android 兑现可拖动的Button

public class DraftTest extends Activity {       @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);                DisplayMetrics dm=getResources().getDisplayMetrics();        final int screenWidth=dm.widthPixels;        final int screenHeight=dm.heightPixels-50;                final Button b=(Button)findViewById(R.id.btn);                b.setOnTouchListener(new OnTouchListener(){         int lastX,lastY;                  @Override   public boolean onTouch(View v, MotionEvent event) {    // TODO Auto-generated method stub          int ea=event.getAction();          Log.i("TAG", "Touch:"+ea);                               switch(ea){          case MotionEvent.ACTION_DOWN:                                 lastX=(int)event.getRawX();//获取触摸事件触摸位置的原始X坐标           lastY=(int)event.getRawY();                      break;                   case MotionEvent.ACTION_MOVE:           int dx=(int)event.getRawX()-lastX;           int dy=(int)event.getRawY()-lastY;                                 int l=v.getLeft()+dx;            int b=v.getBottom()+dy;           int r=v.getRight()+dx;           int t=v.getTop()+dy;　　　　　　//下面判断移动是否超出屏幕           if(l<0){            l=0;                r=l+v.getWidth();           }                      if(t<0){            t=0;            b=t+v.getHeight();           }                      if(r>screenWidth){            r=screenWidth;            l=r-v.getWidth();           }                     if(b>screenHeight){            b=screenHeight;            t=b-v.getHeight();           }           v.layout(l, t, r, b);                      lastX=(int)event.getRawX();           lastY=(int)event.getRawY();           Toast.makeText(DraftTest.this,             "当前位置："+l+","+t+","+r+","+b,              Toast.LENGTH_SHORT).show();           v.postInvalidate();                      break;          case MotionEvent.ACTION_UP:           break;                    }    return false;   }});    }}