refactor(InputCheckboxDirective): fix API doc and refactor ng-true/false-value

- Refactor `NgTrueValue` and `NgFalseValue` so that it smells good!
- Refactor/clean-up InputCheckboxDirective.
- Update `input[type=checkbox]` API doc to reflect AngularDart behavior.

Closes #842

Author: chalin

lib/directive/module.dart

@@ -5,7 +5,6 @@
 * This package is imported for you as part of [angular.dart](#angular/angular),
 * and lists all of the basic directives that are part of Angular.
 *
-*
 */
 library angular.directive;
 
@@ -70,8 +69,8 @@ class NgDirectiveModule extends Module {
     value(ContentEditableDirective, null);
     value(NgModel, null);
     value(NgValue, new NgValue(null));
-    value(NgTrueValue, new NgTrueValue(null));
-    value(NgFalseValue, new NgFalseValue(null));
+    value(NgTrueValue, new NgTrueValue());
+    value(NgFalseValue, new NgFalseValue());
     value(NgSwitchDirective, null);
     value(NgSwitchWhenDirective, null);
     value(NgSwitchDefaultDirective, null);

lib/directive/ng_model.dart

@@ -265,17 +265,32 @@ class NgModel extends NgControl implements NgAttachAware {
 /**
  * Usage:
  *
- *     <input type="checkbox" ng-model="flag">
+ *     <input type="checkbox" 
+ *            ng-model="expr"
+ *            [ng-true-value="t_expr"]
+ *            [ng-false-value="f_expr"]
+ *            >
  *
- * This creates a two way databinding between the boolean expression specified
- * in ng-model and the checkbox input element in the DOM.  If the ng-model value
- * is falsy (i.e. one of `false`, `null`, and `0`), then the checkbox is
- * unchecked. Otherwise, it is checked.  Likewise, when the checkbox is checked,
- * the model value is set to true.  When unchecked, it is set to false.
+ * This creates a two way databinding between the `ng-model` expression
+ * and the checkbox input element state.
+ * 
+ * If the optional `ng-true-value` is absent then: if the model expression
+ * evaluates to true or to a nonzero [num], then the checkbox is checked; 
+ * otherwise, it is unchecked.
+ * 
+ * If `ng-true-value="t_expr"` is present, then: if the model expression
+ * evaluates to the same value as `t_expr` then the checkbox is checked; 
+ * otherwise, it is unchecked.
+ * 
+ * When the checkbox is checked, the model is set to the value of `t_expr` if
+ * present, true otherwise. When unchecked, it is set to the value of
+ * `f_expr` if present, false otherwise.
+ * 
+ * Also see [NgTrueValue] and [NgFalseValue].
  */
 @NgDirective(selector: 'input[type=checkbox][ng-model]')
 class InputCheckboxDirective {
-  final dom.InputElement inputElement;
+  final dom.CheckboxInputElement inputElement;
   final NgModel ngModel;
   final NgTrueValue ngTrueValue;
   final NgFalseValue ngFalseValue;
@@ -285,14 +300,13 @@ class InputCheckboxDirective {
                          this.scope, this.ngTrueValue, this.ngFalseValue) {
     ngModel.render = (value) {
       scope.rootScope.domWrite(() {
-        inputElement.checked = ngTrueValue.isValue(inputElement, value);
+        inputElement.checked = ngTrueValue.isValue(value);
       });
     };
     inputElement
-        ..onChange.listen((value) {
+        ..onChange.listen((_) {
           ngModel.viewValue = inputElement.checked
-              ? ngTrueValue.readValue(inputElement)
-              : ngFalseValue.readValue(inputElement);
+              ? ngTrueValue.value : ngFalseValue.value;
         })
         ..onBlur.listen((e) {
           ngModel.markAsTouched();
@@ -473,44 +487,45 @@ class NgValue {
 }
 
