File watching package.

pkg/watcher/lib/src/directory_watcher.dart

Index: pkg/watcher/lib/src/directory_watcher.dart
diff --git a/pkg/watcher/lib/src/directory_watcher.dart b/pkg/watcher/lib/src/directory_watcher.dart
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+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library watcher.directory_watcher;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:crypto/crypto.dart';
+
+import 'stat.dart';
+import 'watch_event.dart';
+
+/// Watches the contents of a directory and emits [WatchEvent]s when something
+/// in the directory has changed.
+class DirectoryWatcher {
+  /// The directory whose contents are being monitored.
+  final String directory;
+
+  /// The broadcast [Stream] of events that have occurred to files in
+  /// [directory].
+  ///
+  /// Changes will only be monitored while this stream has subscribers. Any
+  /// file changes that occur during periods when there are no subscribers
+  /// will not be reported the next time a subscriber is added.
+  Stream<WatchEvent> get events => _events.stream;
+  StreamController<WatchEvent> _events;
+
+  _WatchState _state = _WatchState.notWatching;
+
+  /// A [Future] that completes when the watcher is initialized and watching
+  /// for file changes.
+  ///
+  /// If the watcher is not currently monitoring the directory (because there
+  /// are no subscribers to [events]), this returns a future that isn't
+  /// complete yet. It will complete when a subscriber starts listening and
+  /// the watcher finishes any initialization work it needs to do.
+  ///
+  /// If the watcher is already monitoring, this returns an already complete
+  /// future.
+  Future get ready => _ready.future;
+  Completer _ready = new Completer();
+
+  /// The previous status of the files in the directory.
+  ///
+  /// Used to tell which files have been modified.
+  final _statuses = new Map<String, _FileStatus>();
+
+  /// Creates a new [DirectoryWatcher] monitoring [directory].
+  DirectoryWatcher(this.directory) {
+    _events = new StreamController<WatchEvent>.broadcast(onListen: () {
+      _state = _state.listen(this);
+    }, onCancel: () {
+      _state = _state.cancel(this);
+    });
+  }
+
+  /// Starts the asynchronous polling process.
+  ///
+  /// Scans the contents of the directory and compares the results to the
+  /// previous scan. Loops to continue monitoring as long as there are
+  /// subscribers to the [events] stream.
+  Future _watch() {
+    var files = new Set<String>();
+
+    var stream = new Directory(directory).list(recursive: true);
+
+    return stream.map((entity) {
+      if (entity is! File) return new Future.value();
+      files.add(entity.path);
+      // TODO(rnystrom): These all run as fast as possible and read the
+      // contents of the files. That means there's a pretty big IO hit all at
+      // once. Maybe these should be queued up and rate limited?
+      return _refreshFile(entity.path);
+    }).toList().then((futures) {
+      // Once the listing is done, make sure to wait until each file is also
+      // done.
+      return Future.wait(futures);
+    }).then((_) {
+      var removedFiles = _statuses.keys.toSet().difference(files);
+      for (var removed in removedFiles) {
+        if (_state.shouldNotify) {
+          _events.add(new WatchEvent(ChangeType.REMOVE, removed));
+        }
+        _statuses.remove(removed);
+      }
+
+      var previousState = _state;
+      _state = _state.finish(this);
+
+      // If we were already sending notifications, add a bit of delay before
+      // restarting just so that we don't whale on the file system.
+      // TODO(rnystrom): Tune this and/or make it tunable?
+      if (_state.shouldNotify) {
+        return new Future.delayed(new Duration(seconds: 1));
+      }
+    }).then((_) {
+      // Make sure we haven't transitioned to a non-watching state during the
+      // delay.
+      if (_state.shouldWatch) _watch();
+    });
+  }
+
+  /// Compares the current state of the file at [path] to the state it was in
+  /// the last time it was scanned.
+  Future _refreshFile(String path) {
+    return getModificationTime(path).then((modified) {
+      var lastStatus = _statuses[path];
+
+      // If it's modification time hasn't changed, assume the file is unchanged.
