big update to observe, template_binding, polymer

pkg/polymer_expressions/lib/polymer_expressions.dart

Index: pkg/polymer_expressions/lib/polymer_expressions.dart
diff --git a/pkg/polymer_expressions/lib/polymer_expressions.dart b/pkg/polymer_expressions/lib/polymer_expressions.dart
index 0d4d06192fe05ad9292a8e0f768e7f5b427f2d3e..36ea3524adbc8d7acfc4d1e2e8598353872a4eb7 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -81,17 +81,23 @@ class PolymerExpressions extends BindingDelegate {
       return null;
     }
 
-    return (model, node) {
+    return (model, node, oneTime) {
       if (model is! Scope) {
         model = new Scope(model: model, variables: globals);
       }
+      var converter = null;
       if (node is Element && name == "class") {
-        return new _Binding(expr, model, _classAttributeConverter);
+        converter = _classAttributeConverter;
       }
       if (node is Element && name == "style") {
-        return new _Binding(expr, model, _styleAttributeConverter);
+        converter = _styleAttributeConverter;
       }
-      return new _Binding(expr, model);
+
+      if (oneTime) {
+        return _Binding._oneTime(expr, model, converter);
+      }
+
+      return new _Binding(expr, model, converter);
     };
   }
 
@@ -99,50 +105,82 @@ class PolymerExpressions extends BindingDelegate {
       model is Scope ? model : new Scope(model: model, variables: globals);
 }
 
-class _Binding extends ChangeNotifier {
+class _Binding extends Bindable {
   final Scope _scope;
-  final ExpressionObserver _expr;
   final _converter;
+  Expression _expr;
+  Function _callback;
+  StreamSubscription _sub;
   var _value;
 
-  _Binding(Expression expr, Scope scope, [this._converter])
-      : _expr = observe(expr, scope),
-        _scope = scope {
-    _expr.onUpdate.listen(_setValue).onError((e) {
-      _logger.warning("Error evaluating expression '$_expr': ${e.message}");
-    });
+  _Binding(this._expr, this._scope, [this._converter]);
+
+  static _oneTime(Expression expr, Scope scope, [converter]) {
     try {
-      update(_expr, _scope);
-      _setValue(_expr.currentValue);
+      return _convertValue(eval(expr, scope), scope, converter);
     } on EvalException catch (e) {
-      _logger.warning("Error evaluating expression '$_expr': ${e.message}");
+      _logger.warning("Error evaluating expression '$expr': ${e.message}");
+      return null;
     }
   }
 
   _setValue(v) {
-    var oldValue = _value;
+    _value = _convertValue(v, _scope, _converter);
+    if (_callback != null) _callback(_value);
+  }
+
+  static _convertValue(v, scope, converter) {
     if (v is Comprehension) {
       // convert the Comprehension into a list of scopes with the loop
       // variable added to the scope
-      _value = v.iterable.map((i) {
+      return v.iterable.map((i) {
         var vars = new Map();
         vars[v.identifier] = i;
-        Scope childScope = new Scope(parent: _scope, variables: vars);
+        Scope childScope = new Scope(parent: scope, variables: vars);
         return childScope;
       }).toList(growable: false);
     } else {
-      _value = (_converter == null) ? v : _converter(v);
+      return converter == null ? v : converter(v);
     }
-    notifyPropertyChange(#value, oldValue, _value);
   }
 
-  @reflectable get value => _value;
+  get value {
+    if (_callback != null) return _value;
+    return _oneTime(_expr, _scope, _converter);
+  }
 
-  @reflectable set value(v) {
+  set value(v) {
     try {
       assign(_expr, v, _scope);
     } on EvalException catch (e) {
       _logger.warning("Error evaluating expression '$_expr': ${e.message}");
     }
   }
+
+  open(callback(value)) {
+    if (_callback != null) throw new StateError('already open');
+
+    _callback = callback;
+    final expr = observe(_expr, _scope);
+    _expr = expr;
+    _sub = expr.onUpdate.listen(_setValue)..onError((e) {
+      _logger.warning("Error evaluating expression '$_expr': ${e.message}");
+    });
+    try {
+      update(expr, _scope);
+      _value = _convertValue(expr.currentValue, _scope, _converter);
+    } on EvalException catch (e) {
+      _logger.warning("Error evaluating expression '$_expr': ${e.message}");
+    }
+    return _value;
+  }
+
+  void close() {
+    if (_callback == null) return;
+
+    _sub.cancel();
+    _sub = null;
+    _expr = (_expr as ExpressionObserver).expression;
+    _callback = null;
+  }
 }