前面有关自定义View中进行了绘图,但View的绘图机制存在如下缺陷:
1、View缺乏双缓冲机制。
2、当程序需要更新View上的图像时,程序必须重绘View上显示的整张图片。
3、新线程无法直接更新View组件。
由于View存在上面缺陷,所以在游戏开发中一般使用SurfaceView来进行绘制,SurfaceView一般会与SurfaceHolder结合使用,SurfaceHolder用于向与之关联的SurfaceView上绘图,调用SurfaceView的getHolder()方法即可获取SurfaceView关联的SurfaceHolder.
SurfaceHolder提供了如下方法来获取Canvas对象:
1、Canvas lockCanvas():锁定整个SurfaceView对象,获取该Surface上的Canvas.
2、Canvas lockCanvas(Rect dirty):锁定SurfaceView上Rect划分的区域,获取该Surface上的Canvas.
两个方法返回的是同一个Canvas,但是第二个方法只对圈出来的区域进行刷新,Canvas绘图完成后通过unlockCanvasAndPost(canvas)方法来释放画布,提交修改。当调用SurfaceHolder的unlockCanvasAndPost方法之后,该方法之前所绘制的图形还处于缓冲之下,下一次lockCanvas()方法锁定的区域可能会“遮挡”它。
package com.example.erweimatest;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceHolder.Callback;
import android.view.SurfaceView;
import android.view.View;
import android.view.View.OnTouchListener;
public class SurfaceViewTest extends Activity {
private SurfaceHolder holder;
private Paint paint;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
paint = new Paint();
SurfaceView surface = (SurfaceView) findViewById(R.id.show);
//初始化SurfaceHolder对象
holder = surface.getHolder();
holder.addCallback(new Callback() {
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
}
@Override
public void surfaceCreated(SurfaceHolder holder) {
//锁定整个SurfaceView
Canvas canvas = holder.lockCanvas();
//绘制背景
Bitmap back = BitmapFactory.decodeResource(SurfaceViewTest.this.getResources(), R.drawable.bg);
//绘制背景
canvas.drawBitmap(back, 0, 0, null);
//绘制完成,释放画布,提交修改
holder.unlockCanvasAndPost(canvas);
//重新锁一次,“持久化”上次所绘制内容
//本次lockCanvas会遮挡上次lockCanvas
holder.lockCanvas(new Rect(0, 0, 0, 0));
holder.unlockCanvasAndPost(canvas);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
// TODO Auto-generated method stub
}
});
surface.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
if(event.getAction() == MotionEvent.ACTION_DOWN){
int cx = (int) event.getX();
int cy = (int) event.getY();
//锁定SurfaceView的布局区域,只更新局部内容
Canvas canvas = holder.lockCanvas(new Rect(cx - 50, cy - 50, cx + 50, cy + 50));
//保存canvas当前状态
canvas.save();
//旋转画布
canvas.rotate(30, cx, cy);
paint.setColor(Color.RED);
//绘制红色方块
canvas.drawRect(cx - 40,cy - 40, cx, cy, paint);
//恢复canvas之前的保存状态
canvas.restore();
paint.setColor(Color.GREEN);
//绘制绿色方块
canvas.drawRect(cx, cy, cx + 40, cy + 40, paint);
//绘制完成,释放画布,提交修改
holder.unlockCanvasAndPost(canvas);
}
return false;
}
});
}
}
main.xml
运行效果:
基于SurfaceView开发的示波器: