When do I need to set the contentsScale property of a CALayer?

Question

In the documentation Apple states that you need to consider the scale factor of a layer when it:

• Creates additional Core Animation layers with

Answer 1

While a layer is almost always used inside a view, the layer may have been created ad-hoc and added as a sublayer of the view's layer, or assigned as another layer's mask. In both instances, the contentsScale property would have to be set. For example:

UIView *myMaskedView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
CALayer *myMaskLayer = [CALayer new];
myMaskLayer.frame = myMaskedView.bounds;
UIImage *maskImage = [UIImage imageNamed:@"some_image"];
myMaskLayer.contents = (id)maskImage.CGImage;
myMaskLayer.contentsScale = maskImage.scale;
myMaskedView.layer.mask = myMaskLayer;

You may also want to build a hierarchy of layers for performing animations, without using each layer to back a view:

UIView *myView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];

CALayer *sublayerA = [CALayer new];
sublayerA.contentsScale = myView.layer.contentsScale;
[myView.layer addSublayer:sublayerA];

CALayer *sublayerB = [CALayer new];
sublayerB.contentsScale = myView.layer.contentsScale;
[myView.layer addSublayer:sublayerB];

// Continue adding layers   

This is especially important for Retina devices, which have a native screen scale of 2.0; a layer with the default scale of 1.0 would display noticeable pixelation.