Re-implement directory polling.

pkg/watcher/lib/src/async_queue.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.async_queue;
+
+import 'dart:async';
+import 'dart:collection';
+
+typedef Future ItemProcessor<T>(T item);
+typedef void ErrorHandler(error);
+
+/// A queue of items that are sequentially, asynchronously processed.
+///
+/// Unlike [Stream.map] or [Stream.forEach], the callback used to process each
+/// item returns a [Future], and it will not advance to the next item until the
+/// current item is finished processing.
+///
+/// Items can be added at any point in time and processing will be started as
+/// needed. When all items are processed, it stops processing until more items
+/// are added.
+class AsyncQueue<T> {
+  final _items = new Queue<T>();
+
+  /// Whether or not the queue is currently waiting on a processing future to
+  /// complete.
+  bool _isProcessing = false;
+
+  /// The callback to invoke on each queued item.
+  ///
+  /// The next item in the queue will not be processed until the [Future]
+  /// returned by this completes.
+  final ItemProcessor<T> _processor;
+
+  /// The handler for errors thrown during processing.
+  ///
+  /// Used to avoid top-leveling asynchronous errors.
+  final ErrorHandler _errorHandler;
+
+  AsyncQueue(this._processor, {ErrorHandler onError})
+      : _errorHandler = onError;
+
+  /// Enqueues [item] to be processed and starts asynchronously processing it
+  /// if a process isn't already running.
+  void add(T item) {
+    _items.add(item);
+
+    // Start up the asynchronous processing if not already running.
+    if (_isProcessing) return;
+    _isProcessing = true;
+
+    _processNextItem().catchError(_errorHandler);
+  }
+
+  /// Removes all remaining items to be processed.
+  void clear() {
+    _items.clear();
+  }
+
+  /// Pulls the next item off [_items] and processes it.
+  ///
+  /// When complete, recursively calls itself to continue processing unless
+  /// the process was cancelled.
+  Future _processNextItem() {
+    var item = _items.removeFirst();
+    return _processor(item).then((_) {
+      if (_items.isNotEmpty) return _processNextItem();
+
+      // We have drained the queue, stop processing and wait until something
+      // has been enqueued.
+      _isProcessing = false;
+    });
+  }
+}