双缓冲技术实现Android画板应用

package com.example.phonegaptest; 
 
import android.content.Context; 
import android.graphics.Bitmap; 
import android.graphics.Bitmap.Config; 
import android.graphics.Canvas; 
import android.graphics.Color; 
import android.graphics.Paint; 
import android.graphics.Path; 
import android.util.AttributeSet; 
import android.view.MotionEvent; 
import android.view.View; 
 
public class DrawView extends View { 
 float preX; 
 float preY; 
 private Path path; 
 public Paint paint = null; 
 final int VIEW_WIDTH = 320; 
 final int VIEW_HEIGHT = 480; 
 Bitmap cacheBitmap = null; 
 Canvas cacheCanvas = null; 
 
 public DrawView(Context context, AttributeSet set) { 
  super(context, set); 
  cacheBitmap = Bitmap.createBitmap(VIEW_WIDTH, VIEW_HEIGHT, 
    Config.ARGB_8888); 
  cacheCanvas = new Canvas(); 
 
  path = new Path(); 
  cacheCanvas.setBitmap(cacheBitmap); 
 
  paint = new Paint(Paint.DITHER_FLAG); 
  paint.setColor(Color.RED); 
  paint.setStyle(Paint.Style.STROKE); 
  paint.setStrokeWidth(1); 
  paint.setAntiAlias(true); 
  paint.setDither(true); 
 } 
 
 @Override 
 public boolean onTouchEvent(MotionEvent event) { 
  float x = event.getX(); 
  float y = event.getY(); 
 
  switch (event.getAction()) { 
  case MotionEvent.ACTION_DOWN: 
   path.moveTo(x, y); 
   preX = x; 
   preY = y; 
   break; 
  case MotionEvent.ACTION_MOVE: 
   path.quadTo(preX, preY, x, y); 
   preX = x; 
   preY = y; 
   break; 
  case MotionEvent.ACTION_UP: 
   cacheCanvas.drawPath(path, paint); 
   path.reset(); 
   break; 
  } 
  invalidate(); 
  return true; 
 } 
 
 @Override 
 protected void onDraw(Canvas canvas) { 
  super.onDraw(canvas); 
  Paint bmpPaint = new Paint(); 
  canvas.drawBitmap(cacheBitmap, 0, 0, bmpPaint); 
  canvas.drawPath(path, paint); 
 } 
 
}