如何在Android下载pdf文件？

Question

我想从网址下载pdf文件.为了查看pdf文件,我使用了以下代码.

File file = new File("/sdcard/example.pdf");

if (file.exists()) {
    Uri path = Uri.fromFile(file);
    Intent intent = new Intent(Intent.ACTION_VIEW);
    intent.setDataAndType(path, "application/pdf");
    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

    try {
        startActivity(intent);
    } 
    catch (ActivityNotFoundException e) {
        Toast.makeText(OpenPdf.this, "No Application Available to View PDF",
            Toast.LENGTH_SHORT).show();
    }
}

它正在工作,但我如何从URL(例如http://.../example.pdf)获取pdf文件.我想从这个网址下载 pdf文件.请帮我.提前致谢.

Answer 1

下载pdf:

 startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("www.education.gov.yk.ca/pdf/pdf-test.pdf")));

啊,因为我刚刚发现这取决于设备.

场景

1. 它会将pdf下载到您的浏览器/下载/文件夹中

2. 您有一个Google文档帐户 - 它会要求您登录,然后在浏览器中查看pdf

3. 您安装了pdf阅读器 - 依赖App依赖可能没有

在所有情况下,尽管用户可以使用一行代码访问PDF :-)

Answer 2

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.

1. AsyncTask在对话框中使用并显示下载进度

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);
    }
});

该AsyncTask会是这样的:

// 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;
    }

上面的方法(doInBackground)始终在后台线程上运行.你不应该在那里做任何UI任务.另一方面,onProgressUpdate并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" />

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？.在下一个例子中,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我们的线程的那个; IntentService是一个子类,Service它产生一个线程从那里做后台工作(你应该知道一个Service实际运行在你的应用程序的同一个线程;当你扩展时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 {
            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);
    }
}

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发挥作用:

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库

Groundy是一个基本上可以帮助您在后台服务中运行代码的库,它基于ResultReceiver上面显示的概念.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使用实施Groundy下载文件,并显示进度:

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.

2.2使用https://github.com/koush/ion

3.使用DownloadManager类(GingerBread仅限更新)

GingerBread带来了一项新功能,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.

最后的想法

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:

• 您必须检查用户是否有可用的Internet连接
• 确保您拥有正确的权限(INTERNET和WRITE_EXTERNAL_STORAGE); 同样ACCESS_NETWORK_STATE,如果你想检查网络可用性.
• 确保目录是否要下载存在的文件并具有写入权限.
• 如果下载太大,您可能希望实现一种方法,以便在以前的尝试失败时恢复下载.
• 如果您允许用户中断下载,用户将不胜感激.

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.

Answer 3

下载PDF与下载任何其他二进制文件的工作方式相同.

1. 打开一个 HttpUrlConnection
2. 使用连接的getInputStream()方法读取文件.
3. 创建FileOutputStream并编写输入流.

查看此帖子以获取示例源代码.