Move PathObserver to mdv_observe package

tools/dom/src/PathObserver.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.
-
-part of html;
-
-// This code is inspired by ChangeSummary:
-// https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js
-// ...which underlies MDV. Since we don't need the functionality of
-// ChangeSummary, we just implement what we need for data bindings.
-// This allows our implementation to be much simpler.
-
-// TODO(jmesserly): should we make these types stronger, and require
-// Observable objects? Currently, it is fine to say something like:
-//     var path = new PathObserver(123, '');
-//     print(path.value); // "123"
-//
-// Furthermore this degenerate case is allowed:
-//     var path = new PathObserver(123, 'foo.bar.baz.qux');
-//     print(path.value); // "null"
-//
-// Here we see that any invalid (i.e. not Observable) value will break the
-// path chain without producing an error or exception.
-//
-// Now the real question: should we do this? For the former case, the behavior
-// is correct but we could chose to handle it in the dart:html bindings layer.
-// For the latter case, it might be better to throw an error so users can find
-// the problem.
-
-
-/**
- * A data-bound path starting from a view-model or model object, for example
- * `foo.bar.baz`.
- *
- * When the [values] stream is being listened to, this will observe changes to
- * the object and any intermediate object along the path, and send [values]
- * accordingly. When all listeners are unregistered it will stop observing
- * the objects.
- *
- * This class is used to implement [Node.bind] and similar functionality.
- */
-// TODO(jmesserly): find a better home for this type.
-@Experimental
-class PathObserver {
-  /** The object being observed. */
-  final object;
-
-  /** The path string. */
-  final String path;
-
-  /** True if the path is valid, otherwise false. */
-  final bool _isValid;
-
-  // TODO(jmesserly): same issue here as ObservableMixin: is there an easier
-  // way to get a broadcast stream?
-  StreamController _values;
-  Stream _valueStream;
-
-  _PropertyObserver _observer, _lastObserver;
-
-  Object _lastValue;
-  bool _scheduled = false;
-
-  /**
-   * 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.
-   * See [PathObserver.values] and [PathObserver.value].
-   */
-  PathObserver(this.object, String path)
-    : path = path, _isValid = _isPathValid(path) {
-
-    // TODO(jmesserly): if the path is empty, or the object is! Observable, we
-    // can optimize the PathObserver to be more lightweight.
-
-    _values = new StreamController.broadcast(sync: true,
-                                             onListen: _observe,
-                                             onCancel: _unobserve);
-
-    if (_isValid) {
-      var segments = [];
-      for (var segment in path.trim().split('.')) {
-        if (segment == '') continue;
-        var index = int.parse(segment, onError: (_) {});
-        segments.add(index != null ? index : new Symbol(segment));
-      }
-
-      // Create the property observer linked list.
-      // Note that the structure of a path can't change after it is initially
-      // constructed, even though the objects along the path can change.
-      for (int i = segments.length - 1; i >= 0; i--) {
-        _observer = new _PropertyObserver(this, segments[i], _observer);
-        if (_lastObserver == null) _lastObserver = _observer;
-      }
-    }
-  }
-
-  // TODO(jmesserly): we could try adding the first value to the stream, but
-  // that delivers the first record async.
-  /**
-   * Listens to the stream, and invokes the [callback] immediately with the
-   * current [value]. This is useful for bindings, which want to be up-to-date
-   * immediately.
-   */
-  StreamSubscription bindSync(void callback(value)) {
-    var result = values.listen(callback);
-    callback(value);
-    return result;
-  }
-
-  // TODO(jmesserly): should this be a change record with the old value?
-  // TODO(jmesserly): should this be a broadcast stream? We only need
-  // single-subscription in the bindings system, so single sub saves overhead.
-  /**
-   * Gets the stream of values that were observed at this path.
-   * This returns a single-subscription stream.
-   */
-  Stream get values => _values.stream;
-
-  /** Force synchronous delivery of [values]. */
-  void _deliverValues() {
-    _scheduled = false;
-
-    var newValue = value;
-    if (!identical(_lastValue, newValue)) {
-      _values.add(newValue);
-      _lastValue = newValue;
-    }
-  }
-
-  void _observe() {
-    if (_observer != null) {
-      _lastValue = value;
-      _observer.observe();
-    }
-  }
-
-  void _unobserve() {
-    if (_observer != null) _observer.unobserve();
-  }
-
-  void _notifyChange() {
-    if (_scheduled) return;
-    _scheduled = true;
-
-    // TODO(jmesserly): should we have a guarenteed order with respect to other
-    // paths? If so, we could implement this fairly easily by sorting instances
-    // of this class by birth order before delivery.
