我正在尝试编写一个更新的简单应用程序。为此，我需要一个简单的函数，可以下载文件并在ProgressDialog 显示当前进度 。我知道如何进行ProgressDialog ，但我不知道如何显示当前进度以及如何首先下载文件。
有很多方法可以下载文件。以下我将发布最常见的方式; 您可以自行决定哪种方法更适合您的应用。
AsyncTask并在对话框中显示下载进度此方法将允许您执行一些后台进程并同时更新 UI（在这种情况下，我们将更新进度条）。
这是一个示例代码：
// 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); //cancel the task
}
}); AsyncTask看起来像这样：
// 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;
}上面的方法（ doInBackground ）总是在后台线程上运行。你不应该在那里做任何 UI 任务。另一方面， onProgressUpdate和onPreExecute在 UI 线程上运行，因此您可以更改进度条：
@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();
}要运行此选项，您需要 WAKE_LOCK 权限。
<uses-permission android:name="android.permission.WAKE_LOCK" />这里最大的问题是： 如何从服务更新我的活动？ 。在下一个示例中，我们将使用您可能不知道的两个类： ResultReceiver和IntentService 。 ResultReceiver是允许我们从服务更新线程的那个; IntentService是Service一个子类，它产生一个线程来从那里进行后台工作（你应该知道一个Service实际上在你的 app 的同一个线程中运行; 当你扩展Service ，你必须手动生成新的线程来运行 CPU 阻塞操作） 。
下载服务可能如下所示：
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 {
//create url and connect
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());
String path = "/sdcard/BarcodeScanner-debug.apk" ;
OutputStream output = new FileOutputStream(path);
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);
}
// close streams
output.flush();
output.close();
input.close();
} catch (IOException e) {
e.printStackTrace();
}
Bundle resultData = new Bundle();
resultData.putInt("progress" ,100);
receiver.send(UPDATE_PROGRESS, resultData);
}
}将服务添加到清单：
<service android:name=".DownloadService"/>活动将如下所示：
// 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);以下是ResultReceiver来玩：
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"); //get the progress
dialog.setProgress(progress);
if (progress == 100) {
dialog.dismiss();
}
}
}
} Groundy是一个基本上可以帮助您在后台服务中运行代码段的库，它基于上面显示的ResultReceiver概念。此库已弃用 。这就是整个代码的样子：
您正在显示对话框的活动...
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;
}
}
};
}一个GroundyTask使用Groundy下载文件并显示正在执行：
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;
}
}
}然后将其添加到清单中：
<service android:name="com.codeslap.groundy.GroundyService"/>我想这可不容易。只需从 Github拿起最新的罐子，你就可以开始了。请记住， Groundy的主要目的是在后台服务中调用外部 REST apis，并轻松地将结果发布到 UI。如果你在你的应用程序中做了类似的事情，它可能真的很有用。
DownloadManager类（仅限GingerBread和更新版本） GingerBread 带来了一个新功能DownloadManager ，它允许您轻松下载文件并将处理线程，流等的艰苦工作委派给系统。
首先，让我们看一个实用方法：
/**
* @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) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
return true;
}
return false;
}方法的名称解释了这一切。一旦确定DownloadManager可用，您可以执行以下操作：
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);下载进度将显示在通知栏中。
第一种和第二种方法只是冰山一角。如果您希望自己的应用程序功能强大，那么您需要记住很多事情。这是一个简短的清单：
INTERNET和WRITE_EXTERNAL_STORAGE ）; 如果您想查看互联网可用性，还可以访问ACCESS_NETWORK_STATE 。 除非您需要详细控制下载过程，否则请考虑使用DownloadManager （3），因为它已经处理了上面列出的大多数项目。
但也要考虑您的需求可能会发生变化。例如， DownloadManager 没有响应缓存 。它会盲目地多次下载同一个大文件。事后没有简单的方法来解决它。如果你从一个基本的HttpURLConnection （1,2）开始，那么你只需要添加一个HttpResponseCache 。因此，学习基本标准工具的初步努力可能是一项很好的投资。
此类在 API 级别 26 中已弃用. ProgressDialog 是一个模式对话框，可防止用户与应用程序进行交互。您应该使用 ProgressBar 等进度指示器，而不是使用此类，可以将其嵌入到应用程序的 UI 中。或者，您可以使用通知来通知用户任务的进度。有关详细信息链接
如果您要从互联网上下载内容，请不要忘记为清单文件添加权限！
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.helloandroid"
android:versionCode="1"
android:versionName="1.0">
<uses-sdk android:minSdkVersion="10" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>
<uses-permission android:name="android.permission.READ_PHONE_STATE"></uses-permission>
<application
android:icon="@drawable/icon"
android:label="@string/app_name"
android:debuggable="true">
</application>
</manifest>是的，上面的代码可以正常工作。但是如果你在Asynctask onProgressUpdate中更新你的progressbar Asynctask ，你按下后退按钮或完成你的活动AsyncTask用你的 UI 失去它的跟踪。当你回到你的活动时，即使下载正在运行在后台，您将看不到进度条上的更新。因此，在OnResume()尝试运行一个类似runOnUIThread的线程， runOnUIThread包含一个计时器任务，该任务使用从AsyncTask运行背景更新的值更新您的progressbar 。
private void updateProgressBar(){
Runnable runnable = new updateProgress();
background = new Thread(runnable);
background.start();
}
public class updateProgress implements Runnable {
public void run() {
while(Thread.currentThread()==background)
//while (!Thread.currentThread().isInterrupted()) {
try {
Thread.sleep(1000);
Message msg = new Message();
progress = getProgressPercentage();
handler.sendMessage(msg);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
} catch (Exception e) {
}
}
}
private Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
progress.setProgress(msg.what);
}
};当您的活动不可见时，不要忘记销毁线程。
private void destroyRunningThreads() {
if (background != null) {
background.interrupt();
background=null;
}
}