Wrap Directory.watch on linux for the watcher package.

pkg/watcher/lib/src/directory_watcher/resubscribable.dart

+// 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.resubscribable;
+
+import 'dart:async';
+import 'dart:io';
+
+import '../directory_watcher.dart';
+import '../utils.dart';
+import '../watch_event.dart';
+
+typedef ManuallyClosedDirectoryWatcher WatcherFactory();
+
+/// A wrapper for [ManuallyClosedDirectoryWatcher] that encapsulates support for
+/// closing the watcher when it has no subscribers and re-opening it when it's
+/// re-subscribed.
+///
+/// It's simpler to implement watchers without worrying about this behavior.
+/// This class wraps a watcher class which can be written with the simplifying
+/// assumption that it can continue emitting events until an explicit `close`
+/// method is called, at which point it will cease emitting events entirely. The
+/// [ManuallyClosedDirectoryWatcher] interface is used for these watchers.
+///
+/// This would be more cleanly implemented as a function that takes a class and
+/// emits a new class, but Dart doesn't support that sort of thing. Instead it
+/// takes a factory function that produces instances of the inner class.
+abstract class ResubscribableDirectoryWatcher implements DirectoryWatcher {
+  /// The factory function that produces instances of the inner class.
+  final WatcherFactory _factory;
+
+  final String directory;
+
+  Stream<WatchEvent> get events => _eventsController.stream;
+  StreamController<WatchEvent> _eventsController;
+
+  bool get isReady => _readyCompleter.isCompleted;
+
+  Future get ready => _readyCompleter.future;
+  var _readyCompleter = new Completer();
+
+  /// Creates a new [ResubscribableDirectoryWatcher] wrapping the watcher

[Bob Nystrom, 2013/11/06 19:24:04]

"watcher" - "watchers".

[nweiz, 2013/11/07 00:46:37]

Done.

+  /// emitted by [_factory].
+  ResubscribableDirectoryWatcher(this.directory, this._factory) {
+    var watcher;
+    var subscription;
+
+    _eventsController = new StreamController<WatchEvent>.broadcast(
+        onListen: () {
+      watcher = _factory();
+      subscription = watcher.events.listen(_eventsController.add,
+          onError: _eventsController.addError,
+          onDone: _eventsController.close);
+
+      // It's important that this completes the current completer at the time
+      // it's called, since a new completer may be created if this subscription

[Bob Nystrom, 2013/11/06 19:24:04]

This comment is confusing me. I'm not sure what the first "this" and the second
"it's" refer to. Can you clarify it?

[nweiz, 2013/11/07 00:46:37]

Done.

+      // is canceled before [watcher.ready] fires.
+      watcher.ready.then(_readyCompleter.complete);
+    }, onCancel: () {
+      // Cancel the subscription before closing the watcher so that the
+      // watcher's `onDone` event doesn't close [events].
+      subscription.cancel();
+      watcher.close();
+      _readyCompleter = new Completer();
+    }, sync: true);
+  }
+}
+
+/// An interface for watchers with an explicit, manual [close] method.
+///
+/// See [ResubscribableDirectoryWatcher].
+abstract class ManuallyClosedDirectoryWatcher implements DirectoryWatcher {
+  /// Closes the watcher.
+  ///
+  /// Subclasses should close their [events] stream and release any internal
+  /// resources.
+  void close();
+}