Android中如何下载文件并显示下载进度

原文地址：http://jcodecraeer.com/a/anzhuokaifa/androidkaifa/2014/1125/2057.html

这里主要讨论三种方式：AsyncTask、Service和使用DownloadManager。

一、使用AsyncTask并在进度对话框中显示下载进度

这种方式的优势是你可以在后台执行下载任务的同时，也可以更新UI（这里我们用progress bar来更新下载进度）

下面的代码是使用的例子

 // declare the dialog as a member field of your activity
 ProgressDialog mProgressDialog;
 // instantiate it within the onCreate method
 mProgressDialog = new ProgressDialog(YourActivity.this);
 mProgressDialog.setMessage("A message");
 mProgressDialog.setIndeterminate(true);
 mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
 mProgressDialog.setCancelable(true);
 // execute this when the downloader must be fired
 final DownloadTask downloadTask = new DownloadTask(YourActivity.this);
 downloadTask.execute("the url to the file you want to download");
 mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
     @Override
     public void onCancel(DialogInterface dialog) {
         downloadTask.cancel(true);
     }
 });

DownloadTask继承自AsyncTask，按照如下框架定义，你需要将代码中的某些参数替换成你自己的。

 // usually, subclasses of AsyncTask are declared inside the activity class.
 // that way, you can easily modify the UI thread from here
 private class DownloadTask extends AsyncTask<String, Integer, String> {
     private Context context;
     private PowerManager.WakeLock mWakeLock;
     public DownloadTask(Context context) {
         this.context = context;
     }
     @Override
     protected String doInBackground(String... sUrl) {
         InputStream input = null;
         OutputStream output = null;
         HttpURLConnection connection = null;
         try {
             URL url = new URL(sUrl[0]);
             connection = (HttpURLConnection) url.openConnection();
             connection.connect();
             // expect HTTP 200 OK, so we don't mistakenly save error report
             // instead of the file
             if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
                 return "Server returned HTTP " + connection.getResponseCode()
                         + " " + connection.getResponseMessage();
             }
             // this will be useful to display download percentage
             // might be -1: server did not report the length
             int fileLength = connection.getContentLength();
             // download the file
             input = connection.getInputStream();
             output = new FileOutputStream("/sdcard/file_name.extension");
             byte data[] = new byte[4096];
             long total = 0;
             int count;
             while ((count = input.read(data)) != -1) {
                 // allow canceling with back button
                 if (isCancelled()) {
                     input.close();
                     return null;
                 }
                 total += count;
                 // publishing the progress....
                 if (fileLength > 0) // only if total length is known
                     publishProgress((int) (total * 100 / fileLength));
                 output.write(data, 0, count);
             }
         } catch (Exception e) {
             return e.toString();
         } finally {
             try {
                 if (output != null)
                     output.close();
                 if (input != null)
                     input.close();
             } catch (IOException ignored) {
             }
             if (connection != null)
                 connection.disconnect();
         }
         return null;
     }

上面的代码只包含了doInBackground，这是执行后台任务的代码块，不能在这里做任何的UI操作，但是onProgressUpdate和onPreExecute是运行在UI线程中的，所以我们应该在这两个方法中更新progress bar。

接上面的代码：

 @Override
 protected void onPreExecute() {
     super.onPreExecute();
     // take CPU lock to prevent CPU from going off if the user
     // presses the power button during download
     PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
     mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
          getClass().getName());
     mWakeLock.acquire();
     mProgressDialog.show();
 }
 @Override
 protected void onProgressUpdate(Integer... progress) {
     super.onProgressUpdate(progress);
     // if we get here, length is known, now set indeterminate to false
     mProgressDialog.setIndeterminate(false);
     mProgressDialog.setMax(100);
     mProgressDialog.setProgress(progress[0]);
 }
 @Override
 protected void onPostExecute(String result) {
     mWakeLock.release();
     mProgressDialog.dismiss();
     if (result != null)
         Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();
     else
         Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();
 }

