Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/path_observer.dart

Index: observatory_pub_packages/observe/src/path_observer.dart
===================================================================
--- observatory_pub_packages/observe/src/path_observer.dart	(revision 0)
+++ observatory_pub_packages/observe/src/path_observer.dart	(working copy)
@@ -0,0 +1,929 @@
+// 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.path_observer;
+
+import 'dart:async';
+import 'dart:collection';
+import 'dart:math' show min;
+
+import 'package:logging/logging.dart' show Logger, Level;
+import 'package:observe/observe.dart';
+import 'package:smoke/smoke.dart' as smoke;
+
+import 'package:utf/utf.dart' show stringToCodepoints;
+
+/// A data-bound path starting from a view-model or model object, for example
+/// `foo.bar.baz`.
+///
+/// When [open] is called, this will observe changes to the object and any
+/// intermediate object along the path, and send updated values accordingly.
+/// When [close] is called it will stop observing the objects.
+///
+/// This class is used to implement `Node.bind` and similar functionality in
+/// the [template_binding](pub.dartlang.org/packages/template_binding) package.
+class PathObserver extends _Observer implements Bindable {
+  PropertyPath _path;
+  Object _object;
+  _ObservedSet _directObserver;
+
+  /// Observes [path] on [object] for changes. This returns an object
+  /// that can be used to get the changes and get/set the value at this path.
+  ///
+  /// The path can be a [PropertyPath], or a [String] used to construct it.
+  ///
+  /// See [open] and [value].
+  PathObserver(Object object, [path])
+      : _object = object,
+        _path = new PropertyPath(path);
+
+  PropertyPath get path => _path;
+
+  /// Sets the value at this path.
+  void set value(Object newValue) {
+    if (_path != null) _path.setValueFrom(_object, newValue);
+  }
+
+  int get _reportArgumentCount => 2;
+
+  /// Initiates observation and returns the initial value.
+  /// The callback will be passed the updated [value], and may optionally be
+  /// declared to take a second argument, which will contain the previous value.
+  open(callback) => super.open(callback);
+
+  void _connect() {
+    _directObserver = new _ObservedSet(this, _object);
+    _check(skipChanges: true);
+  }
+
+  void _disconnect() {
+    _value = null;
+    if (_directObserver != null) {
+      _directObserver.close(this);
+      _directObserver = null;
+    }
+    // Dart note: the JS impl does not do this, but it seems consistent with
+    // CompoundObserver. After closing the PathObserver can't be reopened.
+    _path = null;
+    _object = null;
+  }
+
+  void _iterateObjects(void observe(obj, prop)) {
+    _path._iterateObjects(_object, observe);
+  }
+
+  bool _check({bool skipChanges: false}) {
+    var oldValue = _value;
+    _value = _path.getValueFrom(_object);
+    if (skipChanges || _value == oldValue) return false;
+
+    _report(_value, oldValue, this);
+    return true;
+  }
+}
+
+/// A dot-delimieted property path such as "foo.bar" or "foo.10.bar".
+///
+/// The path specifies how to get a particular value from an object graph, where
+/// the graph can include arrays and maps. Each segment of the path describes
+/// how to take a single step in the object graph. Properties like 'foo' or
+/// 'bar' are read as properties on objects, or as keys if the object is a [Map]
+/// or a [Indexable], while integer values are read as indexes in a [List].
+// TODO(jmesserly): consider specialized subclasses for:
+// * empty path
+// * "value"
+// * single token in path, e.g. "foo"
+class PropertyPath {
+  /// The segments of the path.
+  final List<Object> _segments;
+
+  /// Creates a new [PropertyPath]. These can be stored to avoid excessive
+  /// parsing of path strings.
+  ///
+  /// The provided [path] should be a String or a List. If it is a list it
+  /// should contain only Symbols and integers. This can be used to avoid
+  /// parsing.
+  ///
+  /// Note that this constructor will canonicalize identical paths in some cases
+  /// to save memory, but this is not guaranteed. Use [==] for comparions
+  /// purposes instead of [identical].
