android控件跟随手势滑动改变位置

要求：
1.通过手指移动来拖动图片  
2.控制图片不能超出屏幕显示区域

技术点:
1.MotionEvent处理
2.对View进行动态定位(layout)

activity_main.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" >

    <ImageView
        android:id="@+id/iv_main"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/test"/>

</RelativeLayout>

MainActivity:

public class MainActivity extends Activity implements OnTouchListener {

	private ImageView iv_main;
	private RelativeLayout parentView;

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

		iv_main = (ImageView) findViewById(R.id.iv_main);
		parentView = (RelativeLayout) iv_main.getParent();
		/*
		int right = parentView.getRight(); //0
		int bottom = parentView.getBottom();   //0
		Toast.makeText(this, right+"---"+bottom, 1).show();
		*/
		//设置touch监听
		iv_main.setOnTouchListener(this);
	}

	private int lastX;
	private int lastY;
	private int maxRight;
	private int maxBottom;

	@Override
	public boolean onTouch(View v, MotionEvent event) {
		//得到事件的坐标
		int eventX = (int) event.getRawX();
		int eventY = (int) event.getRawY();
		switch (event.getAction()) {
		case MotionEvent.ACTION_DOWN:

			//得到父视图的right/bottom
			if(maxRight==0) {//保证只赋一次值
				maxRight = parentView.getRight();
				maxBottom = parentView.getBottom();
			}

			//第一次记录lastX/lastY
			lastX =eventX;
			lastY = eventY;
			break;
		case MotionEvent.ACTION_MOVE:
			//计算事件的偏移
			int dx = eventX-lastX;
			int dy = eventY-lastY;
			//根据事件的偏移来移动imageView
			int left = iv_main.getLeft()+dx;
			int top = iv_main.getTop()+dy;
			int right = iv_main.getRight()+dx;
			int bottom = iv_main.getBottom()+dy;

			//限制left  >=0
			if(left<0) {
				right += -left;
				left = 0;
			}
			//限制top
			if(top<0) {
				bottom += -top;
				top = 0;
			}
			//限制right <=maxRight
			if(right>maxRight) {
				left -= right-maxRight;
				right = maxRight;
			}
			//限制bottom <=maxBottom
			if(bottom>maxBottom) {
				top -= bottom-maxBottom;
				bottom = maxBottom;
			}

			iv_main.layout(left, top, right, bottom);
			//再次记录lastX/lastY
			lastX = eventX;
			lastY = eventY;
			break;

		default:
			break;
		}
		return true;//所有的motionEvent都交给imageView处理
	}
}