Slight efficiency improvement on all page transitions (#9356)

Remove intermediate animation listener

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

@@ -20,14 +20,17 @@ final FractionalOffsetTween _kMiddleLeftTween = new FractionalOffsetTween(
   end: const FractionalOffset(-1.0/3.0, 0.0),
 );
 
+// Fractional offset from offscreen below to fully on screen.
+final FractionalOffsetTween _kBottomUpTween = new FractionalOffsetTween(
+  begin: FractionalOffset.bottomLeft,
+  end: FractionalOffset.topLeft,
+);
+
 /// Provides the native iOS page transition animation.
 ///
-/// Takes in a page widget and a route animation from a [TransitionRoute] and produces an
-/// AnimatedWidget wrapping that animates the page transition.
-///
 /// The page slides in from the right and exits in reverse. It also shifts to the left in
 /// a parallax motion when another page enters to cover it.
-class CupertinoPageTransition extends AnimatedWidget {
+class CupertinoPageTransition extends StatelessWidget {
   CupertinoPageTransition({
     Key key,
     // Linear route animation from 0.0 to 1.0 when this screen is being pushed.
@@ -55,13 +58,7 @@ class CupertinoPageTransition extends AnimatedWidget {
           reverseCurve: Curves.easeIn,
         )
       ),
-      super(
-        key: key,
-        // Trigger a rebuild whenever any of the 2 animation route happens.
-        listenable: new Listenable.merge(
-          <Listenable>[incomingRouteAnimation, outgoingRouteAnimation]
-        ),
-      );
+      super(key: key);
 
   // When this page is coming in to cover another page.
   final Animation<FractionalOffset> _incomingPositionAnimation;
@@ -91,32 +88,26 @@ class CupertinoPageTransition extends AnimatedWidget {
 
 /// Transitions used for summoning fullscreen dialogs in iOS such as creating a new
 /// calendar event etc by bringing in the next screen from the bottom.
-class CupertinoFullscreenDialogTransition extends AnimatedWidget {
+class CupertinoFullscreenDialogTransition extends StatelessWidget {
   CupertinoFullscreenDialogTransition({
     Key key,
     @required Animation<double> animation,
     @required this.child,
-  }) : super(
-    key: key,
-    listenable: _kBottomUpTween.animate(
+  }) : _positionAnimation = _kBottomUpTween.animate(
          new CurvedAnimation(
            parent: animation,
            curve: Curves.easeInOut,
          )
        ),
-  );
-
-  static final FractionalOffsetTween _kBottomUpTween = new FractionalOffsetTween(
-    begin: FractionalOffset.bottomLeft,
-    end: FractionalOffset.topLeft,
-  );
+       super(key: key);
 
+  final Animation<FractionalOffset> _positionAnimation;
   final Widget child;
 
   @override
   Widget build(BuildContext context) {
     return new SlideTransition(
-      position: listenable,
+      position: _positionAnimation,
       child: child,
     );
   }

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

@@ -8,38 +8,39 @@ import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
 
+// Fractional offset from 1/4 screen below the top to fully on screen. 
+final FractionalOffsetTween _kBottomUpTween = new FractionalOffsetTween(
+  begin: const FractionalOffset(0.0, 0.25),
+  end: FractionalOffset.topLeft
+);
+
 // Used for Android and Fuchsia.
-class _MountainViewPageTransition extends AnimatedWidget {
+class _MountainViewPageTransition extends StatelessWidget {
   _MountainViewPageTransition({
     Key key,
-    this.routeAnimation,
-    this.child,
-  }) : super(
-    key: key,
-    listenable: _kTween.animate(new CurvedAnimation(
+    @required Animation<double> routeAnimation,
+    @required this.child,
+  }) : _positionAnimation = _kBottomUpTween.animate(new CurvedAnimation(
          parent: routeAnimation, // The route's linear 0.0 - 1.0 animation.
-      curve: Curves.fastOutSlowIn
-    )
-  ));
-
-  static final FractionalOffsetTween _kTween = new FractionalOffsetTween(
-    begin: const FractionalOffset(0.0, 0.25),
-    end: FractionalOffset.topLeft
-  );
+         curve: Curves.fastOutSlowIn,
+       )),
+       _opacityAnimation = new CurvedAnimation(
+         parent: routeAnimation,
+         curve: Curves.easeIn, // Eyeballed from other Material apps.
+       ),
+       super(key: key);
 
+  final Animation<FractionalOffset> _positionAnimation;
+  final Animation<double> _opacityAnimation;
   final Widget child;
-  final Animation<double> routeAnimation;
 
   @override
   Widget build(BuildContext context) {
     // TODO(ianh): tell the transform to be un-transformed for hit testing
     return new SlideTransition(
-      position: listenable,
+      position: _positionAnimation,
       child: new FadeTransition(
-        opacity: new CurvedAnimation(
-          parent: routeAnimation,
-          curve: Curves.easeIn, // Eyeballed from other Material apps.
-        ),
+        opacity: _opacityAnimation,
         child: child,
       ),
     );