Android：小试图片拖动

package com.example.imgdrag;

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;
import android.widget.Toast;

public class MainActivity extends Activity {
private ImageView imageview;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		imageview = (ImageView) findViewById(R.id.imageView1);
		class MyTouchListener implements OnTouchListener{
                //Q1
				int lastX = 0,lastY = 0;
			@Override
			public boolean onTouch(View v, MotionEvent event) {
				switch(event.getAction()){
				case MotionEvent.ACTION_DOWN:
					lastX = (int) event.getRawX();
					lastY = (int) event.getRawY();
					break;
				case MotionEvent.ACTION_MOVE:
				    //不越界
					int nowX = (int) event.getRawX();
					int nowY = (int) event.getRawY();
					int moveX = nowX - lastX;
					int moveY = nowY - lastY;
//					Toast.makeText(getApplicationContext(), "MOVE "+event.getRawX()+" "+event.getRawY(), Toast.LENGTH_LONG).show();
				int left = v.getLeft() + moveX;
				if(left<0) left = 0;
				int top = v.getTop() + moveY;
				if(top < 0)top = 0;
				int right = left + v.getWidth();
				int bottom = top +v.getHeight();
					v.layout(left, top, right, bottom);
					//Q2
					v.postInvalidate();
					lastX = nowX;
					lastY = nowY;
					break;
				case MotionEvent.ACTION_UP:
				    break;
				default:
					break;
				}
				return true;
			}
			
		}
		imageview.setOnTouchListener(new MyTouchListener());
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

 

Q1:lastX与Y用来记录移动前坐标，但每次事件触发都回调用该函数而还私有类对象只创建一次，需要将其设为私有成员

Q2：图片移动后不显示是由于未重绘View。