path: root/core/java/com/android/internal/content/FileSystemProvider.java

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * 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.
 */

package com.android.internal.content;

import android.annotation.CallSuper;
import android.annotation.NonNull;
import android.annotation.Nullable;
import android.content.ContentResolver;
import android.content.Context;
import android.content.Intent;
import android.content.res.AssetFileDescriptor;
import android.database.Cursor;
import android.database.MatrixCursor;
import android.database.MatrixCursor.RowBuilder;
import android.graphics.Point;
import android.net.Uri;
import android.os.Binder;
import android.os.Bundle;
import android.os.CancellationSignal;
import android.os.FileObserver;
import android.os.FileUtils;
import android.os.Handler;
import android.os.ParcelFileDescriptor;
import android.provider.DocumentsContract;
import android.provider.DocumentsContract.Document;
import android.provider.DocumentsProvider;
import android.provider.MediaStore;
import android.provider.MetadataReader;
import android.system.Int64Ref;
import android.text.TextUtils;
import android.util.ArrayMap;
import android.util.Log;
import android.webkit.MimeTypeMap;

import com.android.internal.annotations.GuardedBy;
import com.android.internal.util.ArrayUtils;

import libcore.io.IoUtils;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.FileSystems;
import java.nio.file.FileVisitResult;
import java.nio.file.FileVisitor;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.function.Predicate;
import java.util.regex.Pattern;

/**
 * A helper class for {@link android.provider.DocumentsProvider} to perform file operations on local
 * files.
 */
public abstract class FileSystemProvider extends DocumentsProvider {

    private static final String TAG = "FileSystemProvider";

    private static final boolean LOG_INOTIFY = false;

    protected static final String SUPPORTED_QUERY_ARGS = joinNewline(
            DocumentsContract.QUERY_ARG_DISPLAY_NAME,
            DocumentsContract.QUERY_ARG_FILE_SIZE_OVER,
            DocumentsContract.QUERY_ARG_LAST_MODIFIED_AFTER,
            DocumentsContract.QUERY_ARG_MIME_TYPES);

    private static String joinNewline(String... args) {
        return TextUtils.join("\n", args);
    }

    private String[] mDefaultProjection;

    @GuardedBy("mObservers")
    private final ArrayMap<File, DirectoryObserver> mObservers = new ArrayMap<>();

    private Handler mHandler;

    protected abstract File getFileForDocId(String docId, boolean visible)
            throws FileNotFoundException;

    protected abstract String getDocIdForFile(File file) throws FileNotFoundException;

    protected abstract Uri buildNotificationUri(String docId);

    /**
     * Callback indicating that the given document has been modified. This gives
     * the provider a hook to invalidate cached data, such as {@code sdcardfs}.
     */
    protected void onDocIdChanged(String docId) {
        // Default is no-op
    }

    /**
     * Callback indicating that the given document has been deleted or moved. This gives
     * the provider a hook to revoke the uri permissions.
     */
    protected void onDocIdDeleted(String docId) {
        // Default is no-op
    }

    @Override
    public boolean onCreate() {
        throw new UnsupportedOperationException(
                "Subclass should override this and call onCreate(defaultDocumentProjection)");
    }

    @CallSuper
    protected void onCreate(String[] defaultProjection) {
        mHandler = new Handler();
        mDefaultProjection = defaultProjection;
    }

    @Override
    public boolean isChildDocument(String parentDocId, String docId) {
        try {
            final File parent = getFileForDocId(parentDocId).getCanonicalFile();
            final File doc = getFileForDocId(docId).getCanonicalFile();
            return FileUtils.contains(parent, doc);
        } catch (IOException e) {
            throw new IllegalArgumentException(
                    "Failed to determine if " + docId + " is child of " + parentDocId + ": " + e);
        }
    }

    @Override
    public @Nullable Bundle getDocumentMetadata(String documentId)
            throws FileNotFoundException {
        File file = getFileForDocId(documentId);

        if (!file.exists()) {
            throw new FileNotFoundException("Can't find the file for documentId: " + documentId);
        }

        final String mimeType = getDocumentType(documentId);
        if (Document.MIME_TYPE_DIR.equals(mimeType)) {
            final Int64Ref treeCount = new Int64Ref(0);
            final Int64Ref treeSize = new Int64Ref(0);
            try {
                final Path path = FileSystems.getDefault().getPath(file.getAbsolutePath());
                Files.walkFileTree(path, new FileVisitor<Path>() {
                    @Override
                    public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
                        return FileVisitResult.CONTINUE;
                    }

