<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.example.paint.MainActivity" >

    <Button

        android:onClick="save"

        android:id="@+id/btn"

        android:layout_width="fill_parent"

        android:layout_height="wrap_content"

        android:text="保存" />

    <Button

        android:onClick="selectImage"

        android:id="@+id/select"

        android:layout_width="fill_parent"

        android:layout_height="wrap_content"

        android:text="选择一个图片" />

    <ImageView

        android:layout_above="@id/select"

        android:id="@+id/iv"

        android:layout_width="fill_parent"

        android:layout_height="fill_parent"

        />

</LinearLayout>

package com.example.paint;

import java.io.File;

import java.io.FileOutputStream;

import android.content.Intent;

import android.graphics.Bitmap;

import android.graphics.Bitmap.CompressFormat;

import android.graphics.Bitmap.Config;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.net.Uri;

import android.os.Bundle;

import android.os.Environment;

import android.support.v7.app.ActionBarActivity;

import android.view.MotionEvent;

import android.view.View;

import android.view.View.OnTouchListener;

import android.widget.ImageView;

import android.widget.Toast;

public class MainActivity extends ActionBarActivity {

    private ImageView iv;

    private Bitmap baseBitmap;

    private Canvas canvas; //画布

    private Paint paint; //画笔

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        paint = new Paint();

        //画笔宽度

        paint.setStrokeWidth(5);

        //画笔颜色

        paint.setColor(Color.GREEN);

        //创建一个可以修改的bitmap

        baseBitmap = Bitmap.createBitmap(320, 350, Config.ARGB_8888);

        //创建一个画布

        canvas = new Canvas(baseBitmap);

        //设置背景色

        canvas.drawColor(Color.WHITE);

        iv = (ImageView)findViewById(R.id.iv);

        //知道用户手指在平美国上移动的轨迹

        iv.setOnTouchListener(new OnTouchListener() {

            //获取手指开始位置的坐标

            int startX;

            int startY;

            @Override

            public boolean onTouch(View v, MotionEvent event) {

                switch(event.getAction())

                {

                    case MotionEvent.ACTION_DOWN: //手指第一次接触屏幕

                        startX = (int)event.getX();

                        startY = (int)event.getY();

                        break;

                    case MotionEvent.ACTION_MOVE: //手指在屏幕上移动

                        int newX = (int)event.getX();

                        int newY = (int)event.getY();

                        //画线

                        canvas.drawLine(startX, startY, newY, newY, paint);

                        //重新更新画笔开始位置

                        startX =  (int)event.getX();

                        startY = (int)event.getY();

                        iv.setImageBitmap(baseBitmap);

                        break;

                    case MotionEvent.ACTION_UP: //手指离开屏幕的动作

                        break;

                }

                //return true 表示画笔一直触动

                return true;

            }

        });

    }

    public void selectImage(View view)

    {

        Intent intent = new Intent();

        intent.setAction(Intent.ACTION_PICK);

        intent.setType("image/*");

        startActivityForResult(intent, 0);

    }

    @Override

    protected void onActivityResult(int arg0, int arg1, Intent data) {

        if(data!=null)

        {

            Uri uri = data.getData();

            Bitmap bitmap = data.getParcelableExtra("data");

            baseBitmap = bitmap;

            iv.setImageURI(uri);

        }

        super.onActivityResult(arg0, arg1, data);

    }

    public void save(View view)

    {

        try {

            File file = new File(Environment.getExternalStorageDirectory(),System.currentTimeMillis()+".jpg");

            FileOutputStream fos = new FileOutputStream(file);

            //保存图片

            baseBitmap.compress(CompressFormat.JPEG, 100, fos);

            Toast.makeText(this, "保存图片成功", 0).show();

            //模拟一个消息通知系统sd卡被重新挂载

            Intent intent = new Intent();

            intent.setAction(Intent.ACTION_MEDIA_MOUNTED);

            intent.setData(Uri.fromFile(Environment.getExternalStorageDirectory()));

            sendBroadcast(intent);

        } catch (Exception e) {

            Toast.makeText(this, "保存图片失败", 0).show();

            e.printStackTrace();

        }

    }

}