android - Custom ContentProvider - openInputStream(), openOutputStream()

Question

The content provider/ resolver APIs provide a complicated, but robust way of transferring data between processes using a URI and the openInputStream() and openOutputStream() methods. Custom content providers have the ability to override the openFile() method with custom code to effectively resolve a URI into a Stream; however, the method signature of openFile() has a ParcelFileDescriptor return type and it is not clear how one might generate a proper representation for dynamically generated content to return from this method.

Returning a memory mapped InputStream from a content provider?

Are there examples of implementing ContentProvider.openFile() method for dynamic content in the existing code base? If not can you suggest source code or process for doing so?

Answer 1

Check out this great example project from the always helpful CommonsWare. It lets you create a ParcelFileDescriptor pipe with whatever InputStream you want on one side, and the receiving application on the other side:

https://github.com/commonsguy/cw-omnibus/tree/master/ContentProvider/Pipe

The key parts are creating the pipe in openFile:

public ParcelFileDescriptor openFile(Uri uri, String mode)
                                                        throws FileNotFoundException {
    ParcelFileDescriptor[] pipe=null;

    try {
      pipe=ParcelFileDescriptor.createPipe();
      AssetManager assets=getContext().getResources().getAssets();

      new TransferThread(assets.open(uri.getLastPathSegment()),
                       new AutoCloseOutputStream(pipe[1])).start();
    }
    catch (IOException e) {
      Log.e(getClass().getSimpleName(), "Exception opening pipe", e);
      throw new FileNotFoundException("Could not open pipe for: "
          + uri.toString());
    }

    return(pipe[0]);
  }

Then create a thread that keeps the pipe full:

static class TransferThread extends Thread {
    InputStream in;
    OutputStream out;

    TransferThread(InputStream in, OutputStream out) {
        this.in = in;
        this.out = out;
    }

    @Override
    public void run() {
        byte[] buf = new byte[8192];
        int len;

        try {
            while ((len = in.read(buf)) > 0) {
                out.write(buf, 0, len);
            }

            in.close();
            out.flush();
            out.close();
        } catch (IOException e) {
            Log.e(getClass().getSimpleName(),
                    "Exception transferring file", e);
        }
    }
}