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How to do a curve/arc animation with CAAnimation?

Asked 2010-10-27T12:30:02.257
18

I have an user interface where an item get deleted, I would like to mimic the "move to folder" effect in iOS mail. The effect where the little letter icon is "thrown" into the folder. Mine will get dumped in a bin instead.

I tried implementing it using a CAAnimation on the layer. As far as I can read in the documentations I should be able to set a byValue and a toValue and CAAnimation should interpolate the values. I am looking to do a little curve, so the item goes through a point a bit above and to the left of the items start position.

    CABasicAnimation* animation = [CABasicAnimation animationWithKeyPath:@"position"];
[animation setDuration:2.0f];
[animation setRemovedOnCompletion:NO];
[animation setFillMode:kCAFillModeForwards];    
[animation setTimingFunction:[CAMediaTimingFunction functionWithName: kCAMediaTimingFunctionEaseOut]];
[animation setFromValue:[NSValue valueWithCGPoint:fromPoint]];
[animation setByValue:[NSValue valueWithCGPoint:byPoint]];
[animation setToValue:[NSValue valueWithCGPoint:CGPointMake(512.0f, 800.0f)]];
[animation setRepeatCount:1.0];

I played around with this for some time, but it seems to me that Apple means linear interpolation. Adding the byValue does not calculate a nice arc or curve and animate the item through it.

How would I go about doing such an animation?

Thanks for any help given.

ios caanimation
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1 Answer

8

You are absolutely correct that animating the position with a CABasicAnimation causes it to go in a straight line. There is another class called CAKeyframeAnimation for doing more advanced animations.

An array of values

Instead of toValue, fromValue and byValue for basic animations you can either use an array of values or a complete path to determine the values along the way. If you want to animate the position first to the side and then down you can pass an array of 3 positions (start, intermediate, end).

CGPoint startPoint = myView.layer.position;
CGPoint endPoint   = CGPointMake(512.0f, 800.0f); // or any point
CGPoint midPoint   = CGPointMake(endPoint.x, startPoint.y);

CAKeyframeAnimation *move = [CAKeyframeAnimation animationWithKeyPath:@"position"];
move.values = @[[NSValue valueWithCGPoint:startPoint],
                [NSValue valueWithCGPoint:midPoint],
                [NSValue valueWithCGPoint:endPoint]];
move.duration = 2.0f;

myView.layer.position = endPoint; // instead of removeOnCompletion
[myView.layer addAnimation:move forKey:@"move the view"];

If you do this you will notice that the view moves from the start point in a straight line to the mid point and in another straight line to the end point. The part that is missing to make it arc from start to end via the mid point is to change the calculationMode of the animation.

move.calculationMode = kCAAnimationCubic;

You can control it arc by changing the tensionValues, continuityValues and biasValues properties. If you want finer control you can define your own path instead of the values array.

A path to follow

You can create any path and specify that the property should follow that. Here I'm using a simple arc

answered 2013-07-11T08:36:27.350

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