android 利用Canvas显示局部图片

package MyBitMap_1.code;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Rect;
import android.util.DisplayMetrics;
import android.util.Log;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.View;

public class MyView extends View {
 
 private static int mScreenHeight;
 private static int mScreenWidth;
 private double fScale = 1.0;
 
 private int xOffset = 0;
 private int yOffset = 0;
 private int xDist = 0;
 private int yDist = 0;
 
 private int xOffset_T = 0;
 private int yOffset_T = 0;
 
 
 private Bitmap bitMap1;

 public MyView(Context context )
 {
  super(context);

  Initialize();
 }
 
 private void Initialize()
 {
  DisplayMetrics dm = getResources().getDisplayMetrics();
  mScreenWidth = dm.widthPixels;
  mScreenHeight = dm.heightPixels;
  
  Bitmap bmp = BitmapFactory.decodeResource(getResources(),R.drawable.trail );
  bitMap1 = bmp;
  
  xDist = mScreenWidth;
  yDist = mScreenHeight;
 }
 
 @Override
 protected void onDraw(Canvas canvas )
 {
  canvas.drawColor(Color.WHITE);
  
  xDist = (int)(mScreenWidth);
  yDist = (int)(mScreenHeight);
    
  Rect src = new Rect((int)(xOffset*fScale),(int)(yOffset*fScale),(int)((xDist+xOffset)*fScale),(int)((yDist+yOffset)*fScale));
  
  Log.i("Rollrock","OnDraw" + xOffset + "  " + xDist );
  
  Rect dest = new Rect(0,0,mScreenWidth,mScreenHeight);
  
  canvas.drawBitmap(bitMap1, src,dest, null);
 }
 
 
 @Override
 public boolean onKeyDown(int keyCode , KeyEvent event )
 {
  Log.i("Rollrock", "" + keyCode + "  " + event.getAction() + " " + event.getKeyCode());
  
  if( KeyEvent.KEYCODE_DPAD_LEFT == keyCode )
  {
   
  }
  else if( KeyEvent.KEYCODE_DPAD_RIGHT == keyCode )
  {
   
  }
  else if( KeyEvent.KEYCODE_DPAD_UP == keyCode )
  {
   fScale += 0.1;
   this.invalidate();
  }
  else if( KeyEvent.KEYCODE_DPAD_DOWN == keyCode )
  {
   fScale -= 0.1;
   this.invalidate();
  }
  
  return super.onKeyDown(keyCode, event);
 }
 
 @Override
 public boolean onTouchEvent(MotionEvent event )
 {
  int action = event.getAction();
  int x = 0;
  int y = 0;
  
  switch(action)
  {
  case MotionEvent.ACTION_DOWN:
   xOffset_T = (int)event.getX();
   yOffset_T = (int)event.getY();
   break;
   
  case MotionEvent.ACTION_MOVE:
   break;
   
  case MotionEvent.ACTION_UP:
   x = (int)event.getX();
   y = (int)event.getY();
   
   xOffset += xOffset_T - x;
   yOffset += yOffset_T - y;
      
   this.invalidate();
   
   break;
  }
  
  return true;
 }
}