+  // Dart note: this is ported from `function getPath`.
+  factory PropertyPath([path]) {
+    if (path is PropertyPath) return path;
+    if (path == null || (path is List && path.isEmpty)) path = '';
+
+    if (path is List) {
+      var copy = new List.from(path, growable: false);
+      for (var segment in copy) {
+        // Dart note: unlike Javascript, we don't support arbitraty objects that
+        // can be converted to a String.
+        // TODO(sigmund): consider whether we should support that here. It might
+        // be easier to add support for that if we switch first to use strings
+        // for everything instead of symbols.
+        if (segment is! int && segment is! String && segment is! Symbol) {
+          throw new ArgumentError(
+              'List must contain only ints, Strings, and Symbols');
+        }
+      }
+      return new PropertyPath._(copy);
+    }
+
+    var pathObj = _pathCache[path];
+    if (pathObj != null) return pathObj;
+
+
+    final segments = new _PathParser().parse(path);
+    if (segments == null) return _InvalidPropertyPath._instance;
+
+    // TODO(jmesserly): we could use an UnmodifiableListView here, but that adds
+    // memory overhead.
+    pathObj = new PropertyPath._(segments.toList(growable: false));
+    if (_pathCache.length >= _pathCacheLimit) {
+      _pathCache.remove(_pathCache.keys.first);
+    }
+    _pathCache[path] = pathObj;
+    return pathObj;
+  }
+
+  PropertyPath._(this._segments);
+
+  int get length => _segments.length;
+  bool get isEmpty => _segments.isEmpty;
+  bool get isValid => true;
+
+  String toString() {
+    if (!isValid) return '<invalid path>';
+    var sb = new StringBuffer();
+    bool first = true;
+    for (var key in _segments) {
+      if (key is Symbol) {
+        if (!first) sb.write('.');
+        sb.write(smoke.symbolToName(key));
+      } else {
+        _formatAccessor(sb, key);
+      }
+      first = false;
+    }
+    return sb.toString();
+  }
+
+  _formatAccessor(StringBuffer sb, Object key) {
+    if (key is int) {
+      sb.write('[$key]');
+    } else {
+      sb.write('["${key.toString().replaceAll('"', '\\"')}"]');
+    }
+  }
+
+  bool operator ==(other) {
+    if (identical(this, other)) return true;
+    if (other is! PropertyPath) return false;
+    if (isValid != other.isValid) return false;
+
+    int len = _segments.length;
+    if (len != other._segments.length) return false;
+    for (int i = 0; i < len; i++) {
+      if (_segments[i] != other._segments[i]) return false;
+    }
+    return true;
+  }
+
+  /// This is the [Jenkins hash function][1] but using masking to keep
+  /// values in SMI range.
+  /// [1]: http://en.wikipedia.org/wiki/Jenkins_hash_function
+  // TODO(jmesserly): should reuse this instead, see
+  // https://code.google.com/p/dart/issues/detail?id=11617
+  int get hashCode {
+    int hash = 0;
+    for (int i = 0, len = _segments.length; i < len; i++) {
+      hash = 0x1fffffff & (hash + _segments[i].hashCode);
+      hash = 0x1fffffff & (hash + ((0x0007ffff & hash) << 10));
+      hash = hash ^ (hash >> 6);
+    }
+    hash = 0x1fffffff & (hash + ((0x03ffffff & hash) <<  3));
+    hash = hash ^ (hash >> 11);
+    return 0x1fffffff & (hash + ((0x00003fff & hash) << 15));
+  }
+
+  /// Returns the current value of the path from the provided [obj]ect.
+  getValueFrom(Object obj) {
+    if (!isValid) return null;
+    for (var segment in _segments) {
+      if (obj == null) return null;
+      obj = _getObjectProperty(obj, segment);
+    }
+    return obj;
+  }
+
+  /// Attempts to set the [value] of the path from the provided [obj]ect.
+  /// Returns true if and only if the path was reachable and set.
