Canvas in Android

CANVAS

The Android framework APIs provides a set 2D drawing APIs that allow you to render your own custom graphics onto a canvas or to modify existing Views to customize their look and feel. When drawing 2D graphics, you'll typically do so in one of two ways:

a.     Draw your graphics or animations into a View object from your layout. In this manner, the drawing of your graphics is handled by the system's normal View hierarchy drawing process — you simply define the graphics to go inside the View.

b.    Draw your graphics directly to a Canvas. This way, you personally call the appropriate class's onDraw() method (passing it your Canvas), or one of the Canvas draw...() methods (likedrawPicture()). In doing so, you are also in control of any animation.

Drawing to a Canvas, is better when your application needs to regularly re-draw itself. Applications such as video games should be drawing to the Canvas on its own.

Drawing With Canvas:

When you're writing an application in which you would like to perform specialized drawing and/or control the animation of graphics, you should do so by drawing through a Canvas. A Canvas works for you as a pretense, or interface, to the actual surface upon which your graphics will be drawn. It holds all of your "draw" calls. Via the Canvas, your drawing is actually performed upon an underlying Bitmap , which is placed into the window. However, if you need to create a new Canvas, then you must define the Bitmap upon which drawing will actually be performed. The Bitmap is always required for a Canvas. You can set up a new Canvas like this:

Bitmap b = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888);

Canvas c = new Canvas(b);

Now your Canvas will draw onto the defined Bitmap.

On a View:

If your application does not require a significant amount of processing or frame-rate speed (perhaps for a chess game, a snake game, or another slowly-animated application), then you should consider creating a custom View component and drawing with a Canvas in View.onDraw(). The most convenient aspect of doing so is that the Android framework will provide you with a pre-defined Canvas to which you will place your drawing calls.

To start, extend the View class (or descendant thereof) and define the onDraw() callback method. This method will be called by the Android framework to request that your View draw itself. This is where you will perform all your calls to draw through the Canvus, which is passed to you through the onDraw() callback.

On a SurfaceView:

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.