在 doInBackground 之前调用 AsyncTask onPostExecute答案

【问题标题】：AsyncTask onPostExecute called before doInBackground在 doInBackground 之前调用 AsyncTask onPostExecute
【发布时间】：2019-10-18 10:42:17
【问题描述】：

我有这个 AsyncTask 类的问题：

public class DownloadPDFTask extends AsyncTask<String, Void, File> {

    private Context mContext;
    private String mDownloadDIR = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).toString();

    public DownloadPDFTask(Context mContext) {
        this.mContext = mContext;
    }

    @Override
    protected synchronized void onPreExecute() {
        super.onPreExecute();
        removeOldFiles();
    }

    @Override
    protected synchronized File doInBackground(String... args) {
        File retFile = null;
        downloadFilePDF(args[0]);
        File directory = new File(mDownloadDIR);
        File[] files = directory.listFiles();
        for (File file : files) {
            if (file.getName().contains(".pdf")) {
                retFile = file;
                break;
            }
        }
        return retFile;

    }

    @Override
    protected synchronized void onPostExecute(File file) {
        super.onPostExecute(file);
        NotificationManager mNotificationManager = (NotificationManager) mContext.getSystemService(Context.NOTIFICATION_SERVICE);
        mNotificationManager.cancelAll();
        Intent mGoToPDFView = new Intent(mContext, PDFViewerActivity.class);
        mGoToPDFView.putExtra("FILE", file);
        mGoToPDFView.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        mContext.startActivity(mGoToPDFView);
    }

    private void downloadFilePDF(String url) {
        DownloadManager mDownloadManager = (DownloadManager) mContext.getSystemService(Context.DOWNLOAD_SERVICE);
        Uri mUri = Uri.parse(url);
        DownloadManager.Request mReq = new DownloadManager.Request(mUri);
        mReq.setTitle("Develop Test");
        mReq.setDescription("PDF downloading....");
        mReq.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE);
        mReq.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "temp.pdf");
        mReq.setMimeType("application/pdf");
        mDownloadManager.enqueue(mReq);
    }


    private void removeOldFiles() {
        File mDirectory = new File(mDownloadDIR);
        if (mDirectory.isDirectory()) {
            String[] children = mDirectory.list();
            for (String child : children) {
                if (child.contains(".pdf")) {
                    new File(mDirectory, child).delete();
                }
            }
        }
    }
}

如果我使用调试，我想清除下载文件夹，下载 PDF 和打开其他通过意图传递的文件创建的活动，然后慢慢进行过程工作，但在运行时，onPostExecute 方法在 doInBackground 方法之前调用。

你能帮帮我吗？

【问题讨论】：

"在 doInBackground 方法之前调用的 onPostExecute 方法" - 你有什么证据？ enqueue() on DownloadManager 不会立即下载任何内容，因此运行速度非常快。这里唯一需要在后台线程上的代码是删除文件，这是您专门不在后台线程上拥有的代码。

标签： java android pdf android-asynctask

【解决方案1】：

感谢@CommonsWare 提供意见！

我用这个解决方案解决了，我改变了AsyncTask：

public class DownloadPDFTask extends AsyncTask<String, Void, Void> {

    @Override
    protected synchronized void onPreExecute() {
        String mDownloadDIR = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).toString();
        super.onPreExecute();
        File mDirectory = new File(mDownloadDIR);
        if (mDirectory.isDirectory()) {
            String[] children = mDirectory.list();
            for (String child : children) {
                if (child.contains(".pdf")) {
                    new File(mDirectory, child).delete();
                }
            }
        }
    }

    @Override
    private void downloadFilePDF(String url) {
        DownloadManager mDownloadManager = (DownloadManager) mContext.getSystemService(Context.DOWNLOAD_SERVICE);
        Uri mUri = Uri.parse(url);
        DownloadManager.Request mReq = new DownloadManager.Request(mUri);
        mReq.setTitle("Develop Test");
        mReq.setDescription("PDF downloading....");
        mReq.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE);
        mReq.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "temp.pdf");
        mReq.setMimeType("application/pdf");
        mDownloadManager.enqueue(mReq);
        return null;
    }

    @Override
    protected synchronized void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);
        NotificationManager mNotificationManager = (NotificationManager) BaseApplication.getAppContext().getSystemService(Context.NOTIFICATION_SERVICE);
        mNotificationManager.cancelAll();
    }
}

我将使用 onPostExecute，但我正在等待带有 BroadcastReceiver 的 DownloadManager：

public class DownloadReceiver extends BroadcastReceiver {

    private String mDownloadDIR = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).toString();
    private Context mContext = BaseApplication.getAppContext();

    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if (DownloadManager.ACTION_DOWNLOAD_COMPLETE.equals(action)) {
            Log.i("DownloadReceiver", "DownloadCompleted!");
            File retFile = null;
            File directory = new File(mDownloadDIR);
            File[] files = directory.listFiles();
            for (File file : files) {
                Log.i("DownloadReceiver", "Searching file in DOWNLOADS");
                if (file.getName().contains(".pdf")) {
                    Log.i("DownloadReceiver", "File found!");
                    retFile = file;
                    break;
                }
            }

            if (retFile != null) {
                Log.i("DownloadReceiver", "Go to PDF viewing...");
                Intent mGoToPDFView = new Intent(mContext, PDFViewerActivity.class);
                mGoToPDFView.putExtra("FILE", retFile);
                mGoToPDFView.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                mContext.startActivity(mGoToPDFView);
            }

        }
    }
}

在 AndroidManifest.xml 更新之后……它可以工作了！

【讨论】：