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.
说明及理解：
1.在View的事件回调方法onDraw()中，要做的仅仅是直接使用draw方法（canvas已经提供给你了，不需要再考虑canvas问题）。对于SurfaceView，可以通过surfaceHolder.lockCanvas()方法获得特定的canvas。
2.如果要自己新建一个Canvas，必需定义一个Bitmap。这样在canvas上调用的各种draw方法画出的内容都将承载在bitmap上。
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 =newCanvas(b);
3.你可以把你自己的Bitmap放到其它Canvas上。 通过Canvas.drawBitmap(Bitmap,...)
但建议，你最终的图像是画在View.onDraw(）的画布上或者surfaceHolder.lockCanvas()的画布上。
4.Canvas类有自己的画画方法集。如drawBitmap(...)，drawRect(...)等等。你可能用到的其他的类，它们也有自己的画画方法。如你画在Canvas上的Drawable对象，也有draw()方法。它们是把canvas作为draw()方法的参数。