Merge pull request #3015 from abarth/snackbar_fade

Snackbar opacity animation shouldn't trigger on reverse

packages/flutter/lib/src/animation/curves.dart

@@ -95,6 +95,25 @@ class Interval extends Curve {
   }
 }
 
+/// A curve that is 0.0 until it hits the threshold, then it jumps to 1.0.
+class Step extends Curve {
+  const Step(this.threshold);
+
+  /// The value before which the curve is 0.0 and after which the curve is 1.0.
+  ///
+  /// At exactly step, the curve has the value 1.0.
+  final double threshold;
+
+  @override
+  double transform(double t) {
+    assert(threshold >= 0.0);
+    assert(threshold <= 1.0);
+    if (t == 0.0 || t == 1.0)
+      return t;
+    return t < threshold ? 0.0 : 1.0;
+  }
+}
+
 /// A cubic polynomial mapping of the unit interval.
 class Cubic extends Curve {
   const Cubic(this.a, this.b, this.c, this.d);

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

@@ -82,7 +82,7 @@ class _DropDownMenu<T> extends StatusTransitionWidget {
     for (int itemIndex = 0; itemIndex < route.items.length; ++itemIndex) {
       CurvedAnimation opacity;
       if (itemIndex == route.selectedIndex) {
-        opacity = new CurvedAnimation(parent: route.animation, curve: const Interval(0.0, 0.001), reverseCurve: const Interval(0.75, 1.0));
+        opacity = new CurvedAnimation(parent: route.animation, curve: const Step(0.0), reverseCurve: const Interval(0.75, 1.0));
       } else {
         final double start = (0.5 + (itemIndex + 1) * unit).clamp(0.0, 1.0);
         final double end = (start + 1.5 * unit).clamp(0.0, 1.0);
@@ -112,7 +112,7 @@ class _DropDownMenu<T> extends StatusTransitionWidget {
     final CurvedAnimation resize = new CurvedAnimation(
       parent: route.animation,
       curve: const Interval(0.25, 0.5),
-      reverseCurve: const Interval(0.0, 0.001)
+      reverseCurve: const Step(0.0)
     );
 
     final Tween<double> menuTop = new Tween<double>(

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

@@ -105,7 +105,7 @@ class SnackBar extends StatelessWidget {
     if (action != null)
       children.add(action);
     CurvedAnimation heightAnimation = new CurvedAnimation(parent: animation, curve: _snackBarHeightCurve);
-    CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve);
+    CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve, reverseCurve: const Step(0.0));
     ThemeData theme = Theme.of(context);
     return new ClipRect(
       child: new AnimatedBuilder(