+  bool setValueFrom(Object obj, Object value) {
+    var end = _segments.length - 1;
+    if (end < 0) return false;
+    for (int i = 0; i < end; i++) {
+      if (obj == null) return false;
+      obj = _getObjectProperty(obj, _segments[i]);
+    }
+    return _setObjectProperty(obj, _segments[end], value);
+  }
+
+  void _iterateObjects(Object obj, void observe(obj, prop)) {
+    if (!isValid || isEmpty) return;
+
+    int i = 0, last = _segments.length - 1;
+    while (obj != null) {
+      // _segments[i] is passed to indicate that we are only observing that
+      // property of obj. See observe declaration in _ObservedSet.
+      observe(obj, _segments[i]);
+
+      if (i >= last) break;
+      obj = _getObjectProperty(obj, _segments[i++]);
+    }
+  }
+
+  // Dart note: it doesn't make sense to have compiledGetValueFromFn in Dart.
+}
+
+
+/// Visible only for testing:
+getSegmentsOfPropertyPathForTesting(p) => p._segments;
+
+class _InvalidPropertyPath extends PropertyPath {
+  static final _instance = new _InvalidPropertyPath();
+
+  bool get isValid => false;
+  _InvalidPropertyPath() : super._([]);
+}
+
+bool _changeRecordMatches(record, key) {
+  if (record is PropertyChangeRecord) {
+    return (record as PropertyChangeRecord).name == key;
+  }
+  if (record is MapChangeRecord) {
+    if (key is Symbol) key = smoke.symbolToName(key);
+    return (record as MapChangeRecord).key == key;
+  }
+  return false;
+}
+
+/// Properties in [Map] that need to be read as properties and not as keys in
+/// the map. We exclude methods ('containsValue', 'containsKey', 'putIfAbsent',
+/// 'addAll', 'remove', 'clear', 'forEach') because there is no use in reading
+/// them as part of path-observer segments.
+const _MAP_PROPERTIES = const [#keys, #values, #length, #isEmpty, #isNotEmpty];
+
+_getObjectProperty(object, property) {
+  if (object == null) return null;
+
+  if (property is int) {
+    if (object is List && property >= 0 && property < object.length) {
+      return object[property];
+    }
+  } else if (property is String) {
+    return object[property];
+  } else if (property is Symbol) {
+    // Support indexer if available, e.g. Maps or polymer_expressions Scope.
+    // This is the default syntax used by polymer/nodebind and
+    // polymer/observe-js PathObserver.
+    // TODO(sigmund): should we also support using checking dynamically for
+    // whether the type practically implements the indexer API
+    // (smoke.hasInstanceMethod(type, const Symbol('[]')))?
+    if (object is Indexable<String, dynamic> ||
+        object is Map<String, dynamic> && !_MAP_PROPERTIES.contains(property)) {
+      return object[smoke.symbolToName(property)];
+    }
+    try {
+      return smoke.read(object, property);
+    } on NoSuchMethodError catch (e) {
+      // Rethrow, unless the type implements noSuchMethod, in which case we
+      // interpret the exception as a signal that the method was not found.
+      // Dart note: getting invalid properties is an error, unlike in JS where
+      // it returns undefined.
+      if (!smoke.hasNoSuchMethod(object.runtimeType)) rethrow;
+    }
+  }
+
+  if (_logger.isLoggable(Level.FINER)) {
+    _logger.finer("can't get $property in $object");
+  }
+  return null;
+}
+
+bool _setObjectProperty(object, property, value) {
+  if (object == null) return false;
+
+  if (property is int) {
+    if (object is List && property >= 0 && property < object.length) {
+      object[property] = value;
+      return true;
+    }
+  } else if (property is Symbol) {
+    // Support indexer if available, e.g. Maps or polymer_expressions Scope.
