Merge pull request #170 from UnityTech/slider

Slider

Runtime/animation/tween.cs

         }
     }
 
+    public class NullableFloatTween : Tween<float?> {
+        public NullableFloatTween(float? begin = null, float? end = null) : base(begin: begin, end: end) {
+        }
+
+        public override float? lerp(float t) {
+            D.assert(this.begin != null);
+            D.assert(this.end != null);
+            return this.begin + (this.end - this.begin) * t;
+        }
+    }
+
     public class FloatTween : Tween<float> {
         public FloatTween(float begin, float end) : base(begin: begin, end: end) {
         }

Runtime/widgets/implicit_animations.cs

     }
 
     class _AnimatedOpacityState : ImplicitlyAnimatedWidgetState<AnimatedOpacity> {
-        FloatTween _opacity;
+        NullableFloatTween _opacity;
-            this._opacity = (FloatTween) visitor.visit(this, this._opacity, this.widget.opacity,
-                (float value) => new FloatTween(begin: value, end: 1.0f));
+            this._opacity = (NullableFloatTween) visitor.visit(this, this._opacity, this.widget.opacity,
+                (float? value) => new NullableFloatTween(begin: value));
-            this._opacityAnimation = this.animation.drive(this._opacity);
+            float? endValue = this._opacity.end ?? this._opacity.begin ?? null;
+            D.assert(endValue != null);
+            this._opacityAnimation = this.animation.drive(new FloatTween(begin: this._opacity.begin.Value, end: endValue.Value));
         }
 
         public override Widget build(BuildContext context) {

Samples/UIWidgetSample/txt/TextFieldSample.cs

                 ),
                 body: new Padding(
                     padding: EdgeInsets.all(16.0f),
-                    child: new TextField(controller: this.myController)
+                    child: new TextField(
+                        controller: this.myController,
+                        autofocus: true,
+                        decoration: new InputDecoration(
+                            hintText: "hinthere",
+                            labelText: "pwd",
+                            prefixIcon: new Icon(Unity.UIWidgets.material.Icons.search)))
                 ),
                 floatingActionButton: new FloatingActionButton(
                     // When the user presses the button, show an alert dialog with the