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);  }  }