Visual Effects - iOS Core Animation - page 59

CGMutablePathRef circlePath = CGPathCreateMutable ();
CGPathAddEllipseInRect (circlePath, NULL , self .layerView2. bounds );
self .layerView2. layer . shadowPath = circlePath;
CGPathRelease (circlePath);
}
@end
For something like a rectangle or circle, creating a CGPath manually is fairly straight-
forward. For a more complex shape like a rounded rectangle, you'll probably find it easier
to use the UIBezierPath class, which is an Objective-C wrapper around CGPath
provided by UIKit.
Layer Masking
Using the masksToBounds property, we know that it is possible to clip a layer's contents
to its bounds , and using the cornerRadius property, we can even give it rounded
corners. But sometimes you will want to represent content that is not rectangular or even
rounded-rectangular in shape. For example, you might want to create a star-shaped photo
frame around an image, or you might want the edges of some scrolling text to fade
gracefully into the background instead of clipping to a sharp edge.
Using a 32-bit PNG image with an alpha component, you can specify a backing image that
includes an arbitrary alpha mask, which is usually the simplest way to create a non-
rectangular view. But that approach doesn't allow you to clip images dynamically using
programmatically generated masks or to have sublayers or subviews that also clip to the
same arbitrary shape.
CALayer has a property called mask that can help with this problem. The mask property
is itself a CALayer and has all the same drawing and layout properties of any other layer.
It is used in a similar way to a sublayer in that it is positioned relative to its parent (the layer
that owns it), but it does not appear as a normal sublayer. Instead of being drawn inside the
parent, the mask layer defines the part of the parent layer that is visible.
The color of the mask layer is irrelevant; all that matters is its silhouette. The mask acts
like a cookie cutter; the solid part of the mask layer will be “cut out” of its parent layer and
kept; anything else is discarded (see Figure 4.12).
If the mask layer is smaller than the parent layer, only the parts of the parent (or its sub-
layers) that intersect the mask will be visible. If you are using a mask layer, anything
outside of that layer is implicitly hidden.