+    if (object is Indexable<String, dynamic> ||
+        object is Map<String, dynamic> && !_MAP_PROPERTIES.contains(property)) {
+      object[smoke.symbolToName(property)] = value;
+      return true;
+    }
+    try {
+      smoke.write(object, property, value);
+      return true;
+    } on NoSuchMethodError catch (e, s) {
+      if (!smoke.hasNoSuchMethod(object.runtimeType)) rethrow;
+    }
+  }
+
+  if (_logger.isLoggable(Level.FINER)) {
+    _logger.finer("can't set $property in $object");
+  }
+  return false;
+}
+
+// From: https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js
+
+final _identRegExp = () {
+  const identStart = '[\$_a-zA-Z]';
+  const identPart = '[\$_a-zA-Z0-9]';
+  return new RegExp('^$identStart+$identPart*\$');
+}();
+
+_isIdent(s) => _identRegExp.hasMatch(s);
+
+// Dart note: refactored to convert to codepoints once and operate on codepoints
+// rather than characters.
+class _PathParser {
+  List keys = [];
+  int index = -1;
+  String key;
+
+  final Map<String, List<String>> _pathStateMachine = {
+    'beforePath': {
+      'ws': ['beforePath'],
+      'ident': ['inIdent', 'append'],
+      '[': ['beforeElement'],
+      'eof': ['afterPath']
+    },
+
+    'inPath': {
+      'ws': ['inPath'],
+      '.': ['beforeIdent'],
+      '[': ['beforeElement'],
+      'eof': ['afterPath']
+    },
+
+    'beforeIdent': {
+      'ws': ['beforeIdent'],
+      'ident': ['inIdent', 'append']
+    },
+
+    'inIdent': {
+      'ident': ['inIdent', 'append'],
+      '0': ['inIdent', 'append'],
+      'number': ['inIdent', 'append'],
+      'ws': ['inPath', 'push'],
+      '.': ['beforeIdent', 'push'],
+      '[': ['beforeElement', 'push'],
+      'eof': ['afterPath', 'push']
+    },
+
+    'beforeElement': {
+      'ws': ['beforeElement'],
+      '0': ['afterZero', 'append'],
+      'number': ['inIndex', 'append'],
+      "'": ['inSingleQuote', 'append', ''],
+      '"': ['inDoubleQuote', 'append', '']
+    },
+
+    'afterZero': {
+      'ws': ['afterElement', 'push'],
+      ']': ['inPath', 'push']
+    },
+
+    'inIndex': {
+      '0': ['inIndex', 'append'],
+      'number': ['inIndex', 'append'],
+      'ws': ['afterElement'],
+      ']': ['inPath', 'push']
+    },
+
+    'inSingleQuote': {
+      "'": ['afterElement'],
+      'eof': ['error'],
+      'else': ['inSingleQuote', 'append']
+    },
+
+    'inDoubleQuote': {
+      '"': ['afterElement'],
+      'eof': ['error'],
+      'else': ['inDoubleQuote', 'append']
+    },
+
+    'afterElement': {
+      'ws': ['afterElement'],
+      ']': ['inPath', 'push']
+    }
+  };
+
+  /// From getPathCharType: determines the type of a given [code]point.
+  String _getPathCharType(code) {
+    if (code == null) return 'eof';
+    switch(code) {
+      case 0x5B: // [
+      case 0x5D: // ]
+      case 0x2E: // .
+      case 0x22: // "
+      case 0x27: // '
+      case 0x30: // 0
+        return _char(code);
+
+      case 0x5F: // _
+      case 0x24: // $
+        return 'ident';
+
+      case 0x20: // Space
+      case 0x09: // Tab
+      case 0x0A: // Newline
+      case 0x0D: // Return
+      case 0xA0:  // No-break space
+      case 0xFEFF:  // Byte Order Mark
+      case 0x2028:  // Line Separator
+      case 0x2029:  // Paragraph Separator
+        return 'ws';
+    }
+
+    // a-z, A-Z
+    if ((0x61 <= code && code <= 0x7A) || (0x41 <= code && code <= 0x5A))
+      return 'ident';
+
+    // 1-9
+    if (0x31 <= code && code <= 0x39)
+      return 'number';
+
+    return 'else';
+  }
+
+  static String _char(int codepoint) => new String.fromCharCodes([codepoint]);
+
+  void push() {
+    if (key == null) return;
+
+    // Dart note: we store the keys with different types, rather than
+    // parsing/converting things later in toString.
