Implementing a DocumentsProvier

DocumentsCursor.java

import android.database.MatrixCursor;
import android.os.Bundle;
import android.provider.DocumentsContract;

public class DocumentsCursor extends MatrixCursor {

    private Bundle mExtras;

    public DocumentsCursor(String[] columnNames, int initialCapacity) {
        super(columnNames, initialCapacity);
    }

    public DocumentsCursor(String[] columnNames) {
        super(columnNames);
    }

    public Bundle getExtras() {
        if (mExtras == null) {
            return super.getExtras();
        }
        return mExtras;
    }

    public void setErrorInformation(String errorMessage) {
        if (mExtras == null) {
            this.mExtras = new Bundle();
            mExtras.putString(DocumentsContract.EXTRA_ERROR, errorMessage);
        }
    }

    public void setIsLoading(boolean isLoading) {
        if (mExtras == null) {
            mExtras = new Bundle();
            mExtras.putBoolean(DocumentsContract.EXTRA_LOADING, isLoading);
        }
    }
}

OpenDocumentFile.java

 [...]
 
 @Override
    public ParcelFileDescriptor openDocument(final String documentId, String mode, final CancellationSignal signal) throws FileNotFoundException {
       File file = fileFromDocumentId(documentId);

        if(file.exists()) {
            return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
        }

		/* really dirty like in Drive app
        StrictMode.ThreadPolicy threadpolicy = StrictMode.getThreadPolicy();
        StrictMode.setThreadPolicy((new StrictMode.ThreadPolicy.Builder()).build());
		*/
        try {
           //blocking download
			download(documentId, file, signal);
			
			/*
			StrictMode.setThreadPolicy(threadpolicy);
			*/

			if(file.exists()) {
				return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
			}
			throw new FileNotFoundException("Failed to open document " + documentId + " and mode " + mode);
        } catch (IOException e) {
            throw new FileNotFoundException("Failed to open document " + documentId + " and mode " + mode);
        }
    }
    
    [...]

OpenDocumentPipe.java

[...]
  @Override
    public ParcelFileDescriptor openDocument(final String documentId, String mode, final CancellationSignal signal) throws FileNotFoundException {
        try {
            ParcelFileDescriptor[] pipe = ParcelFileDescriptor.createReliablePipe();

        //use the write part of pipe
            ParcelFileDescriptor.AutoCloseOutputStream outputStream = new ParcelFileDescriptor.AutoCloseOutputStream(pipe[1]);

			//1. download and register a listener on cancellation
            final DownloadTask task = downloadWithYourHttpLayer(documentId, outputStream);

            if(signal != null) {
                signal.setOnCancelListener(new CancellationSignal.OnCancelListener() {
                    @Override
                    public void onCancel() {
                        task.cancel();
                    }
                });
            }
			
			//or
			//2. download and check the signal during the download
            downloadWithYourHttpLayer(documentId, outputStream, signal);
			
	    //return the read part of pipe	
            return pipe[0];
        } catch (IOException e) {
            throw new FileNotFoundException("Failed to open document " + documentId + " in mode " + mode);
        }
    }
[...]

QueryChildDocuments.java

[...]

private final ConcurrentHashMap<Uri, Boolean> mLoadingUris = new ConcurrentHashMap<Uri, Boolean>();

@Override
    public Cursor queryChildDocuments(String parentDocumentId, String[] projection, String sortOrder) throws FileNotFoundException {
        final DocumentsCursor cursor = new DocumentsCursor(resolveDocumentProjection(projection));

        fillChildrenFromLocalCache(parentDocumentId, cursor);

        final Uri uri = DocumentsContract.buildChildDocumentsUri(AUTHORITY, parentDocumentId);

        Boolean active = mLoadingUris.get(uri);

        if(active == null) {
			//no loading request for this uri, launch it
            mLoadingUris.put(uri, Boolean.TRUE);
            cursor.setIsLoading(true);
            cursor.setNotificationUri(getContext().getContentResolver(), uri);

            fillChildrenFromWeb(parentDocumentId, new OnLoadedListener() {
                @Override
                public void onLoaded(List<YourItem> childs) {
                    putChildrenInLocalCache(childs);
					
					//loading request is finished, refresh
                    mLoadingUris.put(uri, Boolean.FALSE);
                    getContext().getContentResolver().notifyChange(uri, null);
                }
            });
        } else if(!active) {
			//loading request is finished and refreshed
            mLoadingUris.remove(uri);
        }
		
        return cursor;
    }
[...]