Most widget animation can be taken care of using the animation APIs in Android. The standard animation APIs are designed to take care of animations with a defined start and end, or animations that form a simple loop. Some animations, however, don't fit into this mold; a good example would be a game. A game has many animations running continuously, and you can even think about the entire game screen as a single, continuous animation.
There are a number of widgets that need to be continuously animated, and your standard Android animation API won't work. In these cases, you'll need a View that can continuously animate and update itself as long as it's visible to the user. In these cases, a slightly different design is called for, as the widget will always be changing.
To illustrate how to write a widget that has a continuous animation, let's write a View class that animates some number of bouncing Drawable objects. Each Drawable will be tracked separately, and when it...