+      if (lastStatus != null && lastStatus.modified == modified) return;
+
+      return _hashFile(path).then((hash) {
+        var status = new _FileStatus(modified, hash);
+        _statuses[path] = status;
+
+        // Only notify if the file contents changed.
+        if (_state.shouldNotify &&
+            (lastStatus == null || !_sameHash(lastStatus.hash, hash))) {
+          var change = lastStatus == null ? ChangeType.ADD : ChangeType.MODIFY;
+          _events.add(new WatchEvent(change, path));
+        }
+      });
+    });
+  }
+
+  /// Calculates the SHA-1 hash of the file at [path].
+  Future<List<int>> _hashFile(String path) {
+    return new File(path).readAsBytes().then((bytes) {
+      var sha1 = new SHA1();
+      sha1.add(bytes);
+      return sha1.close();
+    });
+  }
+
+  /// Returns `true` if [a] and [b] are the same hash value, i.e. the same
+  /// series of byte values.
+  bool _sameHash(List<int> a, List<int> b) {
+    // Hashes should always be the same size.
+    assert(a.length == b.length);
+
+    for (var i = 0; i < a.length; i++) {
+      if (a[i] != b[i]) return false;
+    }
+
+    return true;
+  }
+}
+
+/// An "event" that is sent to the [_WatchState] FSM to trigger state
+/// transitions.
+typedef _WatchState _WatchStateEvent(DirectoryWatcher watcher);
+
+/// The different states that the watcher can be in and the transitions between
+/// them.
+///
+/// This class defines a finite state machine for keeping track of what the
+/// asynchronous file polling is doing. Each instance of this is a state in the
+/// machine and its [listen], [cancel], and [finish] fields define the state
+/// transitions when those events occur.
+class _WatchState {
+  /// The watcher has no subscribers.
+  static final notWatching = new _WatchState(
+      listen: (watcher) {
+    watcher._watch();
+    return _WatchState.scanning;
+  });
+
+  /// The watcher has subscribers and is scanning for pre-existing files.
+  static final scanning = new _WatchState(
+      cancel: (watcher) {
+    // No longer watching, so create a new incomplete ready future.
+    watcher._ready = new Completer();
+    return _WatchState.cancelling;
+  }, finish: (watcher) {
+    watcher._ready.complete();
+    return _WatchState.watching;
+  }, shouldWatch: true);
+
+  /// The watcher was unsubscribed while polling and we're waiting for the poll
+  /// to finish.
+  static final cancelling = new _WatchState(
+      listen: (_) => _WatchState.scanning,
+      finish: (_) => _WatchState.notWatching);
+
+  /// The watcher has subscribers, we have scanned for pre-existing files and
+  /// now we're polling for changes.
+  static final watching = new _WatchState(
+      cancel: (watcher) {
+    // No longer watching, so create a new incomplete ready future.
+    watcher._ready = new Completer();
+    return _WatchState.cancelling;
+  }, finish: (_) => _WatchState.watching,
+      shouldWatch: true, shouldNotify: true);
+
+  /// Called when the first subscriber to the watcher has been added.
+  final _WatchStateEvent listen;
+
+  /// Called when all subscriptions on the watcher have been cancelled.
+  final _WatchStateEvent cancel;
+
+  /// Called when a poll loop has finished.
+  final _WatchStateEvent finish;
+
+  /// If the directory watcher should be watching the file system while in
+  /// this state.
+  final bool shouldWatch;
+
+  /// If a change event should be sent for a file modification while in this
+  /// state.
+  final bool shouldNotify;
+
+  _WatchState({this.listen, this.cancel, this.finish,
+      this.shouldWatch: false, this.shouldNotify: false});
+}
+
+class _FileStatus {
+  /// The last time the file was modified.
+  DateTime modified;
+
+  /// The SHA-1 hash of the contents of the file.
+  List<int> hash;
+
+  _FileStatus(this.modified, this.hash);
+}