Draw with a 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.

In the event that you're drawing within the onDraw() callback method, the Canvas is provided for you and you need only place your drawing calls upon it. You can also acquire a Canvas from SurfaceHolder.lockCanvasn, when dealing with a SurfaceView object. (Both of these scenarios are discussed in the following sections.) 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(10 0, 100, Bitmap.Config.ARGB_8888); Canvas c = new Canvas(b);

Now your Canvas will draw onto the defined Bitmap. After drawing upon it with the Canvas, you can then carry your Bitmap to another Canvas with one of the Canvas.drawBitmap(Bitmap,...) methods. It's recommended that you ultimately draw your final graphics through a Canvas offered to you by view.onDraw() or SurfaceHolder.lockCanvasn (see the following sections).

The Canvas class has its own set of drawing methods that you can use, like drawBitmap(...), drawRect(...), drawText(...), and many more. Other classes that you might use also have draw() methods. For example, you'll probably have some Drawable objects that you want to put on the Canvas. Drawable has its own draw() method that takes your Canvas as an arguement.

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