+    if (_isIdent(key)) {
+      keys.add(smoke.nameToSymbol(key));
+    } else {
+      var index = int.parse(key, radix: 10, onError: (_) => null);
+      keys.add(index != null ? index : key);
+    }
+    key = null;
+  }
+
+  void append(newChar) {
+    key = (key == null) ? newChar : '$key$newChar';
+  }
+
+  bool _maybeUnescapeQuote(String mode, codePoints) {
+    if (index >= codePoints.length) return false;
+    var nextChar = _char(codePoints[index + 1]);
+    if ((mode == 'inSingleQuote' && nextChar == "'") ||
+        (mode == 'inDoubleQuote' && nextChar == '"')) {
+      index++;
+      append(nextChar);
+      return true;
+    }
+    return false;
+  }
+
+  /// Returns the parsed keys, or null if there was a parse error.
+  List<String> parse(String path) {
+    var codePoints = stringToCodepoints(path);
+    var mode = 'beforePath';
+
+    while (mode != null) {
+      index++;
+      var c = index >= codePoints.length ? null : codePoints[index];
+
+      if (c != null &&
+          _char(c) == '\\' && _maybeUnescapeQuote(mode, codePoints)) continue;
+
+      var type = _getPathCharType(c);
+      if (mode == 'error') return null;
+
+      var typeMap = _pathStateMachine[mode];
+      var transition = typeMap[type];
+      if (transition == null) transition = typeMap['else'];
+      if (transition == null) return null; // parse error;
+
+      mode = transition[0];
+      var actionName = transition.length > 1 ? transition[1] : null;
+      if (actionName == 'push' && key != null) push();
+      if (actionName == 'append') {
+        var newChar = transition.length > 2 && transition[2] != null
+            ? transition[2] : _char(c);
+        append(newChar);
+      }
+
+      if (mode == 'afterPath') return keys;
+    }
+    return null; // parse error
+  }
+}
+
+final Logger _logger = new Logger('observe.PathObserver');
+
+
+/// This is a simple cache. It's like LRU but we don't update an item on a
+/// cache hit, because that would require allocation. Better to let it expire
+/// and reallocate the PropertyPath.
+// TODO(jmesserly): this optimization is from observe-js, how valuable is it in
+// practice?
+final _pathCache = new LinkedHashMap<String, PropertyPath>();
+
+/// The size of a path like "foo.bar" is approximately 160 bytes, so this
+/// reserves ~16Kb of memory for recently used paths. Since paths are frequently
+/// reused, the theory is that this ends up being a good tradeoff in practice.
+// (Note: the 160 byte estimate is from Dart VM 1.0.0.10_r30798 on x64 without
+// using UnmodifiableListView in PropertyPath)
+const int _pathCacheLimit = 100;
+
+/// [CompoundObserver] is a [Bindable] object which knows how to listen to
+/// multiple values (registered via [addPath] or [addObserver]) and invoke a
+/// callback when one or more of the values have changed.
+///
+///    var obj = new ObservableMap.from({'a': 1, 'b': 2});
+///    var otherObj = new ObservableMap.from({'c': 3});
+///
+///    var observer = new CompoundObserver()
+///      ..addPath(obj, 'a');
+///      ..addObserver(new PathObserver(obj, 'b'));
+///      ..addPath(otherObj, 'c');
+///      ..open((values) {
+///        for (int i = 0; i < values.length; i++) {
+///          print('The value at index $i is now ${values[i]}');
+///        }
+///      });
+///
+///   obj['a'] = 10; // print will be triggered async
+///
+class CompoundObserver extends _Observer implements Bindable {
+  _ObservedSet _directObserver;
+  bool _reportChangesOnOpen;
+  List _observed = [];
+
+  CompoundObserver([this._reportChangesOnOpen = false]) {
+    _value = [];
+  }
+
+  int get _reportArgumentCount => 3;
+
+  /// Initiates observation and returns the initial value.
