On a Surface View

The SurfaceView is a special subclass of View that offers a dedicated drawing surface within the View hierarchy. The aim is to offer this drawing surface to an application's secondary thread, so that the application isn't required to wait until the system's View hierarchy is ready to draw. Instead, a secondary thread that has reference to a SurfaceView can draw to its own Canvas at its own pace.

To begin, you need to create a new class that extends SurfaceView. The class should also implement SurfaceHolder.Callback. This subclass is an interface that will notify you with information about the underlying Surface, such as when it is created, changed, or destroyed. These events are important so that you know when you can start drawing, whether you need to make adjustments based on new surface properties, and when to stop drawing and potentially kill some tasks. Inside your SurfaceView class is also a good place to define your secondary Thread class, which will perform all the drawing procedures to your Canvas.

Instead of handling the Surface object directly, you should handle it via a SurfaceHolder. So, when your SurfaceView is initialized, get the SurfaceHolder by calling getHolder(). You should then notify the SurfaceHolder that you'd like to receive SurfaceHolder callbacks (from SurfaceHolder.Callback) by calling addCallbackO (pass it this). Then override each of the SurfaceHolder.Callback methods inside your SurfaceView class.

In order to draw to the Surface Canvas from within your second thread, you must pass the thread your SurfaceHandler and retrieve the Canvas with lockCanvas(). You can now take the Canvas given to you by the SurfaceHolder and do your necessary drawing upon it. Once you're done drawing with the Canvas, call unlockCanvasAndPost(), passing it your Canvas object. The Surface will now draw the Canvas as you left it. Perform this sequence of locking and unlocking the canvas each time you want to redraw.

Note: On each pass you retrieve the Canvas from the SurfaceHolder, the previous state of the Canvas will be retained. In order to properly animate your graphics, you must re-paint the entire surface. For example, you can clear the previous state of the Canvas by filling in a color with drawColor() or setting a background image with drawBitmap(). Otherwise, you will see traces of the drawings you previously performed.

For a sample application, see the Lunar Landar game, in the SDK samples folder: <your-sdk-directory>/samples/LunarLander/. Or, browse the source in the Sample Code section.

Except as noted, this content is licensed under Apache 2.0. For details and restrictions, see the Content License.

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