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Merge pull request giantray#15 from troyliu0105/master
翻译完成(Download_a_file_with_Android_and_showing_the_progress_in_a_ProgressDialog.md)
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‎README.md‎

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@@ -53,6 +53,10 @@ stackoverflow-Java-top-qa
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* [如何测试 private 方法,变量或者内部类](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/How_to_test_a_class_that_has_private_methods,_fields_or_inner_classes.md)
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> Android
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* [在Android里面下载文件,并在ProgressDialog显示进度](https://github.com/troyliu0105/stackoverflow-java-top-qa/blob/master/contents/Download_a_file_with_Android_and_showing_the_progress_in_a_ProgressDialog.md)
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### 待翻译问题链接(还剩x问题)
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- [Why is subtracting these two times (in 1927) giving a strange result?](http://stackoverflow.com/questions/6841333/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result)
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- [Proper use cases for Android UserManager.isUserAGoat()?](http://stackoverflow.com/questions/13375357/proper-use-cases-for-android-usermanager-isuseragoat)
@@ -90,7 +94,6 @@ stackoverflow-Java-top-qa
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- [How can I Initialize a static Map?](http://stackoverflow.com/questions/507602/how-can-i-initialize-a-static-map)
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- [How do I discover memory usage of my application in Android?](http://stackoverflow.com/questions/2298208/how-do-i-discover-memory-usage-of-my-application-in-android)
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- [How can I generate an MD5 hash?](http://stackoverflow.com/questions/415953/how-can-i-generate-an-md5-hash)
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- [Download a file with Android, and showing the progress in a ProgressDialog](http://stackoverflow.com/questions/3028306/download-a-file-with-android-and-showing-the-progress-in-a-progressdialog)
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- [Uncatchable ChuckNorrisException](http://stackoverflow.com/questions/13883166/uncatchable-chucknorrisexception)
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- [Can I add jars to maven 2 build classpath without installing them?](http://stackoverflow.com/questions/364114/can-i-add-jars-to-maven-2-build-classpath-without-installing-them)
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- [Update Eclipse with Android development tools v. 23](http://stackoverflow.com/questions/24437564/update-eclipse-with-android-development-tools-v-23)
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## 在Android里面下载文件,并在ProgressDialog显示进度
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### 问题
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尝试写一个可以获得更新的应用程序。 为了达到这个效果,我写了一个可以下载文件并且在一个`ProgressDialog`里面显示进度的简单方法。我知道怎么使用`ProgressDialog`,但是我不太确定怎么显示当前进度和下载文件。
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### 回答
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有很多方式去下载文件。我给出一些最常用的方法;由你来选择选择哪一个最适合你的应用。
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#### 1. 使用AsyncTask,并且在一个dialog里面显示进度
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这种方法允许你执行一些后台任务,并且同时更新UI(在这里,我们是更新进度条progress bar)。
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首先是实例代码
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```java
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// 定义一个dialog为Activity的成员变量
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ProgressDialog mProgressDialog;
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// 在OnCreate()方法里面初始化
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mProgressDialog = new ProgressDialog(YourActivity.this);
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mProgressDialog.setMessage("A message");
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mProgressDialog.setIndeterminate(true);
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mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
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mProgressDialog.setCancelable(true);
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// 执行下载器
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final DownloadTask downloadTask = new DownloadTask(YourActivity.this);
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downloadTask.execute("你要下载文件的Url");
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mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
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@Override
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public void onCancel(DialogInterface dialog) {
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downloadTask.cancel(true);
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}
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});
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```
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`AsyncTask`看起来像这样
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```java
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// 一般我们把AsyncTask的子类定义在Activity的内部
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// 通过这种方式,我们就可以轻松地在这里更改UI线程
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private class DownloadTask extends AsyncTask<String, Integer, String> {
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private Context context;
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private PowerManager.WakeLock mWakeLock;
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public DownloadTask(Context context) {
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this.context = context;
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}
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@Override
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protected String doInBackground(String... sUrl) {
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InputStream input = null;
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OutputStream output = null;
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HttpURLConnection connection = null;
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try {
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URL url = new URL(sUrl[0]);
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connection = (HttpURLConnection) url.openConnection();
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connection.connect();
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// 避免因为接收到非HTTP 200 OK状态,而导致只或者错误代码,而不是要下载的文件
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if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
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return "Server returned HTTP " + connection.getResponseCode()
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+ " " + connection.getResponseMessage();
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}
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// 这对显示下载百分比有帮助
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// 当服务器没有返回文件的大小时,数字可能为-1
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int fileLength = connection.getContentLength();
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// 下载文件
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input = connection.getInputStream();
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output = new FileOutputStream("/sdcard/file_name.extension");
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byte data[] = new byte[4096];
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long total = 0;
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int count;
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while ((count = input.read(data)) != -1) {
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// 允许用返回键取消下载
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if (isCancelled()) {
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input.close();
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return null;
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}
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total += count;
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// 更新下载进度
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if (fileLength > 0) // 只有当 fileLength>0 的时候才会调用
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publishProgress((int) (total * 100 / fileLength));
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output.write(data, 0, count);
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}
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} catch (Exception e) {
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return e.toString();
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} finally {
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try {
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if (output != null)
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output.close();
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if (input != null)
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input.close();
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} catch (IOException ignored) {
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}
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if (connection != null)
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connection.disconnect();
