Android Studio的MyView试图，（小球实现修改背景以及退出程序）

话不多说，直接上硬菜：

1.MainActivity

package com.example.flyyu.third;

import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {


    private Myview myview;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myview = (Myview)findViewById(R.id.myView);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        if(event.getAction()==MotionEvent.ACTION_DOWN){
            float x=event.getX();
        }
        else    if (event.getAction()==MotionEvent.ACTION_MOVE){
            myview.cx=event.getX();
            myview.cy=event.getY();
            if( myview.cx- myview.radius<0){
                myview.cx= myview.radius;
            }
            if( myview.cy- myview.radius<0){
                myview.cy= myview.radius;
            }

            if( myview.cx>myview.wide- myview.radius){
                myview.cx=myview.wide- myview.radius;
            }


            if( myview.cy>myview.height- myview.radius){
                myview.cy=myview.height- myview.radius;
            }
            myview.postInvalidate();
        }
        return true;
    }
}

2.MyView

package com.example.flyyu.third;

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.View;

/**
 * Created by flyyu on 2020/5/20.
 */

public class Myview extends View {

    float cx=50,cy=50;
    float radius=20;

    float wide,height;
    public Myview(Context context, @Nullable AttributeSet attrs) {

        super(context, attrs);
    }
    @Override
    protected void onDraw(Canvas canvas) {
        Paint paint =new Paint();
        paint.setAntiAlias(true);
        paint.setStyle(Paint.Style.FILL);

        paint.setColor(Color.RED);
        canvas.drawCircle(200,200,102,paint);
        canvas.drawRect(150-2,400-2,350+2,600+2,paint);
        paint.setColor(Color.WHITE);
        canvas.drawCircle(200,200,100,paint);
        canvas.drawRect(150,400,350,600,paint);

        paint.setColor(Color.BLACK);
        canvas.drawCircle(cx,cy,radius,paint);
        paint.setTextSize(20);

        paint.setColor(Color.WHITE);
        canvas.drawCircle(cx-5,cy-5,3,paint);
        wide=Myview.this.getWidth();
        height=Myview.this.getHeight();
        float s= (float) (Math.pow(cx-200,2))+(float) (Math.pow(cy-200,2));
        if (s<10000){
            setBackgroundColor(Color.RED);
        }
        else {
            setBackgroundColor(Color.WHITE);
        }
        if(cx>150&&cx<350){
            if(cy>400&&cy<600){
                android.os.Process.killProcess(android.os.Process.myPid());
            }
        }

    }
}

3.activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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.flyyu.third.MainActivity">

    <com.example.flyyu.third.Myview
        android:id="@+id/myView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="1.0" />
</android.support.constraint.ConstraintLayout>