Demo DocumentsProvider by @commonsguy originally posted at http://stackoverflow.com/q/25858032/1893766

DemoDocumentProvider.java

/***
  Copyright (c) 2014 CommonsWare, LLC
  Licensed under the Apache License, Version 2.0 (the "License"); you may not
  use this file except in compliance with the License. You may obtain a copy
  of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required
  by applicable law or agreed to in writing, software distributed under the
  License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
  OF ANY KIND, either express or implied. See the License for the specific
  language governing permissions and limitations under the License.

  From _The Busy Coder's Guide to Android Development_
    http://commonsware.com/Android
 */

package com.commonsware.android.documents.provider;

import android.content.res.AssetManager;
import android.database.Cursor;
import android.database.MatrixCursor;
import android.net.Uri;
import android.os.CancellationSignal;
import android.os.ParcelFileDescriptor;
import android.provider.DocumentsContract.Document;
import android.provider.DocumentsContract.Root;
import android.provider.DocumentsProvider;
import android.util.Log;
import android.webkit.MimeTypeMap;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;

public class DemoDocumentProvider extends DocumentsProvider {
    private static final String TAG = DemoDocumentProvider.class.getSimpleName();

    private static final String[] SUPPORTED_ROOT_PROJECTION = new String[] {
            Root.COLUMN_ROOT_ID, Root.COLUMN_FLAGS, Root.COLUMN_TITLE, Root.COLUMN_DOCUMENT_ID,
            Root.COLUMN_ICON
    };
    private static final String[] SUPPORTED_DOCUMENT_PROJECTION = new String[] {
            Document.COLUMN_DOCUMENT_ID, Document.COLUMN_MIME_TYPE, Document.COLUMN_DISPLAY_NAME,
            Document.COLUMN_FLAGS
    };
    private static final String ROOT_ID = "thisIsMyRoot";
    // private static final String ROOT_DOCUMENT_ID = "thisCannotBeEmpty";
    private static final String DELIMETER = "/";
    private AssetManager assets;

    @Override
    public boolean onCreate() {
        assets = getContext().getAssets();

        return (true);
    }

    @Override
    public Cursor queryRoots(String[] projection) throws FileNotFoundException {
        String[] netProjection = netProjection(projection, SUPPORTED_ROOT_PROJECTION);
        MatrixCursor result = new MatrixCursor(netProjection);
        MatrixCursor.RowBuilder row = result.newRow();

        row.add(Root.COLUMN_ROOT_ID, ROOT_ID);
        row.add(Root.COLUMN_ICON, R.drawable.ic_launcher);
        row.add(Root.COLUMN_FLAGS, Root.FLAG_LOCAL_ONLY);
        row.add(Root.COLUMN_TITLE, getContext().getString(R.string.app_name));
        row.add(Root.COLUMN_DOCUMENT_ID, fixUpDocumentId(ROOT_ID));

        return (result);
    }

    @Override
    public Cursor queryChildDocuments(String parentDocumentId, String[] projection, String sortOrder)
            throws FileNotFoundException {
        String[] netProjection = netProjection(projection, SUPPORTED_DOCUMENT_PROJECTION);
        MatrixCursor result = new MatrixCursor(netProjection);

        parentDocumentId = fixUpDocumentId(parentDocumentId);

        try {
            String[] children = assets.list(getAssetPathFromDocId(parentDocumentId));

            for (String child : children) {
                addDocumentRow(result, child, parentDocumentId + DELIMETER + child);
            }
        } catch (IOException e) {
            Log.e(TAG, "Exception reading asset dir", e);
        }

        return (result);
    }

    @Override
    public Cursor queryDocument(String documentId, String[] projection)
            throws FileNotFoundException {
        String[] netProjection = netProjection(projection, SUPPORTED_DOCUMENT_PROJECTION);
        MatrixCursor result = new MatrixCursor(netProjection);

        documentId = fixUpDocumentId(documentId);

        try {
            addDocumentRow(result, Uri.parse(documentId).getLastPathSegment(), documentId);
        } catch (IOException e) {
            Log.e(TAG, "Exception reading asset dir", e);
        }

        return (result);
    }

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

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

            new TransferThread(assets.open(getAssetPathFromDocId(documentId)),
                    new ParcelFileDescriptor.AutoCloseOutputStream(pipe[1])).start();
        } catch (IOException e) {
            Log.e(TAG, "Exception opening pipe", e);
            throw new FileNotFoundException("Could not open pipe for: " + documentId);
        }

        return (pipe[0]);
    }

    private void addDocumentRow(MatrixCursor result, String child, String documentId)
            throws IOException {
        MatrixCursor.RowBuilder row = result.newRow();

        row.add(Document.COLUMN_DOCUMENT_ID, documentId);

        if (isDirectory(documentId)) {
            row.add(Document.COLUMN_MIME_TYPE, Document.MIME_TYPE_DIR);
        } else {
            String mimeType = MimeTypeMap.getFileExtensionFromUrl(documentId);
            row.add(Document.COLUMN_MIME_TYPE, mimeType);
            row.add(Document.COLUMN_SIZE, lastModified(documentId));
        }

        row.add(Document.COLUMN_DISPLAY_NAME, child);
        row.add(Document.COLUMN_FLAGS, 0);
    }

    private boolean isDirectory(String documentId) throws IOException {
        return assets.list(getAssetPathFromDocId(documentId)).length > 0;
    }

    private long lastModified(String documentId) throws IOException {
        return (assets.openFd(getAssetPathFromDocId(documentId)).getLength());
    }

    private String fixUpDocumentId(String documentId) {
        if (ROOT_ID.equals(documentId)) {
            return documentId;
        }
        return documentId;
    }

    private String getAssetPathFromDocId(String documentId) throws FileNotFoundException {
        if (ROOT_ID.equals(documentId)) {
            return "";
        }

        final int splitIndex = documentId.indexOf(DELIMETER, 1);
        if (splitIndex < 0) {
            throw new FileNotFoundException("Missing root for " + documentId);
        } else {
            return documentId.substring(splitIndex + 1);
        }
    }

    private static String[] netProjection(String[] requested, String[] supported) {
        if (requested == null) {
            return (supported);
        }

        ArrayList<String> result = new ArrayList<String>();

        for (String request : requested) {
            for (String support : supported) {
                if (request.equals(support)) {
                    result.add(request);
                    break;
                }
            }
        }

        return (result.toArray(new String[0]));
    }

    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[1024];
            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(TAG, "Exception transferring file", e);
            }
        }
    }
}