8). Multitask Downloading

我们编写的是Andorid的HTTP多 线程断点下载应用程序。因为之间我们学习的学习积累，直接使用单线程下载HTTP文 件对我们来说是一件非常简单的事。那么，多线程断点下载的难点在哪里？1.多 线程下载，2.支持断点。

 

       多线程下载：
      

       如何才能从文件的指定位置处开始下载文件？（比如从50MB开始）这一点我们可以通过HTTP请 求信息头来设置，还记得HTTP请求信息头的“Range”属性吗？

 

       断点：

       首要问题（多线程下载）已经被我们解决了，支持断点下载想必大家也已经想到了。就是将下载的进度保存到文件中，但在Android中却不能这么做。通过老黎的试验，在Android平台中，我们需要向文件中写出下载的文件数据，还需要向另一 个文件中写出下载进度，这样会出错。这样会导致有一个文件的内容没有被写出。所以我们就不能以文件的方式来保存下载进度，但可以通过数据库的方式保存下载 进度。

 

       这两大问题我们已经有了解决思路，那么就开始动手编写吧！

 

       1.创建Android工程

       Project name:MulThreadDownloader

       BuildTarget:Android2.1

       Application name：多线程断点下载

       Package name:com.changcheng.download

       Create Activity:MulThreadDownloader

       Min SDK Version:7

 

       2.AndroidManifest.xml

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

<manifest xmlns:android=""

      package="com.changcheng.download"

      android:versionCode="1"

      android:versionName="1.0">

    <application android:icon="@drawable/icon" android:label="@string/app_name">

        <activity android:name=".MulThreadDownloader"

                  android:label="@string/app_name">

            <intent-filter>

                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />

            intent-filter>

        activity>

    application>

    <uses-sdk android:minSdkVersion="7" />

        

         <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>

        

         <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

        

         <uses-permission android:name="android.permission.INTERNET"/>       

manifest>

 

       3.strings.xml

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

<resources>

    <string name="hello">Hello World, DownloadActivity!string>

    <string name="app_name">多线程断点下载string>

    <string name="path">下载路径string>

    <string name="downloadbutton">下载string>

    <string name="sdcarderror">SDCard不存在或者写保护string>

resources>

 

       4.main.xml

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

<LinearLayout xmlns:android=""

    android:orientation="vertical"

    android:layout_width="fill_parent"

    android:layout_height="fill_parent"

    >

   

         <TextView 

             android:layout_width="fill_parent"

             android:layout_height="wrap_content"

             android:text="@string/path"

             />

    <EditText

             android:layout_width="fill_parent"

             android:layout_height="wrap_content"

             android:text=""

             android:id="@+id/path"

             />

        

         <Button

                   android:layout_width="wrap_content"

             android:layout_height="wrap_content"

             android:text="@string/downloadbutton"

             android:id="@+id/button"

             />

          

          <ProgressBar

                   android:layout_width="fill_parent"

             android:layout_height="20dip"

             style="?android:attr/progressBarStyleHorizontal"

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

           

       <TextView 

             android:layout_width="fill_parent"

             android:layout_height="wrap_content"

             android:gravity="center"

             android:id="@+id/resultView"

             />

LinearLayout>

 

       5.MulThreadDownloader

package com.changcheng.download;

 

import java.io.File;

import com.changcheng.net.download.DownloadProgressListener;

import com.changcheng.net.download.FileDownloader;

import com.changcheng.download.R;

import android.app.Activity;

import android.os.Bundle;

import android.os.Environment;

import android.os.Handler;

import android.os.Message;

import android.view.View;

import android.widget.Button;

import android.widget.EditText;

import android.widget.ProgressBar;

import android.widget.TextView;

import android.widget.Toast;

 

public class MulThreadDownloader extends Activity {

         private EditText pathText;

         private ProgressBar progressBar;

         private TextView resultView;

         private Handler handler = new Handler(){

                   @Override

                   public void handleMessage(Message msg) {

                            if(!Thread.currentThread().isInterrupted()){

                                     switch (msg.what) {

                                     case 1:

                                               // 获取 当前文件下载的进度

                                               int size = msg.getData().getInt("size");

                                               progressBar.setProgress(size);

                                               int result = (int)(((float)size/(float)progressBar.getMax()) * 100);

                                               resultView.setText(result+ "%");

                                               if(progressBar.getMax() == size){

                                                        Toast.makeText(MulThreadDownloader.this, "文件 下载完成", 1).show();

                                               }

                                               break;

        

                                     case -1:

                                               String error = msg.getData().getString("error");

                                               Toast.makeText(MulThreadDownloader.this, error, 1).show();

                                               break;

                                     }                          

                            }

                            super.handleMessage(msg);

                   }                 

         };

        

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        pathText = (EditText)this.findViewById(R.id.path);

        progressBar = (ProgressBar)this.findViewById(R.id.downloadbar);

        resultView = (TextView)this.findViewById(R.id.resultView);

        Button button = (Button)this.findViewById(R.id.button);

        button.setOnClickListener(new View.OnClickListener() {                           

                            @Override

                            public void onClick(View v) {

                                     String path = pathText.getText().toString();

                                     if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){

                                               //下载文件需要很长的时间，主线程是不能够长时 间被阻塞，如果主线程被长时间阻塞， 那么Android被 回收应用

                                               download(path, Environment.getExternalStorageDirectory());

                                     }else{

                                               Toast.makeText(MulThreadDownloader.this, R.string.sdcarderror, 1).show();

                                     }

                            }

                   });

    }

    /**

     * 下载 文件

     * @param path 下载路径

     * @param saveDir 文件保存目录

     */

  //对于Android的UI控 件，只能由主线程负责显示界面的更新，其他线程不能直接更新UI控 件的显示

    public void download(final String path, final File saveDir){

       new Thread(new Runnable() {                         

                            @Override

                            public void run() {

                                     FileDownloader downer = new FileDownloader(MulThreadDownloader.this, path, saveDir, 3);

                                     progressBar.setMax(downer.getFileSize());//设置进度条的最大刻度

                                     try {

                                               downer.download(new DownloadProgressListener(){

                                                        @Override

                                                        public void onDownloadSize(int size) {

                                                                 Message msg = new Message();

                                                                 msg.what = 1;

                                                                 msg.getData().putInt("size", size);

                                                                 handler.sendMessage(msg);//发送消息

                                                        }});

                                     } catch (Exception e) {

                                               Message msg = new Message();

                                               msg.what = -1;

                                               msg.getData().putString("error", "下载 失败");

                                               handler.sendMessage(msg);

                                     }

                            }

                   }).start();

    }

}