Android Application Development : Drawing 2D and 3D Graphics - Bling (part 3) - Animation - Background animation

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2.2. Background animation

Frame-by-frame animation, as it is called in the Google documentation, is completely straightforward: a set of frames, played in order at regular intervals. This kind of animation is implemented by subclasses of AnimationDrawable.

As subclasses of Drawable, AnimationDrawable objects can be used in any context that any other Drawable is used. The mechanism that animates them, however, is not a part of the Drawable itself. In order to animate, an AnimationDrawable relies on an external service provider—an implementation of the Drawable.Callback interface—to animate it.

The View class implements this interface and can be used to animate an AnimationDraw⁠able. Unfortunately, it will supply animation services only to the one Drawable object that is installed as its background with one of the two methods setBackgroundDrawa⁠ble or setBackgroundResource.

The good news, however, is that this is probably sufficient. A background animation has access to the entire widget canvas. Everything it draws will appear to be behind anything drawn by the View.onDraw method, so it would be hard to use the background to implement full-fledged sprites (animation integrated into a static scene). Still, with clever use of the DrawableContainer class (which allows you to animate several different animations simultaneously) and because the background can be changed at any time, it is possible to accomplish quite a bit without resorting to implementing your own animation framework.

An AnimationDrawable in a view background is entirely sufficient to do anything from, say, indicating that some long-running activity is taking place—maybe winged packets flying across the screen from a phone to a tower—to simply making a button’s background pulse.

The pulsing button example is illustrative and surprisingly easy to implement. Examples Example 5 and Example 6 show all you need. The animation is defined as a resource, and code applies it to the button.

Example 5. Frame-by-frame animation (resource)
<item android:drawable="@drawable/throbber_f0" android:duration="70" />
<item android:drawable="@drawable/throbber_f1" android:duration="70" />
<item android:drawable="@drawable/throbber_f2" android:duration="70" />
<item android:drawable="@drawable/throbber_f3" android:duration="70" />
<item android:drawable="@drawable/throbber_f4" android:duration="70" />
<item android:drawable="@drawable/throbber_f5" android:duration="70" />
<item android:drawable="@drawable/throbber_f6" android:duration="70" />
Example 6. Frame-by-frame animation (code)
// w is a button that will "throb"

//!!! This is necessary, but should not be so in Cupcake
button.setOnClickListener(new OnClickListener() {
@Override public void onClick(View v) {
AnimationDrawable animation
= (AnimationDrawable) v.getBackground();
if (animation.isRunning()) { animation.stop(); }
else { animation.start(); }
// button action.
} });

There are several gotchas here, though. First of all, as of this writing, the animation-list example in the Google documentation does not quite work. There is a problem with the way it identifies the animation-list resource. To make it work, don’t define an android:id in that resource. Instead, simply refer to the object by its filename (R.drawable.throbber), as Example 6 demonstrates.

The second issue is that a bug in the V1_r2 release of the toolkit prevents a background animation from being started in the Activity.onCreate method. If your application’s background should be animated whenever it is visible, you’ll have to use trickery to start it. The example implementation uses an onClick handler. There are suggestions on the Web that the animation can also be started successfully from a thread that pauses briefly before calling AnimationDrawable.start. The Android development team has a fix for this problem, so the constraint should be relaxed with the release of Cupcake.

Finally, if you have worked with other UI frameworks, especially Mobile UI frameworks, you may be accustomed to painting the view background in the first couple of lines of the onDraw method (or equivalent). If you do that in Android, however, you will paint over your animation. It is, in general, a good idea to get into the habit of using setBackground to control the View background, whether it is a solid color, a gradient, an image, or an animation.

Specifying an AnimationDrawable by resource is very flexible. You can specify a list of drawable resources—any images you like—that comprise the animation. If your animation needs to be dynamic, AnimationDrawable is a straightforward recipe for creating a dynamic drawable that can be animated in the background of a View.

2.3. Surface view animation

Full-on animation requires a SurfaceView. The SurfaceView provides a node in the view tree (and, therefore, space on the display) on which any process at all can draw. The SurfaceView node is laid out, sized, and receives clicks and updates, just like any other widget. Instead of drawing, however, it simply reserves space on the screen, preventing other widgets from affecting any of the pixels within its frame.

Drawing on a SurfaceView requires implementing the SurfaceHolder.Callback interface. The two methods surfaceCreated and surfaceDestroyed inform the implementor that the drawing surface is available for drawing and that it has become unavailable, respectively. The argument to both of the calls is an instance of yet a third class, SurfaceHolder. In the interval between these two calls, a drawing routine can call the SurfaceView methods lockCanvas and unlockCanvasAndPost to edit the pixels there.

If this seems complex, even alongside some of the elaborate animation discussed previously…well, it is. As usual, concurrency increases the likelihood of nasty, hard-to-find bugs. The client of a SurfaceView must be sure that access to any state shared across threads is properly synchronized, and also that it never touches the SurfaceView, Surface, or Canvas except in the interval between the calls to surfaceCreated and surfaceDestroyed. The toolkit could clearly benefit from a more complete framework support for SurfaceView animation.

If you are considering SurfaceView animation, you are probably also considering OpenGL graphics. As we’ll see, there is an extension available for OpenGL animation on a SurfaceView. It will turn up in a somewhat out-of-the-way place, though.

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