                    @Override
                    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
                        treeCount.value += 1;
                        treeSize.value += attrs.size();
                        return FileVisitResult.CONTINUE;
                    }

                    @Override
                    public FileVisitResult visitFileFailed(Path file, IOException exc) {
                        return FileVisitResult.CONTINUE;
                    }

                    @Override
                    public FileVisitResult postVisitDirectory(Path dir, IOException exc) {
                        return FileVisitResult.CONTINUE;
                    }
                });
            } catch (IOException e) {
                Log.e(TAG, "An error occurred retrieving the metadata", e);
                return null;
            }

            final Bundle res = new Bundle();
            res.putLong(DocumentsContract.METADATA_TREE_COUNT, treeCount.value);
            res.putLong(DocumentsContract.METADATA_TREE_SIZE, treeSize.value);
            return res;
        }

        if (!file.isFile()) {
            Log.w(TAG, "Can't stream non-regular file. Returning empty metadata.");
            return null;
        }
        if (!file.canRead()) {
            Log.w(TAG, "Can't stream non-readable file. Returning empty metadata.");
            return null;
        }
        if (!MetadataReader.isSupportedMimeType(mimeType)) {
            Log.w(TAG, "Unsupported type " + mimeType + ". Returning empty metadata.");
            return null;
        }

        InputStream stream = null;
        try {
            Bundle metadata = new Bundle();
            stream = new FileInputStream(file.getAbsolutePath());
            MetadataReader.getMetadata(metadata, stream, mimeType, null);
            return metadata;
        } catch (IOException e) {
            Log.e(TAG, "An error occurred retrieving the metadata", e);
            return null;
        } finally {
            IoUtils.closeQuietly(stream);
        }
    }

    protected final List<String> findDocumentPath(File parent, File doc)
            throws FileNotFoundException {

        if (!doc.exists()) {
            throw new FileNotFoundException(doc + " is not found.");
        }

        if (!FileUtils.contains(parent, doc)) {
            throw new FileNotFoundException(doc + " is not found under " + parent);
        }

        List<String> path = new ArrayList<>();
        while (doc != null && FileUtils.contains(parent, doc)) {
            path.add(0, getDocIdForFile(doc));

            doc = doc.getParentFile();
        }

        return path;
    }

    @Override
    public String createDocument(String docId, String mimeType, String displayName)
            throws FileNotFoundException {
        displayName = FileUtils.buildValidFatFilename(displayName);

        final File parent = getFileForDocId(docId);
        if (!parent.isDirectory()) {
            throw new IllegalArgumentException("Parent document isn't a directory");
        }

        final File file = FileUtils.buildUniqueFile(parent, mimeType, displayName);
        final String childId;
        if (Document.MIME_TYPE_DIR.equals(mimeType)) {
            if (!file.mkdir()) {
                throw new IllegalStateException("Failed to mkdir " + file);
            }
            childId = getDocIdForFile(file);
            onDocIdChanged(childId);
        } else {
            try {
                if (!file.createNewFile()) {
                    throw new IllegalStateException("Failed to touch " + file);
                }
                childId = getDocIdForFile(file);
                onDocIdChanged(childId);
            } catch (IOException e) {
                throw new IllegalStateException("Failed to touch " + file + ": " + e);
            }
        }
        updateMediaStore(getContext(), file);
        return childId;
    }

    @Override
    public String renameDocument(String docId, String displayName) throws FileNotFoundException {
        // Since this provider treats renames as generating a completely new
        // docId, we're okay with letting the MIME type change.
        displayName = FileUtils.buildValidFatFilename(displayName);

        final File before = getFileForDocId(docId);
        final File beforeVisibleFile = getFileForDocId(docId, true);
        final File after = FileUtils.buildUniqueFile(before.getParentFile(), displayName);
        if (!before.renameTo(after)) {
            throw new IllegalStateException("Failed to rename to " + after);
        }

        final String afterDocId = getDocIdForFile(after);
        onDocIdChanged(docId);
        onDocIdDeleted(docId);
        onDocIdChanged(afterDocId);

        final File afterVisibleFile = getFileForDocId(afterDocId, true);

        updateMediaStore(getContext(), beforeVisibleFile);
        updateMediaStore(getContext(), afterVisibleFile);

        if (!TextUtils.equals(docId, afterDocId)) {
            return afterDocId;
        } else {
            return null;
        }
    }

