简易的画画板的实现

* 原理：两点之间用线连接起来
* 编写步骤：
　　1. 创建背景图的副本
　　2. 设置imageview的触摸监听
　　3. 把手指按下的坐标点和手指移动的坐标点连接起来
　　4. 把修改后的副本显示在imageview

package com.example.draw_picture;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.Bitmap.CompressFormat;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.os.Bundle;
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 Activity {
   private Paint   paint;
   private Bitmap  copyBmp;
   private Canvas  canvas;
   private ImageView  iv;
   /**
    * 1. 创建背景图的副本
      2. 设置imageview的触摸监听
      3. 把手指按下的坐标点和手指移动的坐标点连接起来
      4. 把修改后的副本显示在imageview
    */
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      iv = (ImageView) findViewById(R.id.iv);
      //创建背景图的副本
      Bitmap srcBmp = BitmapFactory.decodeResource(getResources(), R.drawable.bg);
      paint = new Paint();
      copyBmp = Bitmap.createBitmap(srcBmp.getWidth(), srcBmp.getHeight(), srcBmp.getConfig());
      canvas = new Canvas(copyBmp);
      canvas.drawBitmap(srcBmp, new Matrix(), paint);
      //设置imageview的触摸监听
      iv.setOnTouchListener(new OnTouchListener() {
         private float   startX;
         private float   startY;
         @Override
         public boolean onTouch(View v, MotionEvent event) {
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN://按下
                   //获取手指按下的xy坐标点
                   startX = event.getX();
                   startY = event.getY();
                   break;
                case MotionEvent.ACTION_MOVE://移动
                   //获取手指移动的xy坐标点
                   float stopX = event.getX();
                   float stopY = event.getY();
                   //把手指按下的坐标点和手指移动的坐标点连接起来
                   canvas.drawLine(startX, startY, stopX, stopY, paint);
                   //5. 更新起始点的坐标
                   startX = stopX;
                   startY =stopY;             
                   break;
                default:
                   break;
            }

            //把修改后的副本显示在imageview
            iv.setImageBitmap(copyBmp);
            return true;
         }
      });
   }
   public void red(View v){
      paint.setColor(Color.RED);
   }
   public void green(View v){
      paint.setColor(Color.GREEN);
   }
   public void blue(View v){
      paint.setColor(Color.BLUE);
   }
   /**
    * 调整画笔的粗细
    */
   public void cx(View view){
      paint.setStrokeWidth(5);
   }
   public void save(View v){
      FileOutputStream os;
      try {
         os = new FileOutputStream(new File("/mnt/sdcard/jiezuo.jpg"));
         copyBmp.compress(CompressFormat.JPEG,100, os);
         Toast.makeText(this, "大师，您的杰作保存在：/mnt/sdcard/jiezuo.jpg", 0).show();
      } catch (FileNotFoundException e) {
         e.printStackTrace();
         Toast.makeText(this, "sorry,大师，您的杰作保存失败", 0).show();
      }
   }
}

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添加SD卡的权限:<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />