实现画图板

package com.pingyijinren.helloworld;

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

import java.util.ArrayList;
import java.util.Iterator;

/**
 * Created by Administrator on 2016/6/2 0002.
 */
public class MyPaintView extends View {
    private ArrayList<Point> allPoints=new ArrayList<Point>();
    public MyPaintView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setBackgroundColor(Color.WHITE);
        setOnTouchListener(new OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                Point point=new Point((int)event.getX(),(int)event.getY());
                if(event.getAction()==MotionEvent.ACTION_DOWN){
                    allPoints=new ArrayList<Point>();
                    allPoints.add(point);
                }
                else if(event.getAction()==MotionEvent.ACTION_UP){
                    allPoints.add(point);
                }
                else if(event.getAction()==MotionEvent.ACTION_MOVE){
                    allPoints.add(point);
                    MyPaintView.this.postInvalidate();
                }
                return true;
            }
        });
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint paint=new Paint();
        paint.setColor(Color.RED);
        if(allPoints.size()>1){
            Iterator<Point> iterator=allPoints.iterator();
            Point firstPoint=null;
            Point lastPoint=null;
            while(iterator.hasNext()){
                if(firstPoint==null){
                    firstPoint=iterator.next();
                }
                else{
                    if(lastPoint!=null){
                        firstPoint=lastPoint;
                    }
                    lastPoint=iterator.next();
                    canvas.drawLine(firstPoint.x,firstPoint.y,lastPoint.x,lastPoint.y,paint);
                }
            }
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    tools:context="com.pingyijinren.helloworld.MainActivity">

    <com.pingyijinren.helloworld.MyPaintView
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</LinearLayout>

package com.pingyijinren.helloworld;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}