Add Form.onChanged, and rename FormState.onChanged (#15405)

examples/flutter_gallery/lib/demo/material/expansion_panels_demo.dart

@@ -254,7 +254,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                               new Radio<_Location>(
                                 value: _Location.Bahamas,
                                 groupValue: field.value,
-                                onChanged: field.onChanged,
+                                onChanged: field.didChange,
                               ),
                               const Text('Bahamas')
                             ]
@@ -265,7 +265,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                               new Radio<_Location>(
                                 value: _Location.Barbados,
                                 groupValue: field.value,
-                                onChanged: field.onChanged,
+                                onChanged: field.didChange,
                               ),
                               const Text('Barbados')
                             ]
@@ -276,7 +276,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                               new Radio<_Location>(
                                 value: _Location.Bermuda,
                                 groupValue: field.value,
-                                onChanged: field.onChanged,
+                                onChanged: field.didChange,
                               ),
                               const Text('Bermuda')
                             ]
@@ -320,7 +320,7 @@ class _ExpansionPanelsDemoState extends State<ExpansionPanelsDemo> {
                         activeColor: Colors.orange[100 + (field.value * 5.0).round()],
                         label: '${field.value.round()}',
                         value: field.value,
-                        onChanged: field.onChanged,
+                        onChanged: field.didChange,
                       );
                     },
                   ),

packages/flutter/lib/src/material/text_form_field.dart

@@ -98,7 +98,7 @@ class TextFormField extends FormField<String> {
         maxLengthEnforced: maxLengthEnforced,
         maxLines: maxLines,
         maxLength: maxLength,
-        onChanged: field.onChanged,
+        onChanged: field.didChange,
         onSubmitted: onFieldSubmitted,
         inputFormatters: inputFormatters,
       );
@@ -173,6 +173,6 @@ class _TextFormFieldState extends FormFieldState<String> {
     // example, the reset() method. In such cases, the FormField value will
     // already have been set.
     if (_effectiveController.text != value)
-      onChanged(_effectiveController.text);
+      didChange(_effectiveController.text);
   }
 }

packages/flutter/lib/src/widgets/form.dart

@@ -26,6 +26,7 @@ class Form extends StatefulWidget {
     @required this.child,
     this.autovalidate: false,
     this.onWillPop,
+    this.onChanged,
   }) : assert(child != null),
        super(key: key);
 
@@ -66,6 +67,12 @@ class Form extends StatefulWidget {
   ///    back button.
   final WillPopCallback onWillPop;
 
+  /// Called when one of the form fields changes.
+  ///
+  /// In addition to this callback being invoked, all the form fields themselves
+  /// will rebuild.
+  final VoidCallback onChanged;
+
   @override
   FormState createState() => new FormState();
 }
@@ -83,6 +90,12 @@ class FormState extends State<Form> {
   // Called when a form field has changed. This will cause all form fields
   // to rebuild, useful if form fields have interdependencies.
   void _fieldDidChange() {
+    if (widget.onChanged != null)
+      widget.onChanged();
+    _forceRebuild();
+  }
+
+  void _forceRebuild() {
     setState(() {
       ++_generation;
     });
@@ -117,7 +130,12 @@ class FormState extends State<Form> {
   }
 
   /// Resets every [FormField] that is a descendant of this [Form] back to its
-  /// initialState.
+  /// [FormField.initialState].
+  ///
+  /// The [Form.onChanged] callback will be called.
+  ///
+  /// If the form's [Form.autovalidate] property is true, the fields will all be
+  /// revalidated after being reset.
   void reset() {
     for (FormFieldState<dynamic> field in _fields)
       field.reset();
@@ -126,8 +144,10 @@ class FormState extends State<Form> {
 
   /// Validates every [FormField] that is a descendant of this [Form], and
   /// returns true if there are no errors.
+  ///
+  /// The form will rebuild to report the results.
   bool validate() {
-    _fieldDidChange();
+    _forceRebuild();
     return _validate();
   }
 
@@ -213,7 +233,7 @@ class FormField<T> extends StatefulWidget {
        super(key: key);
 
   /// An optional method to call with the final value when the form is saved via
-  /// Form.save().
+  /// [FormState.save].
   final FormFieldSetter<T> onSaved;
 
   /// An optional method that validates an input. Returns an error string to
@@ -289,8 +309,11 @@ class FormFieldState<T> extends State<FormField<T>> {
   }
 
   /// Updates this field's state to the new value. Useful for responding to
-  /// child widget changes, e.g. [Slider]'s onChanged argument.
-  void onChanged(T value) {
+  /// child widget changes, e.g. [Slider]'s [Slider.onChanged] argument.
+  ///
+  /// Triggers the [Form.onChanged] callback and, if the [Form.autovalidate]
+  /// field is set, revalidates all the fields of the form.
+  void didChange(T value) {
     setState(() {
       _value = value;
     });
@@ -302,7 +325,7 @@ class FormFieldState<T> extends State<FormField<T>> {
   /// This method should be only be called by subclasses that need to update
   /// the form field value due to state changes identified during the widget
   /// build phase, when calling `setState` is prohibited. In all other cases,
-  /// the value should be set by a call to [onChanged], which ensures that
+  /// the value should be set by a call to [didChange], which ensures that
   /// `setState` is called.
   @protected
   void setValue(T value) {