    @Override
    public String moveDocument(String sourceDocumentId, String sourceParentDocumentId,
            String targetParentDocumentId)
            throws FileNotFoundException {
        final File before = getFileForDocId(sourceDocumentId);
        final File after = new File(getFileForDocId(targetParentDocumentId), before.getName());
        final File visibleFileBefore = getFileForDocId(sourceDocumentId, true);

        if (after.exists()) {
            throw new IllegalStateException("Already exists " + after);
        }
        if (!before.renameTo(after)) {
            throw new IllegalStateException("Failed to move to " + after);
        }

        final String docId = getDocIdForFile(after);
        onDocIdChanged(sourceDocumentId);
        onDocIdDeleted(sourceDocumentId);
        onDocIdChanged(docId);
        // update the database
        updateMediaStore(getContext(), visibleFileBefore);
        updateMediaStore(getContext(), getFileForDocId(docId, true));
        return docId;
    }

    private static void updateMediaStore(@NonNull Context context, File file) {
        if (file != null) {
            final ContentResolver resolver = context.getContentResolver();
            final String noMedia = ".nomedia";
            // For file, check whether the file name is .nomedia or not.
            // If yes, scan the parent directory to update all files in the directory.
            if (!file.isDirectory() && file.getName().toLowerCase(Locale.ROOT).endsWith(noMedia)) {
                MediaStore.scanFile(resolver, file.getParentFile());
            } else {
                MediaStore.scanFile(resolver, file);
            }
        }
    }

    @Override
    public void deleteDocument(String docId) throws FileNotFoundException {
        final File file = getFileForDocId(docId);
        final File visibleFile = getFileForDocId(docId, true);

        final boolean isDirectory = file.isDirectory();
        if (isDirectory) {
            FileUtils.deleteContents(file);
        }
        // We could be deleting pending media which doesn't have any content yet, so only throw
        // if the file exists and we fail to delete it.
        if (file.exists() && !file.delete()) {
            throw new IllegalStateException("Failed to delete " + file);
        }

        onDocIdChanged(docId);
        onDocIdDeleted(docId);
        updateMediaStore(getContext(), visibleFile);
    }

    @Override
    public Cursor queryDocument(String documentId, String[] projection)
            throws FileNotFoundException {
        final MatrixCursor result = new MatrixCursor(resolveProjection(projection));
        includeFile(result, documentId, null);
        return result;
    }

    /**
     * This method is similar to
     * {@link DocumentsProvider#queryChildDocuments(String, String[], String)}. This method returns
     * all children documents including hidden directories/files.
     *
     * <p>
     * In a scoped storage world, access to "Android/data" style directories are hidden for privacy
     * reasons. This method may show privacy sensitive data, so its usage should only be in
     * restricted modes.
     *
     * @param parentDocumentId the directory to return children for.
     * @param projection list of {@link Document} columns to put into the
     *            cursor. If {@code null} all supported columns should be
     *            included.
     * @param sortOrder how to order the rows, formatted as an SQL
     *            {@code ORDER BY} clause (excluding the ORDER BY itself).
     *            Passing {@code null} will use the default sort order, which
     *            may be unordered. This ordering is a hint that can be used to
     *            prioritize how data is fetched from the network, but UI may
     *            always enforce a specific ordering
     * @throws FileNotFoundException when parent document doesn't exist or query fails
     */
    protected Cursor queryChildDocumentsShowAll(
            String parentDocumentId, String[] projection, String sortOrder)
            throws FileNotFoundException {
        return queryChildDocuments(parentDocumentId, projection, sortOrder, File -> true);
    }

    @Override
    public Cursor queryChildDocuments(
            String parentDocumentId, String[] projection, String sortOrder)
            throws FileNotFoundException {
        // Access to some directories is hidden for privacy reasons.
        return queryChildDocuments(parentDocumentId, projection, sortOrder, this::shouldShow);
    }

    private Cursor queryChildDocuments(
            String parentDocumentId, String[] projection, String sortOrder,
            @NonNull Predicate<File> filter) throws FileNotFoundException {
        final File parent = getFileForDocId(parentDocumentId);
        final MatrixCursor result = new DirectoryCursor(
                resolveProjection(projection), parentDocumentId, parent);

        if (!filter.test(parent)) {
            Log.w(TAG, "No permission to access parentDocumentId: " + parentDocumentId);
            return result;
        }

        if (parent.isDirectory()) {
            for (File file : FileUtils.listFilesOrEmpty(parent)) {
                if (filter.test(file)) {
                    includeFile(result, null, file);
                }
            }
        } else {
            Log.w(TAG, "parentDocumentId '" + parentDocumentId + "' is not Directory");
        }
        return result;
    }