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}
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return null;
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}
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```
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上面的`doInBackground`方法总是在后台线程中运行。你不能在这里做任何UI线程相关的任务。另一方面,`onProgressUpdate`和`onPreExecute`是在UI线程里面运行的,所以你可以在这里更改进度条。
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```java
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@Override
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protected void onPreExecute() {
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super.onPreExecute();
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// 取得CPU锁,避免因为用户在下载过程中按了电源键而导致的失效
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PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
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mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
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getClass().getName());
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mWakeLock.acquire();
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mProgressDialog.show();
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}
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@Override
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protected void onProgressUpdate(Integer... progress) {
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super.onProgressUpdate(progress);
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// 如果到了这里,文件长度是确定的,设置indeterminate为false
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mProgressDialog.setIndeterminate(false);
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mProgressDialog.setMax(100);
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mProgressDialog.setProgress(progress[0]);
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}
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@Override
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protected void onPostExecute(String result) {
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mWakeLock.release();
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mProgressDialog.dismiss();
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if (result != null)
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Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();
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else
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Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();
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}
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```
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为了可正常运行,你还要取得WAKE_LOCK权限
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```
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<uses-permission android:name="android.permission.WAKE_LOCK" />
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```
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#### 2. 从服务器上下载文件
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这里有个最大的问题:*我怎么从service来更新我的activity?*。
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在下一个例子当中我们会使用两个你可能不熟悉的类:`ResultReceiver`和`IntentService`。`ResultReceiver`是一个可以允许我们用Service来更新线程的类;`IntentService`是一个可以生成用来处理后台任务的线程的`Service`子类(你需要知道,`Service`实际上是和你的应用运行在同一个线程的;当你继承了`Service`之后,你必须手动生成一个新的线程来处理费时操作)。
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一个提供下载功能的`Service`看起来像这样:
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```java
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public class DownloadService extends IntentService {
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public static final int UPDATE_PROGRESS = 8344;
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public DownloadService() {
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super("DownloadService");
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}
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@Override
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protected void onHandleIntent(Intent intent) {
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String urlToDownload = intent.getStringExtra("url");
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ResultReceiver receiver = (ResultReceiver) intent.getParcelableExtra("receiver");
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try {
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URL url = new URL(urlToDownload);
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URLConnection connection = url.openConnection();
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connection.connect();
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// 这对你在进度条上面显示百分比很有用
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int fileLength = connection.getContentLength();
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// download the file
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InputStream input = new BufferedInputStream(connection.getInputStream());
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OutputStream output = new FileOutputStream("/sdcard/BarcodeScanner-debug.apk");
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byte data[] = new byte[1024];
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long total = 0;
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int count;
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while ((count = input.read(data)) != -1) {
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total += count;
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// 更新进度条....
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Bundle resultData = new Bundle();
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resultData.putInt("progress" ,(int) (total * 100 / fileLength));
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receiver.send(UPDATE_PROGRESS, resultData);
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output.write(data, 0, count);
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}
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output.flush();
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output.close();
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input.close();
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} catch (IOException e) {
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e.printStackTrace();
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}
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Bundle resultData = new Bundle();
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resultData.putInt("progress" ,100);
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receiver.send(UPDATE_PROGRESS, resultData);
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}
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}
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```
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把这个`Service`添加到清单文件中:
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```
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<service android:name=".DownloadService"/>
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```
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activity里面的代码:
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```java
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// 像第一个例子里面一样初始化ProgressBar
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// 在这里启动下载
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mProgressDialog.show();
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Intent intent = new Intent(this, DownloadService.class);
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intent.putExtra("url", "url of the file to download");
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intent.putExtra("receiver", new DownloadReceiver(new Handler()));
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startService(intent);
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```
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然后像这样来使用`ResultReceiver`:
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```java
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private class DownloadReceiver extends ResultReceiver{
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public DownloadReceiver(Handler handler) {
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super(handler);
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}
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@Override
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protected void onReceiveResult(int resultCode, Bundle resultData) {
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super.onReceiveResult(resultCode, resultData);
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if (resultCode == DownloadService.UPDATE_PROGRESS) {
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int progress = resultData.getInt("progress");
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mProgressDialog.setProgress(progress);
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if (progress == 100) {
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mProgressDialog.dismiss();
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}
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}
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}
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}
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```
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##### 2.1 使用Groundy库