注意需要添加如下权限：

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

二、在service中执行下载

在service中执行下载任务的麻烦之处在于如何通知activity更新UI。下面的代码中我们将用ResultReceiver和IntentService来实现下载。ResultReceiver允许我们接收来自service中发出的广播，IntentService继承自service，这IntentService中我们开启一个线程开执行下载任务（service和你的app其实是在一个线程中，因此不想阻塞主线程的话必须开启新的线程）。

 public class DownloadService extends IntentService {
     public static final int UPDATE_PROGRESS = 8344;
     public DownloadService() {
         super("DownloadService");
     }
     @Override
     protected void onHandleIntent(Intent intent) {
         String urlToDownload = intent.getStringExtra("url");
         ResultReceiver receiver = (ResultReceiver) intent.getParcelableExtra("receiver");
         try {
             URL url = new URL(urlToDownload);
             URLConnection connection = url.openConnection();
             connection.connect();
             // this will be useful so that you can show a typical 0-100% progress bar
             int fileLength = connection.getContentLength();
             // download the file
             InputStream input = new BufferedInputStream(connection.getInputStream());
             OutputStream output = new FileOutputStream("/sdcard/BarcodeScanner-debug.apk");
             byte data[] = new byte[1024];
             long total = 0;
             int count;
             while ((count = input.read(data)) != -1) {
                 total += count;
                 // publishing the progress....
                 Bundle resultData = new Bundle();
                 resultData.putInt("progress" ,(int) (total * 100 / fileLength));
                 receiver.send(UPDATE_PROGRESS, resultData);
                 output.write(data, 0, count);
             }
             output.flush();
             output.close();
             input.close();
         } catch (IOException e) {
             e.printStackTrace();
         }
         Bundle resultData = new Bundle();
         resultData.putInt("progress" ,100);
         receiver.send(UPDATE_PROGRESS, resultData);
     }
 }

注册DownloadService：

 <service android:name=".DownloadService"/>

activity中这样调用DownloadService

 // initialize the progress dialog like in the first example
 // this is how you fire the downloader
 mProgressDialog.show();
 Intent intent = new Intent(this, DownloadService.class);
 intent.putExtra("url", "url of the file to download");
 intent.putExtra("receiver", new DownloadReceiver(new Handler()));
 startService(intent);

使用ResultReceiver接收来自DownloadService的下载进度通知

 private class DownloadReceiver extends ResultReceiver{
     public DownloadReceiver(Handler handler) {
         super(handler);
     }
     @Override
     protected void onReceiveResult(int resultCode, Bundle resultData) {
         super.onReceiveResult(resultCode, resultData);
         if (resultCode == DownloadService.UPDATE_PROGRESS) {
             int progress = resultData.getInt("progress");
             mProgressDialog.setProgress(progress);
             if (progress == 100) {
                 mProgressDialog.dismiss();
             }
         }
     }
 }

三、使用DownloadManager

其实这才是解决下载问题的终极方法，因为他使用起来实在是太简单了。可惜只有在GingerBread 之后才能使用。

先判断能不能使用DownloadManager：

 /**
  * @param context used to check the device version and DownloadManager information
  * @return true if the download manager is available
  */
 public static boolean isDownloadManagerAvailable(Context context) {
     try {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD) {
             return false;
         }
         Intent intent = new Intent(Intent.ACTION_MAIN);
         intent.addCategory(Intent.CATEGORY_LAUNCHER);
         intent.setClassName("com.android.providers.downloads.ui", "com.android.providers.downloads.ui.DownloadList");
         List<ResolveInfo> list = context.getPackageManager().queryIntentActivities(intent,
                 PackageManager.MATCH_DEFAULT_ONLY);
         return list.size() > 0;
     } catch (Exception e) {
         return false;
     }
 }

如果能，那么只需要这样就可以开始下载一个文件了：

 String url = "url you want to download";
 DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
 request.setDescription("Some descrition");
 request.setTitle("Some title");
 // in order for this if to run, you must use the android 3.2 to compile your app
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
     request.allowScanningByMediaScanner();
     request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
 }
 request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "name-of-the-file.ext");
 // get download service and enqueue file
 DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
 manager.enqueue(request);

下载的进度会在消息通知中显示。

总结

前两种方法需要你考虑的东西很多，除非是你想完全控制下载的整个过程，否则用最后一种比较省事。

Demo下载