Merge pull request #1183 from Hixie/dismissable

Be more explicit about what's state in dismissable

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

@@ -75,14 +75,6 @@ class Dismissable extends StatefulComponent {
       _startResizePerformance();
   }
 
-  Point get _activeCardDragEndPoint {
-    if (!_isActive)
-      return Point.origin;
-    assert(_size != null);
-    double extent = _directionIsYAxis ? _size.height : _size.width;
-    return new Point(_dragExtent.sign * extent * _kDismissCardThreshold, 0.0);
-  }
-
   bool get _isActive {
     return _size != null && (_dragUnderway || _fadePerformance.isAnimating);
   }
@@ -120,9 +112,11 @@ class Dismissable extends StatefulComponent {
   void _handleDragStart() {
     if (_fadePerformance.isAnimating)
       return;
+    setState(() {
       _dragUnderway = true;
       _dragExtent = 0.0;
       _fadePerformance.progress = 0.0;
+    });
   }
 
   void _handleDragUpdate(double scrollOffset) {
@@ -149,8 +143,14 @@ class Dismissable extends StatefulComponent {
         break;
     }
 
-    if (oldDragExtent.sign != _dragExtent.sign)
-      setState(() {}); // Rebuild to update the new drag endpoint.
+    if (oldDragExtent.sign != _dragExtent.sign) {
+      setState(() {
+        // Rebuild to update the new drag endpoint.
+        // The sign of _dragExtent is part of our build state;
+        // the actual value is not, it's just used to configure
+        // the performances.
+      });
+    }
     if (!_fadePerformance.isAnimating)
       _fadePerformance.progress = _dragExtent.abs() / (_size.width * _kDismissCardThreshold);
   }
@@ -188,6 +188,7 @@ class Dismissable extends StatefulComponent {
     if (!_isActive || _fadePerformance.isAnimating)
       return;
 
+    setState(() {
       _dragUnderway = false;
       if (_fadePerformance.isCompleted) {
         _startResizePerformance();
@@ -198,6 +199,7 @@ class Dismissable extends StatefulComponent {
       } else {
         _fadePerformance.reverse();
       }
+    });
   }
 
   void _handleSizeChanged(Size newSize) {
@@ -206,6 +208,14 @@ class Dismissable extends StatefulComponent {
     });
   }
 
+  Point get _activeCardDragEndPoint {
+    if (!_isActive)
+      return Point.origin;
+    assert(_size != null);
+    double extent = _directionIsYAxis ? _size.height : _size.width;
+    return new Point(_dragExtent.sign * extent * _kDismissCardThreshold, 0.0);
+  }
+
   Widget build() {
     if (_resizePerformance != null) {
       AnimatedValue<double> squashAxisExtent = new AnimatedValue<double>(