自定义小球跟随手指移动

package com.example.ballviewdemo;

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 Administrator on 2017/11/2 0002.
 */
//自定义类继承视图
public class BallDemo extends View{
    private int x = 100;
    private int y = 100;
    Context context;
    public BallDemo(Context context) {
        this(context,null);
    }

    public BallDemo(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs,0);
    }

    public BallDemo(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        this.context = context;
    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint paint = new Paint();
        //设置画笔颜色
        paint.setColor(Color.RED);
        //画圆
        canvas.drawCircle(x,y,20,paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction())
        {
            case MotionEvent.ACTION_DOWN:
            case MotionEvent.ACTION_MOVE:
            case MotionEvent.ACTION_UP:
                x = (int) event.getX();
                y = (int) event.getY();
            break;
        }
        WindowManager manager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        int width = manager.getDefaultDisplay().getWidth();
        int height = manager.getDefaultDisplay().getHeight();
        if(x>=20&&y>=20&&x<=width-20&&y<=height-90)
        {
            invalidate();
        }
        return true;
    }
}

//在主方法的布局中调用视图

<?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" tools:context="com.example.ballviewdemo.BallActivity">
    <com.example.ballviewdemo.BallDemo
        android:id="@+id/circle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />


</RelativeLayout>