Android菜鸟的成长笔记（27）

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;@Overrideprotected 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() {@Overridepublic void surfaceDestroyed(SurfaceHolder holder) {}@Overridepublic void surfaceCreated(SurfaceHolder holder) {//锁定整个SurfaceViewCanvas 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lockCanvasholder.lockCanvas(new Rect(0, 0, 0, 0));holder.unlockCanvasAndPost(canvas);}@Overridepublic void surfaceChanged(SurfaceHolder holder, int format, int width,int height) {// TODO Auto-generated method stub}});surface.setOnTouchListener(new OnTouchListener() {@Overridepublic 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<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android=""android:orientation="vertical"android:layout_width="fill_parent"android:layout_height="fill_parent"><SurfaceView android:id="@+id/show"android:layout_width="fill_parent" android:layout_height="fill_parent" /></LinearLayout>运行效果：

可以看出来，第一次绘制的图形会被第二次的区域遮挡，第三次绘制的图形可能遮挡第二次绘制的区域，但不会遮挡第一次的区域。如果第二次绘制的区域被第三次的区域所遮挡，第一次所绘制的图形可能显露出来。

基于SurfaceView开发的示波器：

package com.example.erweimatest;import java.util.Timer;import java.util.TimerTask;import android.app.Activity;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Rect;import android.os.Bundle;import android.view.SurfaceHolder;import android.view.SurfaceHolder.Callback;import android.view.SurfaceView;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class ShowVawe extends Activity{private SurfaceHolder holder;private Paint paint;final int HEIGHT = 320;final int WIDTH = 320;final int X_OFFSET = 5;private int cx = X_OFFSET;//实际的Y轴的位置int centerY = HEIGHT / 2;Timer timer = new Timer();TimerTask task = null;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);final SurfaceView surface = (SurfaceView) findViewById(R.id.show);//初始化SurfaceHolder对象holder = surface.getHolder();paint = new Paint();paint.setColor(Color.GREEN);paint.setStrokeWidth(3);Button sin = (Button) findViewById(R.id.sin);Button cos = (Button) findViewById(R.id.cos);OnClickListener listener = (new OnClickListener() {@Overridepublic void onClick(final View source) {drawBack(holder);cx = X_OFFSET;if(task != null){task.cancel();}task = new TimerTask() {@Overridepublic void run() {int cy = source.getId() == R.id.sin ? centerY – (int)(100 * Math.sin((cx – 5) * 2 * Math.PI / 150)): centerY – (int)(100 * Math.cos((cx – 5) * 2 * Math.PI / 150));Canvas canvas = holder.lockCanvas(new Rect(cx, cy – 2, cx+2, cy + 2));canvas.drawPoint(cx, cy, paint);cx ++;if(cx > WIDTH){task.cancel();task = null;}holder.unlockCanvasAndPost(canvas);}};timer.schedule(task, 0, 30);}});sin.setOnClickListener(listener);cos.setOnClickListener(listener);holder.addCallback(new Callback() {@Overridepublic void surfaceDestroyed(SurfaceHolder holder) {// TODO Auto-generated method stub}@Overridepublic void surfaceCreated(SurfaceHolder holder) {// TODO Auto-generated method stub}@Overridepublic void surfaceChanged(SurfaceHolder holder, int format, int width,int height) {// TODO Auto-generated method stub}});}private void drawBack(SurfaceHolder holder){Canvas canvas = holder.lockCanvas();//绘制白色背景canvas.drawColor(Color.WHITE);Paint p = new Paint();p.setColor(Color.BLACK);p.setStrokeWidth(2);//绘制坐标轴canvas.drawLine(X_OFFSET, centerY, WIDTH, centerY, p);canvas.drawLine(X_OFFSET, 40, X_OFFSET, HEIGHT, p);holder.unlockCanvasAndPost(canvas);holder.lockCanvas(new Rect(0, 0, 0, 0));holder.unlockCanvasAndPost(canvas);}}activity_main.xml<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android=""android:orientation="vertical"android:layout_width="fill_parent"android:layout_height="fill_parent"><LinearLayout android:orientation="horizontal"android:layout_width="fill_parent"android:layout_height="wrap_content"android:gravity="center"><Button android:id="@+id/sin"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="正旋曲线"/><Button android:id="@+id/cos"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="余旋曲线"/></LinearLayout><SurfaceView android:id="@+id/show"android:layout_width="fill_parent"android:layout_height="fill_parent"android:gravity="center"/></LinearLayout>运行结果：

当程序每次绘制正旋波、余旋波上的当前点时，程序无须重绘整个画面，SurfaceHolder只要锁定当前绘制点的小范围即可，系统更新画面时也只要更新这个范围即可。

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，那些无法讲述的悲伤和苍凉，