 /**
- * `ng-true-value` allows you to select any expression to be set to
- * `ng-model` when checkbox is selected on `<input type="checkbox">`.
+ * Usage:
+ * 
+ *     <input type=checkbox
+ *            ng-model=model
+ *            [ng-true-value=expr]>
+ * 
+ * The initial value of the expression bound to this directive is assigned to
+ * the model when the input is checked. Note that the expression can be of any
+ * type, not just [String]. Also see [InputCheckboxDirective], [NgFalseValue].
  */
-@NgDirective(selector: '[ng-true-value]')
+@NgDirective(selector: 'input[type=checkbox][ng-model][ng-true-value]')
 class NgTrueValue {
   final dom.Element element;
   @NgOneWay('ng-true-value')
-  var value;
-
-  NgTrueValue(this.element);
+  var value = true;
 
-  readValue(dom.Element element) {
-    assert(this.element == null || element == this.element);
-    return this.element == null ? true : value;
-  }
+  NgTrueValue([this.element]);
 
-  bool isValue(dom.Element element, value) {
-    assert(this.element == null || element == this.element);
-    return this.element == null ? toBool(value) : value == this.value;
-  }
+  bool isValue(val) => element == null ? toBool(val) : val == value;
 }
 
 /**
- * `ng-false-value` allows you to select any expression to be set to
- * `ng-model` when checkbox is deselected <input type="checkbox">`.
+ * Usage:
+ * 
+ *     <input type=checkbox 
+ *            ng-model=model
+ *            [ng-false-value=expr]>
+ * 
+ * The initial value of the expression bound to this directive is assigned to
+ * the model when the input is unchecked. Note that the expression can be of any
+ * type, not just [String]. Also see [InputCheckboxDirective], [NgTrueValue].
  */
-@NgDirective(selector: '[ng-false-value]')
+@NgDirective(selector: 'input[type=checkbox][ng-model][ng-false-value]')
 class NgFalseValue {
   final dom.Element element;
   @NgOneWay('ng-false-value')
-  var value;
-
-  NgFalseValue(this.element);
+  var value = false;
 
-  readValue(dom.Element element) {
-    assert(this.element == null || element == this.element);
-    return this.element == null ? false : value;
-  }
+  NgFalseValue([this.element]);
 }
 
 /**

test/directive/ng_model_spec.dart

@@ -631,7 +631,6 @@ void main() {
         expect(model.dirty).toEqual(true);
       });
 
-
       it('should update input value from model using ng-true-value/false', (Scope scope) {
         var element = _.compile('<input type="checkbox" ng-model="model" ng-true-value="1" ng-false-value="0">');
 
@@ -654,7 +653,6 @@ void main() {
         expect(scope.context['model']).toBe(0);
       });
 
-
       it('should allow non boolean values like null, 0, 1', (Scope scope) {
         var element = _.compile('<input type="checkbox" ng-model="model">');
 
@@ -674,7 +672,6 @@ void main() {
         expect(element.checked).toBe(false);
       });
 
-
       it('should update model from the input value', (Scope scope) {
         var element = _.compile('<input type="checkbox" ng-model="model">');
 
@@ -687,6 +684,23 @@ void main() {
         expect(scope.context['model']).toBe(false);
       });
 
+      it('should update model from the input using ng-true-value/false', (Scope scope) {
+        var element = _.compile('<input type="checkbox" ng-model="model" '
+                                'ng-true-value="yes" ng-false-value="no">');
+        scope.apply(() {
+          scope.context['yes'] = 'yes sir!';
+          scope.context['no'] = 'no, sorry';
+        });
+
+        element.checked = true;
+        _.triggerEvent(element, 'change');
+        expect(scope.context['model']).toEqual('yes sir!');
+
+        element.checked = false;
+        _.triggerEvent(element, 'change');
+        expect(scope.context['model']).toEqual('no, sorry');
+      });
+
       it('should only render the input value upon the next digest', (Scope scope) {
         _.compile('<input type="checkbox" ng-model="model" probe="p">');
         Probe probe = _.rootScope.context['p'];