+  /// The callback will be passed the updated [value], and may optionally be
+  /// declared to take a second argument, which will contain the previous value.
+  ///
+  /// Implementation note: a third argument can also be declared, which will
+  /// receive a list of objects and paths, such that `list[2 * i]` will access
+  /// the object and `list[2 * i + 1]` will access the path, where `i` is the
+  /// order of the [addPath] call. This parameter is only used by
+  /// `package:polymer` as a performance optimization, and should not be relied
+  /// on in new code.
+  open(callback) => super.open(callback);
+
+  void _connect() {
+    for (var i = 0; i < _observed.length; i += 2) {
+      var object = _observed[i];
+      if (!identical(object, _observerSentinel)) {
+        _directObserver = new _ObservedSet(this, object);
+        break;
+      }
+    }
+
+    _check(skipChanges: !_reportChangesOnOpen);
+  }
+
+  void _disconnect() {
+    for (var i = 0; i < _observed.length; i += 2) {
+      if (identical(_observed[i], _observerSentinel)) {
+        _observed[i + 1].close();
+      }
+    }
+
+    _observed = null;
+    _value = null;
+
+    if (_directObserver != null) {
+      _directObserver.close(this);
+      _directObserver = null;
+    }
+  }
+
+  /// Adds a dependency on the property [path] accessed from [object].
+  /// [path] can be a [PropertyPath] or a [String]. If it is omitted an empty
+  /// path will be used.
+  void addPath(Object object, [path]) {
+    if (_isOpen || _isClosed) {
+      throw new StateError('Cannot add paths once started.');
+    }
+
+    path = new PropertyPath(path);
+    _observed..add(object)..add(path);
+    if (!_reportChangesOnOpen) return;
+    _value.add(path.getValueFrom(object));
+  }
+
+  void addObserver(Bindable observer) {
+    if (_isOpen || _isClosed) {
+      throw new StateError('Cannot add observers once started.');
+    }
+
+    _observed..add(_observerSentinel)..add(observer);
+    if (!_reportChangesOnOpen) return;
+    _value.add(observer.open((_) => deliver()));
+  }
+
+  void _iterateObjects(void observe(obj, prop)) {
+    for (var i = 0; i < _observed.length; i += 2) {
+      var object = _observed[i];
+      if (!identical(object, _observerSentinel)) {
+        (_observed[i + 1] as PropertyPath)._iterateObjects(object, observe);
+      }
+    }
+  }
+
+  bool _check({bool skipChanges: false}) {
+    bool changed = false;
+    _value.length = _observed.length ~/ 2;
+    var oldValues = null;
+    for (var i = 0; i < _observed.length; i += 2) {
+      var object = _observed[i];
+      var path = _observed[i + 1];
+      var value;
+      if (identical(object, _observerSentinel)) {
+        var observable = path as Bindable;
+        value = _state == _Observer._UNOPENED ?
+            observable.open((_) => this.deliver()) :
+            observable.value;
+      } else {
+        value = (path as PropertyPath).getValueFrom(object);
+      }
+
+      if (skipChanges) {
+        _value[i ~/ 2] = value;
+        continue;
+      }
+
+      if (value == _value[i ~/ 2]) continue;
+
+      // don't allocate this unless necessary.
+      if (_notifyArgumentCount >= 2) {
+        if (oldValues == null) oldValues = new Map();
+        oldValues[i ~/ 2] = _value[i ~/ 2];
+      }
+
+      changed = true;
+      _value[i ~/ 2] = value;
+    }
+
+    if (!changed) return false;
+
+    // TODO(rafaelw): Having _observed as the third callback arg here is
+    // pretty lame API. Fix.
+    _report(_value, oldValues, _observed);
+    return true;
+  }
+}
+
+/// An object accepted by [PropertyPath] where properties are read and written
+/// as indexing operations, just like a [Map].
+abstract class Indexable<K, V> {
+  V operator [](K key);
+  operator []=(K key, V value);
+}
+
+const _observerSentinel = const _ObserverSentinel();
+class _ObserverSentinel { const _ObserverSentinel(); }
+
+// Visible for testing
+get observerSentinelForTesting => _observerSentinel;
+
+// A base class for the shared API implemented by PathObserver and
+// CompoundObserver and used in _ObservedSet.
