1、当测量好一个View后就可以重写onDraw()方法，并在Canvas对象上绘制所需的图形。
public class
Canvas
extends Object| java.lang.Object | |
| ↳ | android.graphics.Canvas |
| Public Constructors | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Canvas() Construct an empty raster canvas. | |||||||||||
| Canvas(Bitmap bitmap) Construct a canvas with the specified bitmap to draw into. | |||||||||||
Class Overview
The Canvas class holds the "draw" calls. To draw something, you need 4 basic components: A Bitmap to hold the pixels, a Canvas to host the draw calls (writing into the bitmap), a drawing primitive (e.g. Rect, Path, text, Bitmap), and a paint (to describe the colors and styles for the drawing).
2、onDraw
protected void onDraw (Canvas canvas)
Implement this to do your drawing.
参数Canvas canvas对象用来进行绘图。
Canvas canvas = new Canvas(bitmap);bitmap用来存储所有绘制在Canvas上的像素信息。
由源码：View的绘制过程有以下几步：
1）绘制背景（background.draw(canvas)）
2)绘制自己（omDraw()）
3)绘制children(dispatchDraw)
4)绘制装饰（onDrawScrollBars(for instance)）
View绘制过程的传递通过dispatchDraw来实现，遍历调用所有子元素的draw方法，draw时间一层层传递下去。
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