Android 自定义组件随着手指自动画圆

package com.example.androidtest0.myView;

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 = 40;
    public float currentY = 50;
    //定义、并创建画笔
    Paint p = new Paint();
    public DrawView(Context context) {
        super(context);
    }

    public DrawView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //设置画笔的颜色
        p.setColor(Color.RED);
        //绘制一个小球
        canvas.drawCircle(currentX, currentY, 15, p);
    }
    
    /**
     * 为该组件的触碰事件重写事件处理方法
     */
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        //修改currentX、currentY两个属性
        currentX = event.getX();
        currentY = event.getY();
        //通知当前组件重绘自己
        invalidate();
        return true;
    }
}

<pre name="code" class="java"><?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/root"
    android:orientation="vertical" >

</LinearLayout>

package com.example.androidtest0;

import com.example.androidtest0.myView.DrawView;

import android.app.Activity;
import android.os.Bundle;
import android.widget.LinearLayout;

public class CustomView extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.custom_layout);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/root"
    android:orientation="vertical" >

    <com.example.androidtest0.myView.DrawView 
        android:layout_width="match_parent" android:layout_height="match_parent"
        />
</LinearLayout>

package com.example.androidtest0;

import com.example.androidtest0.myView.DrawView;

import android.app.Activity;
import android.os.Bundle;
import android.widget.LinearLayout;

public class CustomView extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.custom_layout);
    }
}