AnimatedBuilder should have a child

Providing a pre-built child is more efficient because we don't need to rebuild
the child every tick of the animation.

examples/material_gallery/lib/demo/page_selector_demo.dart

@@ -15,7 +15,7 @@ class PageSelectorDemo extends StatelessComponent {
     CurvedAnimation animation = new CurvedAnimation(parent: selection.animation, curve: Curves.ease);
     return new AnimatedBuilder(
       animation: animation,
-      builder: (BuildContext context) {
+      builder: (BuildContext context, Widget child) {
         Color background = selection.value == iconName ? _selectedColor.end : _selectedColor.begin;
         if (selection.valueIsChanging) {
           // Then the selection's performance is animating from previousValue to value.

examples/material_gallery/lib/demo/progress_indicator_demo.dart

@@ -47,7 +47,7 @@ class _ProgressIndicatorDemoState extends State<ProgressIndicatorDemo> {
     controller.play(direction);
   }
 
-  Widget buildIndicators(BuildContext context) {
+  Widget buildIndicators(BuildContext context, Widget child) {
     List<Widget> indicators = <Widget>[
         new SizedBox(
           width: 200.0,

examples/widgets/smooth_resize.dart

@@ -41,7 +41,7 @@ class CardTransition extends StatelessComponent {
   Widget build(BuildContext context) {
     return new AnimatedBuilder(
       animation: animation,
-      builder: (BuildContext context) {
+      builder: (BuildContext context, Widget child) {
         double currentScale = scale.evaluate(animation);
         Matrix4 transform = new Matrix4.identity()
           ..translate(x.evaluate(animation))
@@ -53,7 +53,8 @@ class CardTransition extends StatelessComponent {
             child: child
           )
         );
-      }
+      },
+      child: child
     );
   }
 }

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

@@ -134,7 +134,7 @@ class _ModalBottomSheetState extends State<_ModalBottomSheet> {
       onTap: () => Navigator.pop(context),
       child: new AnimatedBuilder(
         animation: config.route.animation,
-        builder: (BuildContext context) {
+        builder: (BuildContext context, Widget child) {
           return new ClipRect(
             child: new CustomOneChildLayout(
               delegate: new _ModalBottomSheetLayout(config.route.animation.value),

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

@@ -131,7 +131,7 @@ class _DropDownMenu<T> extends StatusTransitionComponent {
       opacity: opacity,
       child: new AnimatedBuilder(
         animation: resize,
-        builder: (BuildContext context) {
+        builder: (BuildContext context, Widget child) {
           return new CustomPaint(
             painter: new _DropDownMenuPainter(
               color: Theme.of(context).canvasColor,
@@ -142,7 +142,8 @@ class _DropDownMenu<T> extends StatusTransitionComponent {
             ),
             child: child
           );
-        }
+        },
+        child: child
       )
     );
   }

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

@@ -96,7 +96,7 @@ class _PopupMenu<T> extends StatelessComponent {
 
     return new AnimatedBuilder(
       animation: route.animation,
-      builder: (BuildContext context) {
+      builder: (BuildContext context, Widget child) {
         return new Opacity(
           opacity: opacity.evaluate(route.animation),
           child: new Material(
@@ -110,7 +110,8 @@ class _PopupMenu<T> extends StatelessComponent {
             )
           )
         );
-      }
+      },
+      child: child
     );
   }
 }

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

@@ -63,7 +63,7 @@ class _ProgressIndicatorState extends State<ProgressIndicator> {
 
     return new AnimatedBuilder(
       animation: _animation,
-      builder: (BuildContext context) {
+      builder: (BuildContext context, Widget child) {
         return config._buildIndicator(context, _animation.value);
       }
     );

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

@@ -473,20 +473,20 @@ class _PersistentBottomSheetState extends State<_PersistentBottomSheet> {
   }
 
   Widget build(BuildContext context) {
-    Widget child = new BottomSheet(
-      animationController: config.animationController,
-      onClosing: config.onClosing,
-      builder: config.builder
-    );
     return new AnimatedBuilder(
       animation: config.animationController,
-      builder: (BuildContext context) {
+      builder: (BuildContext context, Widget child) {
         return new Align(
           alignment: const FractionalOffset(0.0, 0.0),
           heightFactor: config.animationController.value,
           child: child
         );
-      }
+      },
+      child: new BottomSheet(
+        animationController: config.animationController,
+        onClosing: config.onClosing,
+        builder: config.builder
+      )
     );
   }
 

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

@@ -85,38 +85,38 @@ class SnackBar extends StatelessComponent {
     CurvedAnimation heightAnimation = new CurvedAnimation(parent: animation, curve: _snackBarHeightCurve);
     CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve);
     ThemeData theme = Theme.of(context);
-    Widget child = new Material(
-      elevation: 6,
-      color: _kSnackBackground,
-      child: new Container(
-        margin: const EdgeDims.symmetric(horizontal: _kSideMargins),
-        child: new Theme(
-          data: new ThemeData(
-            brightness: ThemeBrightness.dark,
-            accentColor: theme.accentColor,
-            accentColorBrightness: theme.accentColorBrightness,
-            text: Typography.white
-          ),
-          child: new FadeTransition(
-            opacity: fadeAnimation,
-            child: new Row(
-              children: children,
-              alignItems: FlexAlignItems.center
-            )
-          )
-        )
-      )
-    );
     return new ClipRect(
       child: new AnimatedBuilder(
         animation: heightAnimation,
-        builder: (BuildContext context) {
+        builder: (BuildContext context, Widget child) {
           return new Align(
             alignment: const FractionalOffset(0.0, 0.0),
             heightFactor: heightAnimation.value,
             child: child
           );
-        }
+        },
+        child: new Material(
+          elevation: 6,
+          color: _kSnackBackground,
+          child: new Container(
+            margin: const EdgeDims.symmetric(horizontal: _kSideMargins),
+            child: new Theme(
+              data: new ThemeData(
+                brightness: ThemeBrightness.dark,
+                accentColor: theme.accentColor,
+                accentColorBrightness: theme.accentColorBrightness,
+                text: Typography.white
+              ),
+              child: new FadeTransition(
+                opacity: fadeAnimation,
+                child: new Row(
+                  children: children,
+                  alignItems: FlexAlignItems.center
+                )
+              )
+            )
+          )
+        )
       )
     );
   }

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

@@ -360,16 +360,20 @@ class BuilderTransition extends TransitionComponent {
   }
 }
 
+typedef Widget TransitionBuilder(BuildContext context, Widget child);
+
 class AnimatedBuilder extends AnimatedComponent {
   AnimatedBuilder({
     Key key,
     Animated<Object> animation,
-    this.builder
+    this.builder,
+    this.child
   }) : super(key: key, animation: animation);
 
-  final WidgetBuilder builder;
+  final TransitionBuilder builder;
+  final Widget child;
 
   Widget build(BuildContext context) {
-    return builder(context);
+    return builder(context, child);
   }
 }