Programming iOS 6, 3rd Edition

Emitter Layers

Emitter layers (CAEmitterLayer) are, to some extent, on a par with animated images: once you’ve set up an emitter layer, it just sits there animating all by itself. The nature of this animation is rather narrow: an emitter layer emits particles, which are CAEmitterCell instances. However, by clever setting of the properties of an emitter layer and its emitter cells, you can achieve some astonishing effects. Moreover, the animation is itself animatable using Core Animation.

It is easiest to understand emitter layers and emitter cells if you start with some stupid settings to achieve a boring effect. Let’s start with the emitter cells. Here are some useful basic properties of a CAEmitterCell:

contents, contentsRect

These are modeled after the eponymous CALayer properties, although CAEmitterLayer is not a CALayer subclass; so, respectively, an image (a CGImageRef) and a CGRect defining a region of that image. They define the image that a cell will portray.

birthrate, lifetime

How many cells per second should be emitted, and how many seconds each cell should live before vanishing, respectively.

velocity

The speed at which a cell moves. The unit of measurement is not documented; perhaps it’s points per second.

emissionLatitude, emissionLongitude

The angle at which the cell is emitted from the emitter, as a variation from the perpendicular. Longitude is an angle within the plane; latitude is an angle out of the plane.

So, here’s code to create a very elementary emitter ...