Android 自定义view实例：跟踪手指小球

package com.example.test;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

public class DrawViewActivity extends Activity {

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

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}

}

package com.ui;

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;

Paint p = new Paint();
public DrawView(Context context){
super(context);
}
public DrawView(Context context, AttributeSet set)
{
super(context,set);
}

public void onDraw(Canvas canvas)
{
super.onDraw(canvas);

p.setColor(Color.BLACK);
canvas.drawCircle(currentX, currentY, 15, p);
}

public boolean onTouchEvent(MotionEvent event)
{
currentX = event.getX();
currentY = event.getY();
invalidate();
return true;
}
}

<?xml version="1.0" encoding="utf-8"?>