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

 1 // declare the dialog as a member field of your activity
 2 ProgressDialog mProgressDialog;
 3 // instantiate it within the onCreate method
 4 mProgressDialog = new ProgressDialog(YourActivity.this);
 5 mProgressDialog.setMessage("A message");
 6 mProgressDialog.setIndeterminate(true);
 7 mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
 8 mProgressDialog.setCancelable(true);
 9 // execute this when the downloader must be fired
10 final DownloadTask downloadTask = new DownloadTask(YourActivity.this);
11 downloadTask.execute("the url to the file you want to download");
12 mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
13     @Override
14     public void onCancel(DialogInterface dialog) {
15         downloadTask.cancel(true);
16     }
17 });

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。

 1 // usually, subclasses of AsyncTask are declared inside the activity class.
 2 // that way, you can easily modify the UI thread from here
 3 private class DownloadTask extends AsyncTask<String, Integer, String> {
 4     private Context context;
 5     private PowerManager.WakeLock mWakeLock;
 6     public DownloadTask(Context context) {
 7         this.context = context;
 8     }
 9     @Override
10     protected String doInBackground(String... sUrl) {
11         InputStream input = null;
12         OutputStream output = null;
13         HttpURLConnection connection = null;
14         try {
15             URL url = new URL(sUrl[0]);
16             connection = (HttpURLConnection) url.openConnection();
17             connection.connect();
18             // expect HTTP 200 OK, so we don't mistakenly save error report
19             // instead of the file
20             if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
21                 return "Server returned HTTP " + connection.getResponseCode()
22                         + " " + connection.getResponseMessage();
23             }
24             // this will be useful to display download percentage
25             // might be -1: server did not report the length
26             int fileLength = connection.getContentLength();
27             // download the file
28             input = connection.getInputStream();
29             output = new FileOutputStream("/sdcard/file_name.extension");
30             byte data[] = new byte[4096];
31             long total = 0;
32             int count;
33             while ((count = input.read(data)) != -1) {
34                 // allow canceling with back button
35                 if (isCancelled()) {
36                     input.close();
37                     return null;
38                 }
39                 total += count;
40                 // publishing the progress....
41                 if (fileLength > 0) // only if total length is known
42                     publishProgress((int) (total * 100 / fileLength));
43                 output.write(data, 0, count);
44             }
45         } catch (Exception e) {
46             return e.toString();
47         } finally {
48             try {
49                 if (output != null)
50                     output.close();
51                 if (input != null)
52                     input.close();
53             } catch (IOException ignored) {
54             }
55             if (connection != null)
56                 connection.disconnect();
57         }
58         return null;
59     }

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：

 1 @Override
 2 protected void onPreExecute() {
 3     super.onPreExecute();
 4     // take CPU lock to prevent CPU from going off if the user
 5     // presses the power button during download
 6     PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
 7     mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
 8          getClass().getName());
 9     mWakeLock.acquire();
10     mProgressDialog.show();
11 }
12 @Override
13 protected void onProgressUpdate(Integer... progress) {
14     super.onProgressUpdate(progress);
15     // if we get here, length is known, now set indeterminate to false
16     mProgressDialog.setIndeterminate(false);
17     mProgressDialog.setMax(100);
18     mProgressDialog.setProgress(progress[0]);
19 }
20 @Override
21 protected void onPostExecute(String result) {
22     mWakeLock.release();
23     mProgressDialog.dismiss();
24     if (result != null)
25         Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();
26     else
27         Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();
28 }

注意需要添加如下权限：

1 <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）。

 1 public class DownloadService extends IntentService {
 2     public static final int UPDATE_PROGRESS = 8344;
 3     public DownloadService() {
 4         super("DownloadService");
 5     }
 6     @Override
 7     protected void onHandleIntent(Intent intent) {
 8         String urlToDownload = intent.getStringExtra("url");
 9         ResultReceiver receiver = (ResultReceiver) intent.getParcelableExtra("receiver");
10         try {
11             URL url = new URL(urlToDownload);
12             URLConnection connection = url.openConnection();
13             connection.connect();
14             // this will be useful so that you can show a typical 0-100% progress bar
15             int fileLength = connection.getContentLength();
16             // download the file
17             InputStream input = new BufferedInputStream(connection.getInputStream());
18             OutputStream output = new FileOutputStream("/sdcard/BarcodeScanner-debug.apk");
19             byte data[] = new byte[1024];
20             long total = 0;
21             int count;
22             while ((count = input.read(data)) != -1) {
23                 total += count;
24                 // publishing the progress....
25                 Bundle resultData = new Bundle();
26                 resultData.putInt("progress" ,(int) (total * 100 / fileLength));
27                 receiver.send(UPDATE_PROGRESS, resultData);
28                 output.write(data, 0, count);
29             }
30             output.flush();
31             output.close();
32             input.close();
33         } catch (IOException e) {
34             e.printStackTrace();
35         }
36         Bundle resultData = new Bundle();
37         resultData.putInt("progress" ,100);
38         receiver.send(UPDATE_PROGRESS, resultData);
39     }
40 }

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

activity中这样调用DownloadService

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

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

 1 private class DownloadReceiver extends ResultReceiver{
 2     public DownloadReceiver(Handler handler) {
 3         super(handler);
 4     }
 5     @Override
 6     protected void onReceiveResult(int resultCode, Bundle resultData) {
 7         super.onReceiveResult(resultCode, resultData);
 8         if (resultCode == DownloadService.UPDATE_PROGRESS) {
 9             int progress = resultData.getInt("progress");
10             mProgressDialog.setProgress(progress);
11             if (progress == 100) {
12                 mProgressDialog.dismiss();
13             }
14         }
15     }
16 }

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：

 1 /**
 2  * @param context used to check the device version and DownloadManager information
 3  * @return true if the download manager is available
 4  */
 5 public static boolean isDownloadManagerAvailable(Context context) {
 6     try {
 7         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD) {
 8             return false;
 9         }
10         Intent intent = new Intent(Intent.ACTION_MAIN);
11         intent.addCategory(Intent.CATEGORY_LAUNCHER);
12         intent.setClassName("com.android.providers.downloads.ui", "com.android.providers.downloads.ui.DownloadList");
13         List<ResolveInfo> list = context.getPackageManager().queryIntentActivities(intent,
14                 PackageManager.MATCH_DEFAULT_ONLY);
15         return list.size() > 0;
16     } catch (Exception e) {
17         return false;
18     }
19 }

如果能，那么只需要这样就可以开始下载一个文件了：

 1 String url = "url you want to download";
 2 DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
 3 request.setDescription("Some descrition");
 4 request.setTitle("Some title");
 5 // in order for this if to run, you must use the android 3.2 to compile your app
 6 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
 7     request.allowScanningByMediaScanner();
 8     request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
 9 }
10 request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "name-of-the-file.ext");
11 // get download service and enqueue file
12 DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
13 manager.enqueue(request);