[Android开发常见问题-24] Android 的 SurfaceView 双缓冲应用

双缓冲是为了防止动画闪烁而实现的一种多线程应用，基于SurfaceView的双缓冲实现很简单，开一条线程并在其中绘图即可。本文介绍基于SurfaceView的双缓冲实现，以及介绍类似的更高效的实现方法。 
本文程序运行截图如下，左边是开单个线程读取并绘图，右边是开两个线程，一个专门读取图片，一个专门绘图：

对比一下，右边动画的帧速明显比左边的快，左右两者都没使用Thread.sleep()。为什么要开两个线程一个读一个画，而不去开两个线程像左边那样都 “边读边画”呢？因为SurfaceView每次绘图都会锁定Canvas,也就是说同一片区域这次没画完下次就不能画，因此要提高双缓冲的效率，就得开一条线程专门画图，开另外一条线程做预处理的工作。

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:layout_width="fill_parent" android:layout_height="fill_parent"
	android:orientation="vertical">

	<LinearLayout android:id="@+id/LinearLayout01"
		android:layout_width="wrap_content" android:layout_height="wrap_content">
		<Button android:id="@+id/Button01" android:layout_width="wrap_content"
			android:layout_height="wrap_content" android:text="单个独立线程"></Button>
		<Button android:id="@+id/Button02" android:layout_width="wrap_content"
			android:layout_height="wrap_content" android:text="两个独立线程"></Button>
	</LinearLayout>
	<SurfaceView android:id="@+id/SurfaceView01"
		android:layout_width="fill_parent" android:layout_height="fill_parent"></SurfaceView>
</LinearLayout>

TestSurfaceView.java

package com.testSurfaceView;

import java.lang.reflect.Field;
import java.util.ArrayList;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.widget.Button;

public class TestSurfaceView extends Activity {
	/** Called when the activity is first created. */
	Button btnSingleThread, btnDoubleThread;
	SurfaceView sfv;
	SurfaceHolder sfh;
	ArrayList<Integer> imgList = new ArrayList<Integer>();
	int imgWidth, imgHeight;
	Bitmap bitmap;//独立线程读取，独立线程绘图

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);

		btnSingleThread = (Button) this.findViewById(R.id.Button01);
		btnDoubleThread = (Button) this.findViewById(R.id.Button02);
		btnSingleThread.setOnClickListener(new ClickEvent());
		btnDoubleThread.setOnClickListener(new ClickEvent());
		sfv = (SurfaceView) this.findViewById(R.id.SurfaceView01);
		sfh = sfv.getHolder();
		sfh.addCallback(new MyCallBack());// 自动运行surfaceCreated以及surfaceChanged
	}

	class ClickEvent implements View.OnClickListener {

		@Override
		public void onClick(View v) {

			if (v == btnSingleThread) {
				new Load_DrawImage(0, 0).start();//开一条线程读取并绘图
			} else if (v == btnDoubleThread) {
				new LoadImage().start();//开一条线程读取
				new DrawImage(imgWidth + 10, 0).start();//开一条线程绘图
			}

		}

	}

	class MyCallBack implements SurfaceHolder.Callback {

		@Override
		public void surfaceChanged(SurfaceHolder holder, int format, int width,
				int height) {
			Log.i("Surface:", "Change");

		}

		@Override
		public void surfaceCreated(SurfaceHolder holder) {
			Log.i("Surface:", "Create");

			// 用反射机制来获取资源中的图片ID和尺寸
			Field[] fields = R.drawable.class.getDeclaredFields();
			for (Field field : fields) {
				if (!"icon".equals(field.getName()))// 除了icon之外的图片
				{
					int index = 0;
					try {
						index = field.getInt(R.drawable.class);
					} catch (IllegalArgumentException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					} catch (IllegalAccessException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
					// 保存图片ID
					imgList.add(index);
				}
			}
			// 取得图像大小
			Bitmap bmImg = BitmapFactory.decodeResource(getResources(),
					imgList.get(0));
			imgWidth = bmImg.getWidth();
			imgHeight = bmImg.getHeight();
		}

		@Override
		public void surfaceDestroyed(SurfaceHolder holder) {
			Log.i("Surface:", "Destroy");

		}

	}

	/**
	 * 读取并显示图片的线程
	 */
	class Load_DrawImage extends Thread {
		int x, y;
		int imgIndex = 0;

		public Load_DrawImage(int x, int y) {
			this.x = x;
			this.y = y;
		}

		public void run() {
			while (true) {
				Canvas c = sfh.lockCanvas(new Rect(this.x, this.y, this.x
						+ imgWidth, this.y + imgHeight));
				Bitmap bmImg = BitmapFactory.decodeResource(getResources(),
						imgList.get(imgIndex));
				c.drawBitmap(bmImg, this.x, this.y, new Paint());
				imgIndex++;
				if (imgIndex == imgList.size())
					imgIndex = 0;

				sfh.unlockCanvasAndPost(c);// 更新屏幕显示内容
			}
		}
	};

	/**
	 * 只负责绘图的线程
	 */
	class DrawImage extends Thread {
		int x, y;

		public DrawImage(int x, int y) {
			this.x = x;
			this.y = y;
		}

		public void run() {
			while (true) {
				if (bitmap != null) {//如果图像有效
					Canvas c = sfh.lockCanvas(new Rect(this.x, this.y, this.x
							+ imgWidth, this.y + imgHeight));

					c.drawBitmap(bitmap, this.x, this.y, new Paint());

					sfh.unlockCanvasAndPost(c);// 更新屏幕显示内容
				}
			}
		}
	};

	/**
	 * 只负责读取图片的线程
	 */
	class LoadImage extends Thread {
		int imgIndex = 0;

		public void run() {
			while (true) {
				bitmap = BitmapFactory.decodeResource(getResources(),
						imgList.get(imgIndex));
				imgIndex++;
				if (imgIndex == imgList.size())//如果到尽头则重新读取
					imgIndex = 0;
			}
		}
	};
}