+abstract class _Observer extends Bindable {
+  Function _notifyCallback;
+  int _notifyArgumentCount;
+  var _value;
+
+  // abstract members
+  void _iterateObjects(void observe(obj, prop));
+  void _connect();
+  void _disconnect();
+  bool _check({bool skipChanges: false});
+
+  static int _UNOPENED = 0;
+  static int _OPENED = 1;
+  static int _CLOSED = 2;
+  int _state = _UNOPENED;
+  bool get _isOpen => _state == _OPENED;
+  bool get _isClosed => _state == _CLOSED;
+
+  /// The number of arguments the subclass will pass to [_report].
+  int get _reportArgumentCount;
+
+  open(callback) {
+    if (_isOpen || _isClosed) {
+      throw new StateError('Observer has already been opened.');
+    }
+
+    if (smoke.minArgs(callback) > _reportArgumentCount) {
+      throw new ArgumentError('callback should take $_reportArgumentCount or '
+          'fewer arguments');
+    }
+
+    _notifyCallback = callback;
+    _notifyArgumentCount = min(_reportArgumentCount, smoke.maxArgs(callback));
+
+    _connect();
+    _state = _OPENED;
+    return _value;
+  }
+
+  get value => _discardChanges();
+
+  void close() {
+    if (!_isOpen) return;
+
+    _disconnect();
+    _value = null;
+    _notifyCallback = null;
+    _state = _CLOSED;
+  }
+
+  _discardChanges() {
+    _check(skipChanges: true);
+    return _value;
+  }
+
+  void deliver() {
+    if (_isOpen) _dirtyCheck();
+  }
+
+  bool _dirtyCheck() {
+    var cycles = 0;
+    while (cycles < _MAX_DIRTY_CHECK_CYCLES && _check()) {
+      cycles++;
+    }
+    return cycles > 0;
+  }
+
+  void _report(newValue, oldValue, [extraArg]) {
+    try {
+      switch (_notifyArgumentCount) {
+        case 0: _notifyCallback(); break;
+        case 1: _notifyCallback(newValue); break;
+        case 2: _notifyCallback(newValue, oldValue); break;
+        case 3: _notifyCallback(newValue, oldValue, extraArg); break;
+      }
+    } catch (e, s) {
+      // Deliver errors async, so if a single callback fails it doesn't prevent
+      // other things from working.
+      new Completer().completeError(e, s);
+    }
+  }
+}
+
+/// The observedSet abstraction is a perf optimization which reduces the total
+/// number of Object.observe observations of a set of objects. The idea is that
+/// groups of Observers will have some object dependencies in common and this
+/// observed set ensures that each object in the transitive closure of
+/// dependencies is only observed once. The observedSet acts as a write barrier
+/// such that whenever any change comes through, all Observers are checked for
+/// changed values.
+///
+/// Note that this optimization is explicitly moving work from setup-time to
+/// change-time.
+///
+/// TODO(rafaelw): Implement "garbage collection". In order to move work off
+/// the critical path, when Observers are closed, their observed objects are
+/// not Object.unobserve(d). As a result, it's possible that if the observedSet
+/// is kept open, but some Observers have been closed, it could cause "leaks"
+/// (prevent otherwise collectable objects from being collected). At some
+/// point, we should implement incremental "gc" which keeps a list of
+/// observedSets which may need clean-up and does small amounts of cleanup on a
+/// timeout until all is clean.
+class _ObservedSet {
+  /// To prevent sequential [PathObserver]s and [CompoundObserver]s from
+  /// observing the same object, we check if they are observing the same root
+  /// as the most recently created observer, and if so merge it into the
+  /// existing _ObservedSet.
+  ///
+  /// See <https://github.com/Polymer/observe-js/commit/f0990b1> and
+  /// <https://codereview.appspot.com/46780044/>.
