SurfaceView闪烁问题的根源-双缓存机制

上篇中提到了SurfaceView绘制触摸轨迹时出现的闪烁问题，这里就说明下产生这种困扰的根源——双缓存机制。

Android中的SurfaceView在更新视图时，为了提高更新效率，加强用户体验，采用了双缓存机制。

Android的官方说明有：

Note: On each pass you retrieve the Canvas from the SurfaceHolder, the previous state of the Canvas will be retained. In order to properly animate your graphics, you must re-paint the entire surface. For example, you can clear the previous state of the Canvas by filling in a color with drawColor() or setting a background image with drawBitmap(). Otherwise, you will see traces of the drawings you previously performed.

在运用时可以理解为：SurfaceView在更新视图时用到了两张Canvas，一张frontCanvas和一张backCanvas，每次实际显示的是frontCanvas，backCanvas存储的是上一次更改前的视图，当使用lockCanvas（）获取画布时，得到的实际上是backCanvas而不是正在显示的frontCanvas，之后你在获取到的backCanvas上绘制新视图，再unlockCanvasAndPost（canvas）此视图，那么上传的这张canvas将替换原来的frontCanvas作为新的frontCanvas，原来的frontCanvas将切换到后台作为backCanvas。例如，如果你已经先后两次绘制了视图A和B，那么你再调用lockCanvas（）获取视图，获得的将是A而不是正在显示的B，之后你讲重绘的C视图上传，那么C将取代B作为新的frontCanvas显示在SurfaceView上，原来的B则转换为backCanvas。

如下面的一段代码：

package com.tobacco.touchdraw;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.SurfaceHolder.Callback;

public class TestView extends SurfaceView implements Callback{
private SurfaceHolder sfh;
private Canvas canvas;
private Paint paint;

public TestView(Context context) {
super(context);
sfh=this.getHolder();
sfh.addCallback(this);
paint=new Paint();
paint.setColor(Color.RED);
paint.setAntiAlias(true);
paint.setStrokeWidth(2);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeCap(Paint.Cap.ROUND);
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
// TODO Auto-generated method stub

}

@Override
public void surfaceCreated(SurfaceHolder holder) {
for(int i=0;i<10;i++){
canvas=sfh.lockCanvas();
if(canvas!=null){
canvas.drawText(""+i,10,20*i,paint);
}
sfh.unlockCanvasAndPost(canvas);
}

}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
// TODO Auto-generated method stub

}

}
最终在屏幕上显示的，不会是0到9的10个数字，而是1，3,5,7,9五个奇数数字，原因是最后绘制的是带有这些奇数的那张缓冲画布，而所有的偶数数字将绘制到另一张缓冲画布上。

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