    /**
     * Searches documents under the given folder.
     *
     * To avoid runtime explosion only returns the at most 23 items.
     *
     * @param folder the root folder where recursive search begins
     * @param query the search condition used to match file names
     * @param projection projection of the returned cursor
     * @param exclusion absolute file paths to exclude from result
     * @param queryArgs the query arguments for search
     * @return cursor containing search result. Include
     *         {@link ContentResolver#EXTRA_HONORED_ARGS} in {@link Cursor}
     *         extras {@link Bundle} when any QUERY_ARG_* value was honored
     *         during the preparation of the results.
     * @throws FileNotFoundException when root folder doesn't exist or search fails
     *
     * @see ContentResolver#EXTRA_HONORED_ARGS
     */
    protected final Cursor querySearchDocuments(
            File folder, String[] projection, Set<String> exclusion, Bundle queryArgs)
            throws FileNotFoundException {
        final MatrixCursor result = new MatrixCursor(resolveProjection(projection));
        final List<File> pending = new ArrayList<>();
        pending.add(folder);
        while (!pending.isEmpty() && result.getCount() < 24) {
            final File file = pending.remove(0);
            if (shouldHide(file)) continue;

            if (file.isDirectory()) {
                for (File child : FileUtils.listFilesOrEmpty(file)) {
                    pending.add(child);
                }
            }
            if (!exclusion.contains(file.getAbsolutePath()) && matchSearchQueryArguments(file,
                    queryArgs)) {
                includeFile(result, null, file);
            }
        }

        final String[] handledQueryArgs = DocumentsContract.getHandledQueryArguments(queryArgs);
        if (handledQueryArgs.length > 0) {
            final Bundle extras = new Bundle();
            extras.putStringArray(ContentResolver.EXTRA_HONORED_ARGS, handledQueryArgs);
            result.setExtras(extras);
        }
        return result;
    }

    @Override
    public String getDocumentType(String documentId) throws FileNotFoundException {
        return getDocumentType(documentId, getFileForDocId(documentId));
    }

    private String getDocumentType(final String documentId, final File file)
            throws FileNotFoundException {
        if (file.isDirectory()) {
            return Document.MIME_TYPE_DIR;
        } else {
            final int lastDot = documentId.lastIndexOf('.');
            if (lastDot >= 0) {
                final String extension = documentId.substring(lastDot + 1).toLowerCase();
                final String mime = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
                if (mime != null) {
                    return mime;
                }
            }
            return ContentResolver.MIME_TYPE_DEFAULT;
        }
    }

    @Override
    public ParcelFileDescriptor openDocument(
            String documentId, String mode, CancellationSignal signal)
            throws FileNotFoundException {
        final File file = getFileForDocId(documentId);
        final File visibleFile = getFileForDocId(documentId, true);

        final int pfdMode = ParcelFileDescriptor.parseMode(mode);
        if (visibleFile == null) {
            return ParcelFileDescriptor.open(file, pfdMode);
        } else if (pfdMode == ParcelFileDescriptor.MODE_READ_ONLY) {
            return openFileForRead(visibleFile);
        } else {
            try {
                // When finished writing, kick off media scanner
                return ParcelFileDescriptor.open(
                        file, pfdMode, mHandler, (IOException e) -> {
                            onDocIdChanged(documentId);
                            scanFile(visibleFile);
                        });
            } catch (IOException e) {
                throw new FileNotFoundException("Failed to open for writing: " + e);
            }
        }
    }

    private ParcelFileDescriptor openFileForRead(final File target) throws FileNotFoundException {
        final Uri uri = MediaStore.scanFile(getContext().getContentResolver(), target);
        if (uri == null) {
            Log.w(TAG, "Failed to retrieve media store URI for: " + target);
            return ParcelFileDescriptor.open(target, ParcelFileDescriptor.MODE_READ_ONLY);
        }

