Android圆球随鼠标而移动

XML:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/relativeLayout"
    tools:context="com.example.lianxi.MainActivity">

    <com.example.lianxi.View.BallView
        android:id="@+id/ball"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</RelativeLayout>

Activity：
package com.example.lianxi;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.RelativeLayout;

import com.example.lianxi.View.BallView;

public class MainActivity extends AppCompatActivity {

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

        RelativeLayout container =(RelativeLayout) findViewById(R.id.relativeLayout);

        BallView ballView = new BallView(this);

        container.addView(ballView);

    }
}

BallView:
package com.example.lianxi.View;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.WindowManager;

/**
 * Created by 你家大林哥 on 2018/4/12.
 */

public class BallView extends View {

    private Paint paint;
    Context context;

    private int x =18;
    private int y = 18;
    private int radius = 188;
    public BallView(Context context) {
        super(context);
        this.context = context;
    }

    public BallView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        this.context = context;
    }

    public BallView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        this.context = context;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.drawColor(Color.WHITE);

        paint = new Paint();

        paint.setColor(Color.RED);

        paint.setAntiAlias(true);

        canvas.drawCircle(x,y,radius,paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
       switch (event.getAction()){
           case MotionEvent.ACTION_DOWN:
               x =(int)event.getX();
               y = (int)event.getY();
               System.out.println("按下时:"+"x坐标:"+event.getRawX()+""+"y坐标:"+event.getRawY());
           case MotionEvent.ACTION_MOVE:
               x =(int)event.getX();
               y = (int)event.getY();
               System.out.println("移动时:"+"x坐标:"+event.getRawX()+""+"y坐标:"+event.getRawY());
            case MotionEvent.ACTION_UP:
                x =(int)event.getX();
                y = (int)event.getY();
                System.out.println("抬起时:"+"x坐标:"+event.getRawX()+""+"y坐标:"+event.getRawY());

                break;
       }

        WindowManager manager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        int width = manager.getDefaultDisplay().getWidth();
        int height = manager.getDefaultDisplay().getHeight();


        if (x >= 18 && y >= 18 && x <= width - 18 && y <= height - 18){
            postInvalidate();
        }
       return  true;
    }
}