Question

我的代码一直都是非常奇怪的行为。

基本上，我的代码使用输入/输出流从Internet下载.pdf文件，将其保存到内部存储（使用AsyncTask），然后使用外部“showPdf”库输出。

最奇怪的是它只适用于两个条件：

我运行代码两次（运行或调试没有任何断点）。调用showPdf（）时，第一次运行日志 文件为空 ，但是当自己调用showPdf（）时，第二次运行完全正常运行。
我调试代码并逐步完成程序

作为序言，我是java和android studio的新手，所以我的猜测可能根本不对，但我猜测因为InputStream是“异步”完成的，所以showPdf（）可能在之前调用byte []数组被写入内存。如果是这种情况，我该怎么做才能将Async延迟足够长时间才能工作？

public class pdfView extends AppCompatActivity {
    PDFView pdfView; //pdfView object
    String URL;
    String fileName;
    File directory; //path of created File
    // Container for all parameters of DownloadAsync
    private static class AsyncParameters {
        String URL;
        File directory;
        AsyncParameters(String URL, File directory) {
            this.URL = URL;
            this.directory = directory;
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_pdf_view);

        //Grab the extras from the intent call
        Intent intent = getIntent(); //whatever calls this activity, gather the intent
        URL = intent.getStringExtra("File URL"); // in this case, get the file name of the "extra" passed through
        fileName = intent.getStringExtra("File Name");

        //Grab the internal storage directory and create a folder if it doesn't exist
        File intDirectory = getFilesDir();
        File folder = new File(intDirectory, "pdf");
        boolean isDirectoryCreated = folder.exists();

        //See if the file exists
        if (!isDirectoryCreated) {
            isDirectoryCreated= folder.mkdir();
        }
        if(isDirectoryCreated) {
            directory = new File(folder, fileName);
            try {
                directory.createNewFile();
                if (directory.canWrite())
                    Log.d("hngggggggggggggg", "onCreate: ");
            } catch (IOException e1) {
                e1.printStackTrace();
            }
            //See if file already exists
            boolean empty = directory.length() == 0;
            if (empty){
                /**Call class to create parameter container**/
                AsyncParameters param = new AsyncParameters(URL, directory);
                DownloadAsync Downloader = new DownloadAsync();
                Downloader.execute(param);
                showPdf();
            }
            else
                showPdf();
        }

    }
    public void showPdf()
    {
        pdfView = (PDFView) findViewById(R.id.pdfView);
        pdfView.fromFile(directory).load();
    }

    /**Class for asynchronous tasks**/
    public class DownloadAsync extends AsyncTask<AsyncParameters, Void, Void> {

        // Container for all parameters of DownloadAsync
        ProgressDialog pDialog;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            pDialog = new ProgressDialog(pdfView.this);
            pDialog.setMessage("Downloading Database...");
            String message= "Downloading Files";

            SpannableString ss2 =  new SpannableString(message);
            ss2.setSpan(new RelativeSizeSpan(2f), 0, ss2.length(), 0);
            ss2.setSpan(new ForegroundColorSpan(Color.BLACK), 0, ss2.length(), 0);

            pDialog.setMessage(ss2);
            pDialog.setCancelable(false);
            pDialog.show();
        }

        @Override
        protected Void doInBackground(AsyncParameters... params) {
            Log.d("WE ARE IN DOBACKGROUND", "doInBackground: ");
            String fileURL = params[0].URL;
            File directory = params[0].directory;
            try {

                FileOutputStream f = new FileOutputStream(directory);
                java.net.URL u = new URL(fileURL);
                HttpURLConnection c = (HttpURLConnection) u.openConnection();
                c.connect();
                InputStream in = c.getInputStream();

                byte[] buffer = new byte[8192];
                int len1 = 0;
                while ((len1 = in.read(buffer)) > 0) {
                    f.write(buffer, 0, len1);
                }
                f.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
            onPostExecute();
            return null;
        }

        protected void onPostExecute() {
            pDialog.dismiss();
        }
    }

}

Answer 1

您已经回答了自己的问题。由于下载是在不同线程上运行的asyncTask，因此在调用showPdf()之前无需等待asyncTask完成。您可以从后台任务完成后调用的showPdf()调用onPostExecute()。您的代码应如下所示：

@Override
protected void onCreate(Bundle savedInstanceState) {
    ........
    ........

    AsyncParameters param = new AsyncParameters(URL, directory);
    DownloadAsync Downloader = new DownloadAsync();
    Downloader.execute(param);

    .......
    .......
}

public class DownloadAsync extends AsyncTask<AsyncParameters, Void, Void> {
    .......

    @Override
    protected void onPostExecute() {
        pDialog.dismiss();
        showPdf();
    }
}

Async仅在单步执行调试器时有效

1 个答案: