How do I position `contents` png in CALayer?

Question

Do I have to move the layer frame or apply translate matrix transformation to layer? Or perhaps I can move the contents inside of the layer? If contents is not movable inside of layer, how it would position initially?

Answer 1

A CALayer has a frame (or, equivalently, a bounds and an origin), which is used logically to determine what to draw. When drawInContext: or equivalent is called, it's the frame that determines how the contents are produced.

However, like OS X, iOS adopts a compositing window manager, which means that views know how to draw their output to a buffer and the buffers are combined to create the view, with the window manager figuring out what to do about caching and video memory management in between.

If you adjust the transform property of the view or of the layer class, then you adjust how the compositing happens. However, the results of drawInContext: should explicitly still be the same so the window manager knows it can just use the cached image.

So, for example, if you set a frame of size 128x128 and then a transform that scales the CALayer up to double, you'll occupy a 256x256 area of the screen but the image used for compositing will be only 128x128 in size, making each source pixel into four target pixels. If you set a frame of size 256x256 and the identity transform, you'll cover the same amount of screen space but with each source pixel being 1:1 related to a target pixel.

A side effect is that changing the frame causes a redraw from first principles. Changing the transform doesn't. So the latter is usually faster, and is also the thing to do if you decide to use something like CATiledLayer (as used in Safari, Maps, etc) that draws in a separate thread and may take a while to come up with results.

As a rule of thumb, you use the frame to set the initial position and update the frame for normal work stuff. You play with the transform for transitions and other special effects. However, all of the frame and transform properties of a CATiledLayer are animatable in the CoreAnimation sense, so that's really still at your discretion.

Most people don't work on the level of a CALayer, but prefer to work with UIViews. In which case the comments are mostly the same, with the caveat that you can then adjust the [2d] transform on the view or the [3d] transform on the view's layer and have the compositor figure it all out, but change the frame to prompt a redraw.