        // Passing the calling uid via EXTRA_MEDIA_CAPABILITIES_UID, so that the decision to
        // transcode or not transcode can be made based upon the calling app's uid, and not based
        // upon the Provider's uid.
        final Bundle opts = new Bundle();
        opts.putInt(MediaStore.EXTRA_MEDIA_CAPABILITIES_UID, Binder.getCallingUid());

        final AssetFileDescriptor afd =
                getContext().getContentResolver().openTypedAssetFileDescriptor(uri, "*/*", opts);
        if (afd == null) {
            Log.w(TAG, "Failed to open with media_capabilities uid for URI: " + uri);
            return ParcelFileDescriptor.open(target, ParcelFileDescriptor.MODE_READ_ONLY);
        }

        return afd.getParcelFileDescriptor();
    }

    /**
     * Test if the file matches the query arguments.
     *
     * @param file the file to test
     * @param queryArgs the query arguments
     */
    private boolean matchSearchQueryArguments(File file, Bundle queryArgs) {
        if (file == null) {
            return false;
        }

        final String fileMimeType;
        final String fileName = file.getName();

        if (file.isDirectory()) {
            fileMimeType = DocumentsContract.Document.MIME_TYPE_DIR;
        } else {
            int dotPos = fileName.lastIndexOf('.');
            if (dotPos < 0) {
                return false;
            }
            final String extension = fileName.substring(dotPos + 1);
            fileMimeType = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
        }
        return DocumentsContract.matchSearchQueryArguments(queryArgs, fileName, fileMimeType,
                file.lastModified(), file.length());
    }

    private void scanFile(File visibleFile) {
        final Intent intent = new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
        intent.setData(Uri.fromFile(visibleFile));
        getContext().sendBroadcast(intent);
    }

    @Override
    public AssetFileDescriptor openDocumentThumbnail(
            String documentId, Point sizeHint, CancellationSignal signal)
            throws FileNotFoundException {
        final File file = getFileForDocId(documentId);
        return DocumentsContract.openImageThumbnail(file);
    }

    protected RowBuilder includeFile(final MatrixCursor result, String docId, File file)
            throws FileNotFoundException {
        final String[] columns = result.getColumnNames();
        final RowBuilder row = result.newRow();

        if (docId == null) {
            docId = getDocIdForFile(file);
        } else {
            file = getFileForDocId(docId);
        }

        final String mimeType = getDocumentType(docId, file);
        row.add(Document.COLUMN_DOCUMENT_ID, docId);
        row.add(Document.COLUMN_MIME_TYPE, mimeType);

        final int flagIndex = ArrayUtils.indexOf(columns, Document.COLUMN_FLAGS);
        if (flagIndex != -1) {
            int flags = 0;
            if (file.canWrite()) {
                if (mimeType.equals(Document.MIME_TYPE_DIR)) {
                    flags |= Document.FLAG_DIR_SUPPORTS_CREATE;
                    flags |= Document.FLAG_SUPPORTS_DELETE;
                    flags |= Document.FLAG_SUPPORTS_RENAME;
                    flags |= Document.FLAG_SUPPORTS_MOVE;

                    if (shouldBlockFromTree(docId)) {
                        flags |= Document.FLAG_DIR_BLOCKS_OPEN_DOCUMENT_TREE;
                    }

                } else {
                    flags |= Document.FLAG_SUPPORTS_WRITE;
                    flags |= Document.FLAG_SUPPORTS_DELETE;
                    flags |= Document.FLAG_SUPPORTS_RENAME;
                    flags |= Document.FLAG_SUPPORTS_MOVE;
                }
            }

            if (mimeType.startsWith("image/")) {
                flags |= Document.FLAG_SUPPORTS_THUMBNAIL;
            }

            if (typeSupportsMetadata(mimeType)) {
                flags |= Document.FLAG_SUPPORTS_METADATA;
            }
            row.add(flagIndex, flags);
        }

        final int displayNameIndex = ArrayUtils.indexOf(columns, Document.COLUMN_DISPLAY_NAME);
        if (displayNameIndex != -1) {
            row.add(displayNameIndex, file.getName());
        }

        final int lastModifiedIndex = ArrayUtils.indexOf(columns, Document.COLUMN_LAST_MODIFIED);
        if (lastModifiedIndex != -1) {
            final long lastModified = file.lastModified();
            // Only publish dates reasonably after epoch
            if (lastModified > 31536000000L) {
                row.add(lastModifiedIndex, lastModified);
            }
        }
        final int sizeIndex = ArrayUtils.indexOf(columns, Document.COLUMN_SIZE);
        if (sizeIndex != -1) {
            row.add(sizeIndex, file.length());
        }

