Add location information to watchers to make them more debuggable.

lib/templating.dart

Index: lib/templating.dart
diff --git a/lib/templating.dart b/lib/templating.dart
index d6541fa4d4d0a3324b0b783cdc8dab3970be3163..8eb10382b9940da436ba1da96574c0d0a0ac9dc9 100644
--- a/lib/templating.dart
+++ b/lib/templating.dart
@@ -147,7 +147,8 @@ void changeCssClasses(elem, ChangeRecord change) {
  *         bindCssClasses(e, () => class1);
  *         bindCssClasses(e, () => class2);
  */
-ChangeUnobserver bindCssClasses(Element elem, dynamic exp()) {
+ChangeUnobserver bindCssClasses(Element elem, dynamic exp(),
+    [String debugLocation]) {
   return watchAndInvoke(exp, (e) {
     if (e.changes != null) {
       for (var change in e.changes) changeCssClasses(elem, change);
@@ -155,13 +156,14 @@ ChangeUnobserver bindCssClasses(Element elem, dynamic exp()) {
       updateCssClass(elem, false, e.oldValue);
       updateCssClass(elem, true, e.newValue);
     }
-  }, 'css-class-bind');
+  }, 'css-class-bind', debugLocation);
 }
 
 /** Bind the result of [exp] to the style attribute in [elem]. */
-ChangeUnobserver bindStyle(Element elem, Map<String, String> exp()) {
+ChangeUnobserver bindStyle(Element elem, Map<String, String> exp(),
+    [String debugLocation]) {
   return watchAndInvoke(exp, (e) => updateStyle(elem, e.oldValue, e.newValue),
-      'css-style-bind');
+      'css-style-bind', debugLocation);
 }
 
 /**
@@ -249,22 +251,24 @@ class Listener extends TemplateItem {
   }
 }
 
-/** Represents a generic data binding and a corresponding action. */
-class Binding extends TemplateItem {
+/** Common logic for all bindings. */
+abstract class Binding extends TemplateItem {
   final exp;
-  final ChangeObserver action;
   final bool isFinal;
+  final String debugLocation;
   ChangeUnobserver stopper;
 
-  Binding(this.exp, this.action, this.isFinal);
+  Binding(this.exp, this.isFinal)
+      : debugLocation = verboseDebugMessages && readCurrentStackTrace != null
+            ? readCurrentStackTrace() : null;
 
   void insert() {
     if (isFinal) {
-      action(new ChangeNotification(null, exp()));
+      invokeCallback();
     } else if (stopper != null) {
       throw new StateError('binding already attached');
     } else {
-      stopper = watchAndInvoke(exp, action, 'generic-binding');
+      stopper = registerAndInvoke();
     }
   }
 
@@ -274,109 +278,81 @@ class Binding extends TemplateItem {
       stopper = null;
     }
   }
+
+  /** Invokes the action associated with this binding. */
+  void invokeCallback();
+
+  /**
+   * Registers the watcher and invokes the action associated with this binding.
+   */
+  ChangeUnobserver registerAndInvoke();
+}
+
+/** Represents a generic data binding and a corresponding action. */
+class GenericBinding extends Binding {
+  final ChangeObserver action;
+
+  GenericBinding(exp, this.action, bool isFinal) : super(exp, isFinal);
+
+  void invokeCallback() => action(new ChangeNotification(null, exp()));
+
+  ChangeUnobserver registerAndInvoke() =>
+      watchAndInvoke(exp, action, 'generic-binding', debugLocation);
 }
 
 /** Represents a binding to a style attribute. */
-class StyleAttrBinding extends TemplateItem {
-  final exp;
+class StyleAttrBinding extends Binding {
   final Element elem;
-  final bool isFinal;
-  ChangeUnobserver stopper;
 
-  StyleAttrBinding(this.elem, this.exp, this.isFinal);
+  StyleAttrBinding(this.elem, exp, bool isFinal) : super(exp, isFinal);
 
-  void insert() {
-    if (isFinal) {
-      updateStyle(elem, null, exp());
-    } else if (stopper != null) {
-      throw new StateError('style binding already attached');
-    } else {
-      stopper = bindStyle(elem, exp);
-    }
-  }
+  void invokeCallback() => updateStyle(elem, null, exp());
 
