Android view随触碰滑动效果

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:id="@+id/relativeLayout"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 tools:context="com.xingyi.moveviewwithtouch.MainActivity">
<ImageView
 android:id="@+id/imageView"
 android:layout_width="40dp"
 android:layout_height="40dp"
 android:background="@android:color/black"/>
</RelativeLayout>

package com.xingyi.moveviewwithtouch;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.MotionEvent;
import android.view.View;
import android.widget.ImageView;
import android.widget.RelativeLayout;
public class MainActivity extends AppCompatActivity {
 ImageView imageView;
 RelativeLayout relativeLayout;
 int heightRL,widthRL;
 int halfHeight,halfWidth;
 boolean first=true;
 private int widthImg;
 private int heightImg;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  initView();
 }
 //初始化视图
 private void initView() {
  imageView = (ImageView) findViewById(R.id.imageView);
  relativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout);
  //获取滑动瞬间位置和点击瞬间位置，并移动imageview
  relativeLayout.setOnTouchListener(new View.OnTouchListener() {
   @Override
   public boolean onTouch(View view, MotionEvent motionEvent) {
    switch (motionEvent.getAction()) {
     case MotionEvent.ACTION_MOVE:
      moveView(imageView, motionEvent.getX(), motionEvent.getY());
      break;
     case MotionEvent.ACTION_DOWN:
      getWidthAndHeight();
      moveView(imageView, motionEvent.getX(), motionEvent.getY());
      break;
     default:
      break;
    }
    return true;
   }
  });
 }
 //因为不能在初始化视图时获得长宽，而每次计算一次长宽又影响性能
 private void getWidthAndHeight(){
  if(first){
   widthRL=relativeLayout.getWidth();
   heightRL=relativeLayout.getHeight();
   widthImg=imageView.getWidth();
   heightImg=imageView.getHeight();
   halfWidth = imageView.getWidth() / 2;//imageView宽度的一半
   halfHeight = imageView.getHeight() / 2;//imageView高度的一半
   first=false;
  }
 }
 //滑动瞬间，将x和y分别作imageView的中心点到relativeLayout最左和顶端距离
 private void moveView(View view, float x, float y) {
  RelativeLayout.LayoutParams params = (RelativeLayout.LayoutParams) view.getLayoutParams();
  //设置水平位置
  if (x < halfWidth) {//左边缘
   params.leftMargin = 0;//设置imageview到左端距离为0
  } else if (x > widthRL- halfWidth) {
   params.leftMargin = widthRL-widthImg;//设置imageview左端到左端端距离（params.rightMargin的性能非常糟糕）
  } else {
   params.leftMargin = (int) (x - halfWidth);//imageview左端到relativelayout左端距离
  }
  //设置竖直位置
  if (y < halfHeight) {
   params.topMargin = 0;
  } else if (y > heightRL - halfHeight) {
   params.topMargin = heightRL-widthImg;//params.bottomMargin的性能非常糟糕
  } else {
   params.topMargin = (int) (y - halfHeight);
  }
  view.setLayoutParams(params);
 }
}