Fix form.

Runtime/material/dropdown.cs

         }
     }
 
-    public class DropdownButtonFormField<T> : FormField where T : class {
+    public class DropdownButtonFormField<T> : FormField<T> where T : class {
         public DropdownButtonFormField(
             Key key = null,
             T value = null,
-            FormFieldSetter onSaved = null,
-            FormFieldValidator validator = null,
+            FormFieldSetter<T> onSaved = null,
+            FormFieldValidator<T> validator = null,
             Widget hint = null
         ) : base(
             key: key,
-            builder: (FormFieldState field) => {
+            builder: (FormFieldState<T> field) => {
                 InputDecoration effectiveDecoration = (decoration ?? new InputDecoration())
                     .applyDefaults(Theme.of(field.context).inputDecorationTheme);
                 return new InputDecorator(
         }
     }
 
-    class _DropdownButtonFormFieldState<T> : FormFieldState where T : class {
+    class _DropdownButtonFormFieldState<T> : FormFieldState<T> where T : class {
-        public override void didChange(object value) {
+        public override void didChange(T value) {
-                this.widget.onChanged((T) value);
+                this.widget.onChanged(value);
             }
         }
     }

Runtime/material/text_form_field.cs

 using TextStyle = Unity.UIWidgets.painting.TextStyle;
 
 namespace UIWidgets.Runtime.material {
-    public class TextFormField : FormField {
+    public class TextFormField : FormField<string> {
         public TextFormField(
             Key key = null,
             TextEditingController controller = null,
             int? maxLength = null,
             VoidCallback onEditingComplete = null,
             ValueChanged<string> onFieldSubmitted = null,
-            FormFieldSetter onSaved = null,
-            FormFieldValidator validator = null,
+            FormFieldSetter<string> onSaved = null,
+            FormFieldValidator<string> validator = null,
             List<TextInputFormatter> inputFormatters = null,
             bool enabled = true,
             float cursorWidth = 2.0f,
             validator: validator,
             autovalidate: autovalidate,
             enabled: enabled,
-            builder: (FormFieldState field) => {
+            builder: (FormFieldState<string> field) => {
                 _TextFormFieldState state = (_TextFormFieldState) field;
                 InputDecoration effectiveDecoration = (decoration ?? new InputDecoration())
                     .applyDefaults(Theme.of(field.context).inputDecorationTheme);
         }
     }
 
-    class _TextFormFieldState : FormFieldState {
+    class _TextFormFieldState : FormFieldState<string> {
         TextEditingController _controller;
 
         public TextEditingController _effectiveController {
         public override void initState() {
             base.initState();
             if (this.widget.controller == null) {
-                this._controller = new TextEditingController(text: (string) this.widget.initialValue);
+                this._controller = new TextEditingController(text: this.widget.initialValue);
             }
             else {
                 this.widget.controller.addListener(this._handleControllerChanged);

Runtime/widgets/form.cs

 
     public class FormState : State<Form> {
         int _generation = 0;
-        public readonly HashSet<object> _fields = new HashSet<object>();
+        readonly HashSet<FormFieldState> _fields = new HashSet<FormFieldState>();
 
         public FormState() {
         }
             this.setState(() => { ++this._generation; });
         }
 
-        public void _register(object field) {
+        public void _register(FormFieldState field) {
-        public void _unregister(object field) {
+        public void _unregister(FormFieldState field) {
             this._fields.Remove(field);
         }
 
         }
     }
 
-    public delegate string FormFieldValidator(object value);
+    public delegate string FormFieldValidator<T>(T value);
-    public delegate void FormFieldSetter(object newValue);
+    public delegate void FormFieldSetter<T>(T newValue);
-    public delegate Widget FormFieldBuilder(FormFieldState field);
+    public delegate Widget FormFieldBuilder<T>(FormFieldState<T> field) where T : class;
-    public class FormField : StatefulWidget {
+    public class FormField<T> : StatefulWidget where T : class {
-            FormFieldBuilder builder = null,
-            FormFieldSetter onSaved = null,
-            FormFieldValidator validator = null,
-            object initialValue = null,
+            FormFieldBuilder<T> builder = null,
+            FormFieldSetter<T> onSaved = null,
+            FormFieldValidator<T> validator = null,
+            T initialValue = null,
             bool autovalidate = false,
             bool enabled = true
         ) : base(key: key) {
             this.enabled = enabled;
         }
 
-        public readonly FormFieldSetter onSaved;
+        public readonly FormFieldSetter<T> onSaved;
-        public readonly FormFieldValidator validator;
+        public readonly FormFieldValidator<T> validator;
-        public readonly FormFieldBuilder builder;
+        public readonly FormFieldBuilder<T> builder;
-        public readonly object initialValue;
+        public readonly T initialValue;
 
         public readonly bool autovalidate;
 
-            return new FormFieldState();
+            return new FormFieldState<T>();
-    public class FormFieldState : State<FormField> {
-        object _value;
+    public interface FormFieldState {
+        void save();
+
+        bool validate();
+
+        void reset();
+    }
+
+    public class FormFieldState<T> : State<FormField<T>>, FormFieldState where T : class {
+        T _value;
-        public object value {
+        public T value {
             get { return this._value; }
         }
 
             get { return this._errorText != null; }
         }
 
-        public virtual void save() {
+        public void save() {
             if (this.widget.onSaved != null) {
                 this.widget.onSaved(this.value);
             }
             return !this.hasError;
         }
 
-        public virtual void didChange(object value) {
+        public virtual void didChange(T value) {
-        protected void setValue(object value) {
+        protected void setValue(T value) {
             this._value = value;
         }