fix: floatTween could be nullable in animatedOpacity

Author: zhuxingwei

Runtime/animation/tween.cs

@@ -186,6 +186,15 @@ public override int lerp(float t) {
         }
     }
 
+    public class NullableFloatTween : Tween<float?> {
+        public NullableFloatTween(float? begin = null, float? end = null) : base(begin: begin, end: end) {
+        }
+
+        public override float? lerp(float t) {
+            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

@@ -215,7 +215,13 @@ public override void dispose() {
         }
 
         public bool _shouldAnimateTween<T2>(Tween<T2> tween, T2 targetValue) {
-            return !targetValue.Equals(tween.end == null ? tween.begin : tween.end);
+            var curTargetValue = tween.end;
+            
+            if (tween.end == null || typeof(T2) == typeof(float) &&
+                (float) Convert.ChangeType(tween.end, typeof(float)) == -1.0f) {
+                curTargetValue = tween.begin;
+            }
+            return !targetValue.Equals(curTargetValue);
         }
 
         public void _updateTween<T2>(Tween<T2> tween, T2 targetValue) {
@@ -471,7 +477,7 @@ class _AnimatedOpacityState : ImplicitlyAnimatedWidgetState<AnimatedOpacity> {
 
         protected override void forEachTween(TweenVisitor visitor) {
             this._opacity = (FloatTween) visitor.visit(this, this._opacity, this.widget.opacity,
-                (float value) => new FloatTween(begin: value, end: 1.0f));
+                (float value) => new FloatTween(begin: value, end: -1.0f));
         }
 
         protected override void didUpdateTweens() {

Samples/UIWidgetSample/txt/TextFieldSample.cs

@@ -41,7 +41,13 @@ public override Widget build(BuildContext context) {
                 ),
                 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