+  static _ObservedSet _lastSet;
+
+  /// The root object for a [PathObserver]. For a [CompoundObserver], the root
+  /// object of the first path observed. This is used by the constructor to
+  /// reuse an [_ObservedSet] that starts from the same object.
+  Object _rootObject;
+
+  /// Subset of properties in [_rootObject] that we care about.
+  Set _rootObjectProperties;
+
+  /// Observers associated with this root object, in birth order.
+  final List<_Observer> _observers = [];
+
+  // Dart note: the JS implementation is O(N^2) because Array.indexOf is used
+  // for lookup in this array. We use HashMap to avoid this problem. It
+  // also gives us a nice way of tracking the StreamSubscription.
+  Map<Object, StreamSubscription> _objects;
+
+  factory _ObservedSet(_Observer observer, Object rootObject) {
+    if (_lastSet == null || !identical(_lastSet._rootObject, rootObject)) {
+      _lastSet = new _ObservedSet._(rootObject);
+    }
+    _lastSet.open(observer, rootObject);
+  }
+
+  _ObservedSet._(rootObject)
+      : _rootObject = rootObject,
+        _rootObjectProperties = rootObject == null ? null : new Set();
+
+  void open(_Observer obs, Object rootObject) {
+    if (_rootObject == null) {
+      _rootObject = rootObject;
+      _rootObjectProperties = new Set();
+    }
+
+    _observers.add(obs);
+    obs._iterateObjects(observe);
+  }
+
+  void close(_Observer obs) {
+    if (_observers.isNotEmpty) return;
+
+    if (_objects != null) {
+      for (var sub in _objects) sub.cancel();
+      _objects = null;
+    }
+    _rootObject = null;
+    _rootObjectProperties = null;
+    if (identical(_lastSet, this)) _lastSet = null;
+  }
+
+  /// Observe now takes a second argument to indicate which property of an
+  /// object is being observed, so we don't trigger change notifications on
+  /// changes to unrelated properties.
+  void observe(Object obj, Object prop) {
+    if (identical(obj, _rootObject)) _rootObjectProperties.add(prop);
+    if (obj is ObservableList) _observeStream(obj.listChanges);
+    if (obj is Observable) _observeStream(obj.changes);
+  }
+
+  void _observeStream(Stream stream) {
+    // TODO(jmesserly): we hash on streams as we have two separate change
+    // streams for ObservableList. Not sure if that is the design we will use
+    // going forward.
+
+    if (_objects == null) _objects = new HashMap();
+    if (!_objects.containsKey(stream)) {
+      _objects[stream] = stream.listen(_callback);
+    }
+  }
+
+  /// Whether we can ignore all change events in [records]. This is true if all
+  /// records are for properties in the [_rootObject] and we are not observing
+  /// any of those properties. Changes on objects other than [_rootObject], or
+  /// changes for properties in [_rootObjectProperties] can't be ignored.
+  // Dart note: renamed from `allRootObjNonObservedProps` in the JS code.
+  bool _canIgnoreRecords(List<ChangeRecord> records) {
+    for (var rec in records) {
+      if (rec is PropertyChangeRecord) {
+        if (!identical(rec.object, _rootObject) ||
+            _rootObjectProperties.contains(rec.name)) {
+          return false;
+        }
+      } else if (rec is ListChangeRecord) {
+        if (!identical(rec.object, _rootObject) ||
+            _rootObjectProperties.contains(rec.index)) {
+          return false;
+        }
+      } else {
+        // TODO(sigmund): consider adding object to MapChangeRecord, and make
+        // this more precise.
+        return false;
+      }
+    }
+    return true;
+  }
+
+  void _callback(records) {
+    if (_canIgnoreRecords(records)) return;
+    for (var observer in _observers.toList(growable: false)) {
+      if (observer._isOpen) observer._iterateObjects(observe);
+    }
+
+    for (var observer in _observers.toList(growable: false)) {
+      if (observer._isOpen) observer._check();
+    }
+  }
+}
+
+const int _MAX_DIRTY_CHECK_CYCLES = 1000;