work in progress: observable implementation using detailed change records

lib/src/observe/impl.dart

+// Copyright (c) 2012, 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.
+
+/**
+ * Runtime implementation for the observe library.
+ * Note that this is a runtime library; it is not part of the compiler.
+ */
+library web_ui.src.observe.impl;
+
+import 'package:web_ui/observe/expression.dart';
+import 'package:web_ui/observe/observable.dart';
+
+ExpressionObserverImpl activeObserver;
+
+
+class ReadInfo {
+  /** See [ChangeRecord.type]. */
+  final int type;
+
+  /** See [ChangeRecord.name]. */
+  final name;
+
+  ReadInfo(this.type, this.name);
+
+  bool operator==(other) =>
+      other is ReadInfo && other.type == type && other.name == name;
+
+  int get hashCode => type * 31 + name.hashCode;
+}
+
+class ExpressionObserverImpl {
+  final ObservableExpression _expression;
+  final ExpressionObserver _observer;
+
+  // TODO(jmesserly): should we provide the old value? This will keep it
+  // alive until we have a new value. Watchers needed to keep it alive anyway,
+  // but we don't need it in observers, except to pass it to the
+  // ExpressionObserver.
+  Object _value;
+
+  Observable _deliverHelper;
+
+  final Map<Observable, Map<Object, int>> _reads = new Map();
+  final List<ChangeUnobserver> _unobservers = [];
+
+  ExpressionObserverImpl(this._expression, this._observer);
+
+  void observe() {
+    // If an observe call starts another observation, we need to make sure that
+    // the outer observe is tracked correctly.
+    var parent = activeObserver;
+    activeObserver = this;
+    try {
+      _value = _expression();
+    } catch (e, trace) {
+      onObserveUnhandledError(e, trace, _expression);
+      _value = null;
+    }
+
+    _reads.forEach(_watchForChange);
+    _reads.clear();
+
+    // TODO(jmesserly): should we add our changes to the parent?
+    assert(activeObserver == this);
+    activeObserver = parent;
+
+    _observeValue();
+  }
+
+  void _observeValue() {
+    if (_value is! Observable) return;
+
+    _unobservers.add((_value as Observable).observe((_) {
+      _observer(new ExpressionChange(_value, _value));
+    }));
+  }
+
+  void addRead(Observable target, int type, name) {
+    var reads = _reads.putIfAbsent(target, () => new Map());
+    // We would like to easily match against the name and the type. So use a
+    // mask.
+    int mask = reads[name];
+    if (mask == null) mask = 0;
+    reads[name] = mask | type;
+  }
+
+  void _watchForChange(Observable target, Map<Object, int> reads) {
+    _unobservers.add(target.observe((changes) {
+      if (_deliverHelper != null) return;
+      for (var change in changes) {
+        int mask = reads[change.name];
+        if (mask != null && (mask & change.type) != 0) {
+          // Rather than hook directly into [deliverChangesSync], we use this
+          // object to trigger a ChangeRecord that is handled in the same
+          // deliverChangesSync batch.
+          _deliverHelper = new Observable()
+              ..observe(_deliver)
+              ..notifyChange(ChangeRecord.FIELD, '', 0, 1);
+          break;
+        }
+      }
+    }));
+  }
+
+  void unobserve() {
+    for (var unobserver in _unobservers) {
+      unobserver();
+    }
+    _unobservers.clear();
+    _deliverHelper = null;
+  }
+
+  // _deliver does two things:
+  // 1. Evaluate the expression to compute the new value.
+  // 2. Invoke observer for this expression.
+  //
+  // Note: if you mutate a shared value from one observer, future
+  // observers will see the updated value. Essentially, we collapse
+  // the two change notifications into one.
+  //
+  // We could try someting else, but the current order has benefits:
+  // it preserves the invariant that ExpressionChange.newValue equals the
+  // current value of the expression.
+  void _deliver(_) {
+    var oldValue = _value;
+
+    // Call the expression again to compute the new value, and to get the new
+    // list of dependencies.
+    unobserve();
+    observe();
+
+    bool equal;
+    try {
+      equal = oldValue == _value;
+    } catch (e, trace) {
+      onObserveUnhandledError(e, trace, null);
+      return;
+    }
+
+    if (!equal) {
+      try {
+        _observer(new ExpressionChange(oldValue, _value));
+      } catch (e, trace) {
+        onObserveUnhandledError(e, trace, _observer);
+      }
+    }
+  }
+}