        // Return the row builder just in case any subclass want to add more stuff to it.
        return row;
    }

    private static final Pattern PATTERN_HIDDEN_PATH = Pattern.compile(
            "(?i)^/storage/[^/]+/(?:[0-9]+/)?Android/(?:data|obb|sandbox)$");

    /**
     * In a scoped storage world, access to "Android/data" style directories are
     * hidden for privacy reasons.
     */
    protected boolean shouldHide(@NonNull File file) {
        return (PATTERN_HIDDEN_PATH.matcher(file.getAbsolutePath()).matches());
    }

    private boolean shouldShow(@NonNull File file) {
        return !shouldHide(file);
    }

    protected boolean shouldBlockFromTree(@NonNull String docId) {
        return false;
    }

    protected boolean typeSupportsMetadata(String mimeType) {
        return MetadataReader.isSupportedMimeType(mimeType)
                || Document.MIME_TYPE_DIR.equals(mimeType);
    }

    protected final File getFileForDocId(String docId) throws FileNotFoundException {
        return getFileForDocId(docId, false);
    }

    private String[] resolveProjection(String[] projection) {
        return projection == null ? mDefaultProjection : projection;
    }

    private void startObserving(File file, Uri notifyUri, DirectoryCursor cursor) {
        synchronized (mObservers) {
            DirectoryObserver observer = mObservers.get(file);
            if (observer == null) {
                observer =
                        new DirectoryObserver(file, getContext().getContentResolver(), notifyUri);
                observer.startWatching();
                mObservers.put(file, observer);
            }
            observer.mCursors.add(cursor);

            if (LOG_INOTIFY) Log.d(TAG, "after start: " + observer);
        }
    }

    private void stopObserving(File file, DirectoryCursor cursor) {
        synchronized (mObservers) {
            DirectoryObserver observer = mObservers.get(file);
            if (observer == null) return;

            observer.mCursors.remove(cursor);
            if (observer.mCursors.size() == 0) {
                mObservers.remove(file);
                observer.stopWatching();
            }

            if (LOG_INOTIFY) Log.d(TAG, "after stop: " + observer);
        }
    }

    private static class DirectoryObserver extends FileObserver {
        private static final int NOTIFY_EVENTS = ATTRIB | CLOSE_WRITE | MOVED_FROM | MOVED_TO
                | CREATE | DELETE | DELETE_SELF | MOVE_SELF;

        private final File mFile;
        private final ContentResolver mResolver;
        private final Uri mNotifyUri;
        private final CopyOnWriteArrayList<DirectoryCursor> mCursors;

        DirectoryObserver(File file, ContentResolver resolver, Uri notifyUri) {
            super(file.getAbsolutePath(), NOTIFY_EVENTS);
            mFile = file;
            mResolver = resolver;
            mNotifyUri = notifyUri;
            mCursors = new CopyOnWriteArrayList<>();
        }

        @Override
        public void onEvent(int event, String path) {
            if ((event & NOTIFY_EVENTS) != 0) {
                if (LOG_INOTIFY) Log.d(TAG, "onEvent() " + event + " at " + path);
                for (DirectoryCursor cursor : mCursors) {
                    cursor.notifyChanged();
                }
                mResolver.notifyChange(mNotifyUri, null, false);
            }
        }

        @Override
        public String toString() {
            String filePath = mFile.getAbsolutePath();
            return "DirectoryObserver{file=" + filePath + ", ref=" + mCursors.size() + "}";
        }
    }

    private class DirectoryCursor extends MatrixCursor {
        private final File mFile;

        public DirectoryCursor(String[] columnNames, String docId, File file) {
            super(columnNames);

            final Uri notifyUri = buildNotificationUri(docId);
            boolean registerSelfObserver = false; // Our FileObserver sees all relevant changes.
            setNotificationUris(getContext().getContentResolver(), Arrays.asList(notifyUri),
                    getContext().getContentResolver().getUserId(), registerSelfObserver);

            mFile = file;
            startObserving(mFile, notifyUri, this);
        }

        public void notifyChanged() {
            onChange(false);
        }

        @Override
        public void close() {
            super.close();
            stopObserving(mFile, this);
        }
    }
}