-    queueChangeRecords(_deliverValues);
-  }
-
-  /** Gets the last reported value at this path. */
-  get value {
-    if (!_isValid) return null;
-    if (_observer == null) return object;
-    _observer.ensureValue(object);
-    return _lastObserver.value;
-  }
-
-  /** Sets the value at this path. */
-  void set value(Object value) {
-    // TODO(jmesserly): throw if property cannot be set?
-    // MDV seems tolerant of these error.
-    if (_observer == null || !_isValid) return;
-    _observer.ensureValue(object);
-    var last = _lastObserver;
-    if (_setObjectProperty(last._object, last._property, value)) {
-      // Technically, this would get updated asynchronously via a change record.
-      // However, it is nice if calling the getter will yield the same value
-      // that was just set. So we use this opportunity to update our cache.
-      last.value = value;
-    }
-  }
-}
-
-// TODO(jmesserly): these should go away in favor of mirrors!
-_getObjectProperty(object, property) {
-  if (object is List && property is int) {
-    if (property >= 0 && property < object.length) {
-      return object[property];
-    } else {
-      return null;
-    }
-  }
-
-  // TODO(jmesserly): what about length?
-  if (object is Map) return object[property];
-
-  if (object is Observable) return object.getValueWorkaround(property);
-
-  return null;
-}
-
-bool _setObjectProperty(object, property, value) {
-  if (object is List && property is int) {
-    object[property] = value;
-  } else if (object is Map) {
-    object[property] = value;
-  } else if (object is Observable) {
-    (object as Observable).setValueWorkaround(property, value);
-  } else {
-    return false;
-  }
-  return true;
-}
-
-
-class _PropertyObserver {
-  final PathObserver _path;
-  final _property;
-  final _PropertyObserver _next;
-
-  // TODO(jmesserly): would be nice not to store both of these.
-  Object _object;
-  Object _value;
-  StreamSubscription _sub;
-
-  _PropertyObserver(this._path, this._property, this._next);
-
-  get value => _value;
-
-  void set value(Object newValue) {
-    _value = newValue;
-    if (_next != null) {
-      if (_sub != null) _next.unobserve();
-      _next.ensureValue(_value);
-      if (_sub != null) _next.observe();
-    }
-  }
-
-  void ensureValue(object) {
-    // If we're observing, values should be up to date already.
-    if (_sub != null) return;
-
-    _object = object;
-    value = _getObjectProperty(object, _property);
-  }
-
-  void observe() {
-    if (_object is Observable) {
-      assert(_sub == null);
-      _sub = (_object as Observable).changes.listen(_onChange);
-    }
-    if (_next != null) _next.observe();
-  }
-
-  void unobserve() {
-    if (_sub == null) return;
-
-    _sub.cancel();
-    _sub = null;
-    if (_next != null) _next.unobserve();
-  }
-
-  void _onChange(List<ChangeRecord> changes) {
-    for (var change in changes) {
-      // TODO(jmesserly): what to do about "new Symbol" here?
-      // Ideally this would only preserve names if the user has opted in to
-      // them being preserved.
-      // TODO(jmesserly): should we drop observable maps with String keys?
-      // If so then we only need one check here.
-      if (change.changes(_property)) {
-        value = _getObjectProperty(_object, _property);
-        _path._notifyChange();
-        return;
-      }
-    }
-  }
-}
-
-// From: https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js
-
-const _pathIndentPart = r'[$a-z0-9_]+[$a-z0-9_\d]*';
-final _pathRegExp = new RegExp('^'
-    '(?:#?' + _pathIndentPart + ')?'
-    '(?:'
-      '(?:\\.' + _pathIndentPart + ')'
-    ')*'
-    r'$', caseSensitive: false);
-
-final _spacesRegExp = new RegExp(r'\s');
-
-bool _isPathValid(String s) {
-  s = s.replaceAll(_spacesRegExp, '');
-
-  if (s == '') return true;
-  if (s[0] == '.') return false;
-  return _pathRegExp.hasMatch(s);
-}