Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/list_path_observer.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 observe.src.list_path_observer;
+
+import 'dart:async';
+import 'package:observe/observe.dart';
+
+// Inspired by ArrayReduction at:
+// https://raw.github.com/rafaelw/ChangeSummary/master/util/array_reduction.js
+// The main difference is we support anything on the rich Dart Iterable API.
+
+/// Observes a path starting from each item in the list.
+@deprecated
+class ListPathObserver<E, P> extends ChangeNotifier {
+  final ObservableList<E> list;
+  final String _itemPath;
+  final List<PathObserver> _observers = <PathObserver>[];
+  StreamSubscription _sub;
+  bool _scheduled = false;
+  Iterable<P> _value;
+
+  ListPathObserver(this.list, String path)
+      : _itemPath = path {
+
+    // TODO(jmesserly): delay observation until we are observed.
+    _sub = list.listChanges.listen((records) {
+      for (var record in records) {
+        _observeItems(record.addedCount - record.removed.length);
+      }
+      _scheduleReduce(null);
+    });
+
+    _observeItems(list.length);
+    _reduce();
+  }
+
+  @reflectable Iterable<P> get value => _value;
+
+  void dispose() {
+    if (_sub != null) _sub.cancel();
+    _observers.forEach((o) => o.close());
+    _observers.clear();
+  }
+
+  void _reduce() {
+    _scheduled = false;
+    var newValue = _observers.map((o) => o.value);
+    _value = notifyPropertyChange(#value, _value, newValue);
+  }
+
+  void _scheduleReduce(_) {
+    if (_scheduled) return;
+    _scheduled = true;
+    scheduleMicrotask(_reduce);
+  }
+
+  void _observeItems(int lengthAdjust) {
+    if (lengthAdjust > 0) {
+      for (int i = 0; i < lengthAdjust; i++) {
+        int len = _observers.length;
+        var pathObs = new PathObserver(list, '[$len].$_itemPath');
+        pathObs.open(_scheduleReduce);
+        _observers.add(pathObs);
+      }
+    } else if (lengthAdjust < 0) {
+      for (int i = 0; i < -lengthAdjust; i++) {
+        _observers.removeLast().close();
+      }
+    }
+  }
+}