use FadeTransition instead of Opacity where applicable (#75110)

packages/flutter/lib/src/cupertino/context_menu.dart

@@ -758,8 +758,8 @@ class _ContextMenuRoute<T> extends PopupRoute<T> {
             children: <Widget>[
               Positioned.fromRect(
                 rect: sheetRect,
-                child: Opacity(
-                  opacity: _sheetOpacity.value,
+                child: FadeTransition(
+                  opacity: _sheetOpacity,
                   child: Transform.scale(
                     alignment: getSheetAlignment(_contextMenuLocation),
                     scale: sheetScale,
@@ -1028,8 +1028,8 @@ class _ContextMenuRouteStaticState extends State<_ContextMenuRouteStatic> with T
     return Transform.scale(
       alignment: _ContextMenuRoute.getSheetAlignment(widget.contextMenuLocation),
       scale: _sheetScaleAnimation.value,
-      child: Opacity(
-        opacity: _sheetOpacityAnimation.value,
+      child: FadeTransition(
+        opacity: _sheetOpacityAnimation,
         child: child,
       ),
     );

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

@@ -1211,8 +1211,8 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
 
   @override
   Widget build(BuildContext context) {
-    return Opacity(
-      opacity: _opacityAnimation.value,
+    return FadeTransition(
+      opacity: _opacityAnimation,
       child: Transform(
         transform: Matrix4.rotationZ(_orientationOffset + _orientationAnimation.value)
                              ..setTranslationRaw(0.0, _arrowIconBaselineOffset, 0.0),

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

@@ -373,8 +373,8 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
     assert(widget.helperText != null);
     return Semantics(
       container: true,
-      child: Opacity(
-        opacity: 1.0 - _controller.value,
+      child: FadeTransition(
+        opacity: Tween<double>(begin: 1.0, end: 0.0).animate(_controller),
         child: Text(
           widget.helperText!,
           style: widget.helperStyle,
@@ -391,8 +391,8 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
     return Semantics(
       container: true,
       liveRegion: true,
-      child: Opacity(
-        opacity: _controller.value,
+      child: FadeTransition(
+        opacity: _controller,
         child: FractionalTranslation(
           translation: Tween<Offset>(
             begin: const Offset(0.0, -0.25),
@@ -441,8 +441,8 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
     if (widget.errorText != null) {
       return Stack(
         children: <Widget>[
-          Opacity(
-            opacity: 1.0 - _controller.value,
+          FadeTransition(
+            opacity: Tween<double>(begin: 1.0, end: 0.0).animate(_controller),
             child: _helper,
           ),
           _buildError(),
@@ -454,8 +454,8 @@ class _HelperErrorState extends State<_HelperError> with SingleTickerProviderSta
       return Stack(
         children: <Widget>[
           _buildHelper(),
-          Opacity(
-            opacity: _controller.value,
+          FadeTransition(
+            opacity: _controller,
             child: _error,
           ),
         ],

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

@@ -573,6 +573,7 @@ class _RailDestination extends StatelessWidget {
             ),
           );
         } else {
+          final Animation<double> labelFadeAnimation = extendedTransitionAnimation.drive(CurveTween(curve: const Interval(0.0, 0.25)));
           content = Padding(
             padding: padding ?? EdgeInsets.zero,
             child: ConstrainedBox(
@@ -587,9 +588,9 @@ class _RailDestination extends StatelessWidget {
                       heightFactor: 1.0,
                       widthFactor: extendedTransitionAnimation.value,
                       alignment: AlignmentDirectional.centerStart,
-                      child: Opacity(
+                      child: FadeTransition(
                         alwaysIncludeSemantics: true,
-                        opacity: _extendedLabelFadeValue(),
+                        opacity: labelFadeAnimation,
                         child: styledLabel,
                       ),
                     ),
@@ -604,6 +605,8 @@ class _RailDestination extends StatelessWidget {
       case NavigationRailLabelType.selected:
         final double appearingAnimationValue = 1 - _positionAnimation.value;
         final double verticalPadding = lerpDouble(_verticalDestinationPaddingNoLabel, _verticalDestinationPaddingWithLabel, appearingAnimationValue)!;
+        final Interval interval = selected ? const Interval(0.25, 0.75) : const Interval(0.75, 1.0);
+        final Animation<double> labelFadeAnimation = destinationAnimation.drive(CurveTween(curve: interval));
         content = Container(
           constraints: BoxConstraints(
             minWidth: minWidth,
@@ -621,9 +624,9 @@ class _RailDestination extends StatelessWidget {
                   alignment: Alignment.topCenter,
                   heightFactor: appearingAnimationValue,
                   widthFactor: 1.0,
-                  child: Opacity(
+                  child: FadeTransition(
                     alwaysIncludeSemantics: true,
-                    opacity: selected ? _normalLabelFadeInValue() : _normalLabelFadeOutValue(),
+                    opacity: labelFadeAnimation,
                     child: styledLabel,
                   ),
                 ),
@@ -679,28 +682,6 @@ class _RailDestination extends StatelessWidget {
       ),
     );
   }
-
-  double _normalLabelFadeInValue() {
-    if (destinationAnimation.value < 0.25) {
-      return 0;
-    } else if (destinationAnimation.value < 0.75) {
-      return (destinationAnimation.value - 0.25) * 2;
-    } else {
-      return 1;
-    }
-  }
-
-  double _normalLabelFadeOutValue() {
-    if (destinationAnimation.value > 0.75) {
-      return (destinationAnimation.value - 0.75) * 4.0;
-    } else {
-      return 0;
-    }
-  }
-
-  double _extendedLabelFadeValue() {
-    return extendedTransitionAnimation.value < 0.25 ? extendedTransitionAnimation.value * 4.0 : 1.0;
-  }
 }
 
 /// Defines the behavior of the labels of a [NavigationRail].