-  void remove() {
-    if (!isFinal) {
-      stopper();
-      stopper = null;
-    }
-  }
+  ChangeUnobserver registerAndInvoke() => bindStyle(elem, exp, debugLocation);
 }
 
 /** Represents a binding to a class attribute. */
-class ClassAttrBinding extends TemplateItem {
+class ClassAttrBinding extends Binding {
   final Element elem;
-  final exp;
-  final bool isFinal;
-  ChangeUnobserver stopper;
 
-  ClassAttrBinding(this.elem, this.exp, this.isFinal);
+  ClassAttrBinding(this.elem, exp, bool isFinal) : super(exp, isFinal);
 
-  void insert() {
-    if (isFinal) {
-      updateCssClass(elem, true, exp());
-    } else if (stopper != null) {
-      throw new StateError('class binding already attached');
-    } else {
-      stopper = bindCssClasses(elem, exp);
-    }
-  }
+  void invokeCallback() => updateCssClass(elem, true, exp());
 
-  void remove() {
-    if (!isFinal) {
-      stopper();
-      stopper = null;
-    }
-  }
+  ChangeUnobserver registerAndInvoke() =>
+      bindCssClasses(elem, exp, debugLocation);
 }
 
 /**
  * Represents a one-way binding between a dart getter expression and a DOM
  * property, or conversely between a DOM property value and a dart property.
  */
-class DomPropertyBinding extends TemplateItem {
+class DomPropertyBinding extends Binding {
   /** Value updated by this binding. */
   final Setter setter;
 
   /**
-   * Getter that reads the value of the binding, either from a Dart expression
-   * or from a DOM property (which is internally also a Dart expression).
-   */
-  final Getter getter;
-
-  /**
    * Whether this is a binding that assigns a DOM attribute accepting URL
    * values. If so, the value assigned to the attribute needs to be sanitized.
    */
   final bool isUrl;
 
-  final bool isFinal;
-
-  ChangeUnobserver stopper;
-
-  DomPropertyBinding(this.getter, this.setter, this.isUrl, this.isFinal);
+  /**
+   * Creates a DOM property binding, where [getter] reads the value of the
+   * binding, either from a Dart expression or from a DOM property (which is
+   * internally also a Dart expression).
+   */
+  DomPropertyBinding(Getter getter, this.setter, this.isUrl, bool isFinal)
+      : super(getter, isFinal);
 
   void _safeSetter(value) {
     setter(isUrl ? sanitizeUri(value) : value);
   }
+  void invokeCallback() => _safeSetter(exp());
 
-  void insert() {
-    if (isFinal) {
-      _safeSetter(getter());
-    } else if (stopper != null) {
-      throw new StateError('data binding already attached.');
-    } else {
-      stopper = watchAndInvoke(getter, (e) => _safeSetter(e.newValue),
-          'dom-property-binding');
-    }
-  }
-
-  void remove() {
-    if (!isFinal) {
-      stopper();
-      stopper = null;
-    }
-  }
+  ChangeUnobserver registerAndInvoke() =>
+      watchAndInvoke(exp, (e) => _safeSetter(e.newValue),
+          'dom-property-binding', debugLocation);
 }
 
 /** Represents a component added within a template. */
@@ -420,13 +396,13 @@ class Template extends TemplateItem {
 
   /** Run [action] when [exp] changes (while this template is visible).  */
   void bind(exp, ChangeObserver action, bool isFinal) {
-    children.add(new Binding(exp, action, isFinal));
+    children.add(new GenericBinding(exp, action, isFinal));
   }
 
   /** Create and bind a [Node] to [exp] while this template is visible. */
   Node contentBind(Function exp, isFinal) {
     var bindNode = new Text('');
-    children.add(new Binding(() => '${exp()}', (e) {
+    children.add(new GenericBinding(() => '${exp()}', (e) {
       bindNode = updateBinding(exp(), bindNode, e.newValue);
     }, isFinal));
     return bindNode;