android中如何下载文件并显示下载进度
seannell
9年前
最近开发中遇到需要下载文件的问题，对于一般的下载来说不用考虑断点续传，不用考虑多个线程，比如下载一个apk之类的，这篇文章讨论的就是这种情形。
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(); } } } }
2.1使用 Groundy library
Groundy 可以帮助你在后台service中运行一些代码，其实也是基于刚刚用到的 ResultReceiver，下面是使用Groundy的大致代码：
public class MainActivity extends Activity { private ProgressDialog mProgressDialog; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); findViewById(R.id.btn_download).setOnClickListener(new View.OnClickListener() { public void onClick(View view) { String url = ((EditText) findViewById(R.id.edit_url)).getText().toString().trim(); Bundle extras = new Bundler().add(DownloadTask.PARAM_URL, url).build(); Groundy.create(DownloadExample.this, DownloadTask.class) .receiver(mReceiver) .params(extras) .queue(); mProgressDialog = new ProgressDialog(MainActivity.this); mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL); mProgressDialog.setCancelable(false); mProgressDialog.show(); } }); } private ResultReceiver mReceiver = new ResultReceiver(new Handler()) { @Override protected void onReceiveResult(int resultCode, Bundle resultData) { super.onReceiveResult(resultCode, resultData); switch (resultCode) { case Groundy.STATUS_PROGRESS: mProgressDialog.setProgress(resultData.getInt(Groundy.KEY_PROGRESS)); break; case Groundy.STATUS_FINISHED: Toast.makeText(DownloadExample.this, R.string.file_downloaded, Toast.LENGTH_LONG); mProgressDialog.dismiss(); break; case Groundy.STATUS_ERROR: Toast.makeText(DownloadExample.this, resultData.getString(Groundy.KEY_ERROR), Toast.LENGTH_LONG).show(); mProgressDialog.dismiss(); break; } } }; } 其中GroundyTask的定义如下：
public class DownloadTask extends GroundyTask { public static final String PARAM_URL = "com.groundy.sample.param.url"; @Override protected boolean doInBackground() { try { String url = getParameters().getString(PARAM_URL); File dest = new File(getContext().getFilesDir(), new File(url).getName()); DownloadUtils.downloadFile(getContext(), url, dest, DownloadUtils.getDownloadListenerForTask(this)); return true; } catch (Exception pokemon) { return false; } } } 但是请记住要在activity中注册了相关service才行：
<service android:name="com.codeslap.groundy.GroundyService"/>
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。