Programming iOS 6, 3rd Edition

Transforms

The way a layer is drawn on the screen can be modified though a transform. This is not surprising, because a view can have a transform (see Chapter 14), and a view is drawn on the screen by its layer. As with the bounds and other properties, a view and its underlying layer are tightly linked; when you change the transform of one, you are changing the transform of the other. But, as so often happens, the layer’s transform is more powerful than the view’s transform. Thus, you can use the transform of the underlying layer to accomplish things with a view that you can’t accomplish with the view’s transform alone.

In the simplest cases, when a transform is two-dimensional, you can use the setAffineTransform: and affineTransform methods. The value is a CGAffineTransform, familiar from Chapter 14 and Chapter 15. The transform is applied around the anchorPoint. Thus, the anchorPoint has a second purpose that I didn’t tell you about when discussing it earlier.

You now know everything you need to know in order to understand the code that generated Figure 16-6, so here is that code. Notice how the four cardinal point letters are drawn by a CATextLayer and placed using a transform. They are drawn at the same coordinates, but they have different rotation transforms. Moreover, even though the CATextLayers are small (just 40 by 40) and appear near the perimeter of the circle, they are anchored, and so their rotation is centered, at the center of the circle. In this code, self is the CompassLayer; ...