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 }
注册DownloadService：
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);
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。
时间： 2024-10-09 06:26:05