Changing my CALayer's anchorPoint moves the view

Question

I want to alter the anchorPoint, but keep the view in the same place. I've tried NSLog-ing self.layer.position and self.center and they both stay the same regardless of changes to the anchorPoint. Yet my view moves!

Any tips on how to do this?

self.layer.anchorPoint = CGPointMake(0.5, 0.5);
NSLog(@"center point: %f %f", self.layer.position.x, self.layer.position.y);
self.layer.anchorPoint = CGPointMake(1, 1);
NSLog(@"center point: %f %f", self.layer.position.x, self.layer.position.y);

The output is:

2009-12-27 20:43:24.161 Type[11289:207] center point: 272.500000 242.500000
2009-12-27 20:43:24.162 Type[11289:207] center point: 272.500000 242.500000

Answer 1

The key to solving this was to use the frame property, which is weirdly the only thing that changes.

CGRect oldFrame = self.frame;
self.layer.anchorPoint = CGPointMake(1, 1);
self.frame = oldFrame;

Then I do my resize, where it scales from the anchorPoint. Then I have to restore the old anchorPoint;

oldFrame = self.frame;
self.layer.anchorPoint = CGPointMake(0.5,0.5);
self.frame = oldFrame;

EDIT: this flakes out if the view is rotated, as the frame property is undefined if a CGAffineTransform has been applied.

Answer 2

The Layer Geometry and Transforms section of the Core Animation Programming Guide explains the relationship between a CALayer's position and anchorPoint properties. Basically, the position of a layer is specified in terms of the location of the layer's anchorPoint. By default, a layer's anchorPoint is (0.5, 0.5), which lies at the center of the layer. When you set the position of the layer, you are then setting the location of the center of the layer in its superlayer's coordinate system.

Because the position is relative to the anchorPoint of the layer, changing that anchorPoint while maintaining the same position moves the layer. In order to prevent this movement, you would need to adjust the layer's position to account for the new anchorPoint. One way I've done this is to grab the layer's bounds, multiply the bounds' width and height by the old and new anchorPoint's normalized values, take the difference of the two anchorPoints, and apply that difference to the position of the layer.

You might even be able to account for rotation this way by using CGPointApplyAffineTransform() with your UIView's CGAffineTransform.