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[Groundy](http://casidiablo.github.com/groundy)是一个可以帮助你在后台服务中运行代码片段的库,它是基于`ResultReceiver`这一概念。但是这个库现在已经被标记为**过时**了(deprecated)。下面是**完整**代码的样子。
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你要展示dialog的Activity:
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```java
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public class MainActivity extends Activity {
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private ProgressDialog mProgressDialog;
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@Override
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public void onCreate(Bundle savedInstanceState) {
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super.onCreate(savedInstanceState);
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setContentView(R.layout.main);
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findViewById(R.id.btn_download).setOnClickListener(new View.OnClickListener() {
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public void onClick(View view) {
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String url = ((EditText) findViewById(R.id.edit_url)).getText().toString().trim();
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Bundle extras = new Bundler().add(DownloadTask.PARAM_URL, url).build();
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Groundy.create(DownloadExample.this, DownloadTask.class)
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.receiver(mReceiver)
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.params(extras)
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.queue();
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mProgressDialog = new ProgressDialog(MainActivity.this);
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mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
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mProgressDialog.setCancelable(false);
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mProgressDialog.show();
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}
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});
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}
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private ResultReceiver mReceiver = new ResultReceiver(new Handler()) {
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@Override
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protected void onReceiveResult(int resultCode, Bundle resultData) {
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super.onReceiveResult(resultCode, resultData);
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switch (resultCode) {
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case Groundy.STATUS_PROGRESS:
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mProgressDialog.setProgress(resultData.getInt(Groundy.KEY_PROGRESS));
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break;
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case Groundy.STATUS_FINISHED:
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Toast.makeText(DownloadExample.this, R.string.file_downloaded, Toast.LENGTH_LONG);
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mProgressDialog.dismiss();
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break;
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case Groundy.STATUS_ERROR:
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Toast.makeText(DownloadExample.this, resultData.getString(Groundy.KEY_ERROR), Toast.LENGTH_LONG).show();
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mProgressDialog.dismiss();
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break;
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}
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}
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};
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}
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```
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**Groundy**使用一个`GroundyTask`的实现类来下载文件和显示进度:
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```java
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public class DownloadTask extends GroundyTask {
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public static final String PARAM_URL = "com.groundy.sample.param.url";
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@Override
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protected boolean doInBackground() {
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try {
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String url = getParameters().getString(PARAM_URL);
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File dest = new File(getContext().getFilesDir(), new File(url).getName());
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DownloadUtils.downloadFile(getContext(), url, dest, DownloadUtils.getDownloadListenerForTask(this));
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return true;
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} catch (Exception pokemon) {
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return false;
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}
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}
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}
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```
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添加这行代码到清单文件中:
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```
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<service android:name="com.codeslap.groundy.GroundyService"/>
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```
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这实在是太简单了!只需要从[Github](https://github.com/casidiablo/groundy/downloads)上下载最新的jar文件就可以开始了。但是要记住,Groundy的主要用途是在后台服务中调用外部的REST API,然后更简单地在UI上更新结果。如果你要在你的应用里面做类似的事情,这个库将非常有帮助。
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##### 2.2 使用[ion](https://github.com/koush/ion)
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#### 3. 使用`DownloadManager`类(只适用于GingerBread及其以上的系统)
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这个方法很酷炫,你不需要担心手动下载文件、处理线程和流之类等乱七八糟的东西。GingerBread带来一项新功能:`DownloadManager`。`DownloadManager`允许你轻松地下载文件和把复杂计算的任务委托给系统。
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首先,我们来看一下工具方法:
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```java
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/**
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* @param 使用context来检查设备信息和 DownloadManager 的信息
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* @return 如果downloadmanager可用则返回 true
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*/
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public static boolean isDownloadManagerAvailable(Context context) {
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if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
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return true;
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}
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return false;
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}
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```
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方法的名字就已经告诉了我们一切,只有当你确保可以使用`DownloadManager`的时候,你才可以做下面的事情:
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```java
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String url = "url you want to download";
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DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
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request.setDescription("Some descrition");
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request.setTitle("Some title");
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// in order for this if to run, you must use the android 3.2 to compile your app
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// 为了保证这个if语句会运行,你必须使用android 3.2来编译 (译者注:应该是大于android 3.2的版本)
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if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
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request.allowScanningByMediaScanner();
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request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
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}
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request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "name-of-the-file.ext");
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// 获得下载服务和队列文件
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DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
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manager.enqueue(request);
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```
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#### 最后的一些思考
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第一个和第二个方法只是冰山一角。如果你想你的应用更加健壮,你得留意许多事情。这里是一些建议:
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+ 你必须检查用户是否有Internet连接。
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+ 确保你有正确的权限(`Internet`和`WRITE_EXTERNAL_STORAGE`),如果要检查网络可用性,你还需要`ACCESS_NETWORK_STATE`权限。
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+ 确保你要保存下载文件的目录存在,并且有相应的写入权限。
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+ 如果下载的文件太大,你可能需要实现一种方法来确保上次的请求失败后,可以接着从来。
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+ 如果可以有暂停或者取消下载的选项,用户会很感激你的!
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除非你想对下载过程有绝对的控制权,否则我强烈推荐你使用`DownloadManager`。因为他已经处理好了上面的大部分建议。

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