android - 如何在 android 2.2 中创建自己的下载管理器

Question

我知道我们可以在 Android 2.3 及更高版本中使用内置下载管理器，但我的应用适用于 Android 2.2 及更高版本。我的问题是如何在 android 2.2 中创建自己的下载管理器？请给我一些示例答案。

Answer 1

请给我一些示例答案。

Step1 寻找有关如何在 Android 中下载文件的示例

Step2 查找有关如何在 AsyncTask 中执行操作的示例。

步骤 3 查找有关如何在下载时显示下载进度的示例。

Step4 查找任务完成时如何发送自定义广播的示例

Step5 寻找如何在设备旋转时保持 AsysncTask 操作的示例

Step6 查找有关如何在通知中显示下载进度的示例。

下面是示例代码。

1. 使用 AsyncTask 并在对话框中显示下载进度

// 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(false);
mProgressDialog.setMax(100);
mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);

// execute this when the downloader must be fired
DownloadFile downloadFile = new DownloadFile();
downloadFile.execute("the url to the file you want to download");

The AsyncTask will look like this:

// usually, subclasses of AsyncTask are declared inside the activity class.
// that way, you can easily modify the UI thread from here
private class DownloadFile extends AsyncTask<String, Integer, String> {
    @Override
    protected String doInBackground(String... sUrl) {
        try {
            URL url = new URL(sUrl[0]);
            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(url.openStream());
            OutputStream output = new FileOutputStream("/sdcard/file_name.extension");

            byte data[] = new byte[1024];
            long total = 0;
            int count;
            while ((count = input.read(data)) != -1) {
                total += count;
                // publishing the progress....
                publishProgress((int) (total * 100 / fileLength));
                output.write(data, 0, count);
            }

            output.flush();
            output.close();
            input.close();
        } catch (Exception e) {
        }
        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();
        mProgressDialog.show();
    }

    @Override
    protected void onProgressUpdate(Integer... progress) {
        super.onProgressUpdate(progress);
        mProgressDialog.setProgress(progress[0]);
    }
}

2.从服务下载

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 允许我们从服务更新我们的线程；IntentService 是 Service 的一个子类，它产生一个线程来从那里做后台工作（你应该知道一个 Service 实际上在你的应用程序的同一个线程中运行；当你扩展 Service 时，你必须手动产生新线程来运行 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 {
            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(url.openStream());
            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);
    }
}

将服务添加到您的清单中：

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

活动将如下所示：

// 像第一个例子一样初始化进度对话框

// 这就是你触发下载器的方式

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);

这是 ResultReceiver 来玩的：

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();
            }
        }
    }
}