View的绘制过程详解

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.CanvasCanvas()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。