android 控件跟随手指移动

项目中遇到控件跟随手指触摸移动的部分，简单测试了下。

package com.example.viewanimationtest;

/**
 * @author maria
 * 2013-2-16
 */
import android.os.Bundle;
import android.app.Activity;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends Activity implements OnTouchListener {

	Button _view;
	ViewGroup _root;
	private int _xDelta;
	private int _yDelta;

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		_root = (ViewGroup) findViewById(R.id.root);

		_view = (Button) findViewById(R.id.id_text);

		RelativeLayout.LayoutParams layoutParams = new RelativeLayout.LayoutParams(
				150, 50);
		layoutParams.leftMargin = 50;
		layoutParams.topMargin = 50;
		layoutParams.bottomMargin = -250;
		layoutParams.rightMargin = -250;
		_view.setLayoutParams(layoutParams);
		_view.setOnTouchListener(this);
	}

	public boolean onTouch(View view, MotionEvent event) {
		final int X = (int) event.getRawX();
		final int Y = (int) event.getRawY();
		switch (event.getAction() & MotionEvent.ACTION_MASK) {
		case MotionEvent.ACTION_DOWN:
			RelativeLayout.LayoutParams lParams = (RelativeLayout.LayoutParams) view
					.getLayoutParams();
			_xDelta = X - lParams.leftMargin;
			_yDelta = Y - lParams.topMargin;
			break;
		case MotionEvent.ACTION_UP:
			break;
		case MotionEvent.ACTION_POINTER_DOWN:
			break;
		case MotionEvent.ACTION_POINTER_UP:
			break;
		case MotionEvent.ACTION_MOVE:
			RelativeLayout.LayoutParams layoutParams = (RelativeLayout.LayoutParams) view
					.getLayoutParams();
			layoutParams.leftMargin = X - _xDelta;
			layoutParams.topMargin = Y - _yDelta;
			layoutParams.rightMargin = -250;
			layoutParams.bottomMargin = -250;
			view.setLayoutParams(layoutParams);
			break;
		}
		_root.invalidate();
		return true;
	}
}

.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/root"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/id_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:text="@string/hello_world" />

</RelativeLayout>