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

@@ -596,8 +596,8 @@ class _PopupMenu<T> extends StatelessWidget {
     return AnimatedBuilder(
       animation: route.animation!,
       builder: (BuildContext context, Widget? child) {
-        return Opacity(
-          opacity: opacity.evaluate(route.animation!),
+        return FadeTransition(
+          opacity: opacity.animate(route.animation!),
           child: Material(
             shape: route.shape ?? popupMenuTheme.shape,
             color: route.color ?? popupMenuTheme.color,

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

@@ -547,8 +547,8 @@ class _HeroFlight {
           left: offsets.left,
           child: IgnorePointer(
             child: RepaintBoundary(
-              child: Opacity(
-                opacity: _heroOpacity.value,
+              child: FadeTransition(
+                opacity: _heroOpacity,
                 child: child,
               ),
             ),

packages/flutter/test/cupertino/nav_bar_transition_test.dart

@@ -117,7 +117,7 @@ void checkBackgroundBoxHeight(WidgetTester tester, double height) {
 
 void checkOpacity(WidgetTester tester, Finder finder, double opacity) {
   expect(
-    tester.renderObject<RenderAnimatedOpacity>(
+    tester.firstRenderObject<RenderAnimatedOpacity>(
       find.ancestor(
         of: finder,
         matching: find.byType(FadeTransition),

packages/flutter/test/material/navigation_rail_test.dart

@@ -2331,14 +2331,23 @@ Finder _opacityAboveLabel(String text) {
 }
 
 // Only valid when labelType != all.
-double _labelOpacity(WidgetTester tester, String text) {
-  final Opacity opacityWidget = tester.widget<Opacity>(
-    find.ancestor(
+double? _labelOpacity(WidgetTester tester, String text) {
+  // We search for both Opacity and FadeTransition since in some
+  // cases opacity is animated, in other it's not.
+  final Iterable<Opacity> opacityWidgets = tester.widgetList<Opacity>(find.ancestor(
     of: find.text(text),
     matching: find.byType(Opacity),
-    ),
+  ));
+  if (opacityWidgets.isNotEmpty)
+    return opacityWidgets.single.opacity;
+
+  final FadeTransition fadeTransitionWidget = tester.widget<FadeTransition>(
+    find.ancestor(
+      of: find.text(text),
+      matching: find.byType(FadeTransition),
+    ).first, // first because there's also a FadeTransition from the MaterialPageRoute, which is up the tree
   );
-  return opacityWidget.opacity;
+  return fadeTransitionWidget.opacity.value;
 }
 
 Material _railMaterial(WidgetTester tester) {

packages/flutter/test/widgets/heroes_test.dart

@@ -1182,7 +1182,7 @@ Future<void> main() async {
       bool isVisible = true;
       node.visitAncestorElements((Element ancestor) {
         final RenderObject r = ancestor.renderObject!;
-        if (r is RenderOpacity && r.opacity == 0) {
+        if (r is RenderAnimatedOpacity && r.opacity.value == 0) {
           isVisible = false;
           return false;
         }
@@ -2920,12 +2920,12 @@ Future<void> main() async {
     final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
     final ScrollController controller = ScrollController();
 
-    RenderOpacity? findRenderOpacity() {
+    RenderAnimatedOpacity? findRenderAnimatedOpacity() {
       AbstractNode? parent = tester.renderObject(find.byType(Placeholder));
-      while (parent is RenderObject && parent is! RenderOpacity) {
+      while (parent is RenderObject && parent is! RenderAnimatedOpacity) {
         parent = parent.parent;
       }
-      return parent is RenderOpacity ? parent : null;
+      return parent is RenderAnimatedOpacity ? parent : null;
     }
 
     await tester.pumpWidget(
@@ -2977,14 +2977,14 @@ Future<void> main() async {
     // Starts Hero animation and scroll animation almost simultaneously.
     // Scroll to make the Hero invisible.
     await tester.pump();
-    expect(findRenderOpacity()?.opacity, anyOf(isNull, 1.0));
+    expect(findRenderAnimatedOpacity()?.opacity.value, anyOf(isNull, 1.0));
 
     // In this frame the Hero animation finds out the toHero is not paintable,
     // and starts fading.
     await tester.pump();
     await tester.pump(const Duration(milliseconds: 100));
 
-    expect(findRenderOpacity()?.opacity, lessThan(1.0));
+    expect(findRenderAnimatedOpacity()?.opacity.value, lessThan(1.0));
 
     await tester.pumpAndSettle();
     // The Hero on the new route should be invisible.