自定义View，随着手指运动的小球

package com.kale.drawview;


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

public class DrawView extends View{

    public float currentX = 60;
    public float currentY = 60;
    
    //定义，创建画笔
    Paint paint = new Paint();
    public DrawView (Context context) {
        super(context);
    }
    
    public DrawView(Context context,AttributeSet set) {
        super(context,set);
    }
    
    @Override
    public void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //设置画笔颜色
        paint.setColor(Color.RED);
        //绘制一个小圆
        canvas.drawCircle(currentX, currentY, 50, paint);
    }
    
    // 为该组件的触碰时间2重写处理的方法
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        // 修改坐标
        currentX = event.getX();
        currentY = event.getY();
        

        // 通知组件，重新绘制自己
        invalidate();
        // 返回true表明该方法已经处理该事件
        return true;
    }
    
}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/root_relativeLayout_id"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="${relativePackage}.${activityClass}" >

    <com.kale.drawview.DrawView 
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>

package com.kale.drawview;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.RelativeLayout;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
/*        RelativeLayout root  = (RelativeLayout)findViewById(R.id.root_relativeLayout_id);
        
        final DrawView drawView = new DrawView(this);
        //設置組件的最大寬度
        drawView.setMinimumHeight(300);
        drawView.setMinimumWidth(500);
        root.addView(drawView);*/
    }
}