20; frame = (-65.5 0; 451 367); clipsToBounds = YES; autoresize = W+RM+TM+H; animations = { bounds=<CABasicAnimation: 0xec1e7c0>; }; layer = <CALayer: 0x860bbc0>; contentOffset: {246, 0}>

The animations snippet will list all the active animations, in this case { bounds=<CABasicAnimation: 0xec1e7c0>; }.

Hope this helps.