package com.pingyijinren.test;

import android.content.Context;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.util.AttributeSet;

import android.view.View;

/**

 * Created by Administrator on 2016/5/21 0021.

 */

public class DrawView extends View {

    private float x=100;

    private float y=100;

    public DrawView(Context context){

        super(context);

    }   //自定义View时，必须实现View的三个构造函数

    public DrawView(Context context, AttributeSet attrs){

        super(context,attrs);

    }

    public DrawView(Context context, AttributeSet attrs, int defStyle){

        super(context,attrs,defStyle);

    }

    @Override

    public void onDraw(Canvas canvas){  //onDraw不是必需的，按需重写即可

        super.onDraw(canvas);

        Paint paint=new Paint();

        paint.setColor(Color.RED);

        canvas.drawCircle(x,y,100,paint);

    }

    public void setX(float x){

        this.x=x;

    }

    public void setY(float y){

        this.y=y;

    }

}

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

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context="com.pingyijinren.test.MainActivity">

    <com.pingyijinren.test.DrawView

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:id="@+id/drawView"/>

</RelativeLayout>

package com.pingyijinren.test;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.util.Log;

import android.view.MotionEvent;

import android.view.View;

public class MainActivity extends AppCompatActivity {

    private DrawView drawView;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        try{

            setContentView(R.layout.activity_main);

        }

        catch(Exception e){

            e.printStackTrace();

            Log.d("MainActivity",e.getMessage());

        }

        drawView=(DrawView)findViewById(R.id.drawView);

        drawView.setOnTouchListener(new View.OnTouchListener() {    //drawView响应屏幕触摸事件

            @Override

            public boolean onTouch(View v, MotionEvent event) {

                drawView.setX(event.getX());

                drawView.setY(event.getY());

                drawView.invalidate();

                return true;

            }

        });

    }

}