Clean up flipping of ScrollDirection (#8343)

Makes RenderViewport.layoutOneSide more readable.

packages/flutter/lib/src/rendering/sliver.dart

@@ -131,6 +131,27 @@ AxisDirection applyGrowthDirectionToAxisDirection(AxisDirection axisDirection, G
   return null;
 }
 
+/// Flips the [ScrollDirection] if the [GrowthDirection] is [GrowthDirection.reverse].
+///
+/// Specifically, returns `scrollDirection` if `scrollDirection` is
+/// [GrowthDirection.forward], otherwise returns [flipScrollDirection] applied to
+/// `scrollDirection`.
+///
+/// This function is useful in [RenderSliver] subclasses that are given both an
+/// [ScrollDirection] and a [GrowthDirection] and wish to compute the
+/// [ScrollDirection] in which growth will occur.
+ScrollDirection applyGrowthDirecitonToScrollDirection(ScrollDirection scrollDirection, GrowthDirection growthDirection) {
+  assert(scrollDirection != null);
+  assert(growthDirection != null);
+  switch (growthDirection) {
+    case GrowthDirection.forward:
+      return scrollDirection;
+    case GrowthDirection.reverse:
+      return flipScrollDirection(scrollDirection);
+  }
+  return null;
+}
+
 /// Immutable layout constraints for [RenderSliver] layout.
 ///
 /// The [SliverConstraints] describe the current scroll state of the viewport
@@ -524,7 +545,7 @@ class SliverGeometry {
 
   @override
   String toString() {
-    StringBuffer buffer = new StringBuffer();
+    final StringBuffer buffer = new StringBuffer();
     buffer.write('SliverGeometry(');
       buffer.write('scrollExtent: ${scrollExtent.toStringAsFixed(1)}, ');
       if (paintExtent > 0.0) {

packages/flutter/lib/src/rendering/viewport.dart

@@ -129,28 +129,11 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
   ) {
     assert(scrollOffset.isFinite);
     assert(scrollOffset >= 0.0);
-    ScrollDirection adjustedUserScrollDirection;
-    switch (growthDirection) {
-      case GrowthDirection.forward:
-        adjustedUserScrollDirection = offset.userScrollDirection;
-        break;
-      case GrowthDirection.reverse:
-        switch (offset.userScrollDirection) {
-          case ScrollDirection.forward:
-            adjustedUserScrollDirection = ScrollDirection.reverse;
-            break;
-          case ScrollDirection.reverse:
-            adjustedUserScrollDirection = ScrollDirection.forward;
-            break;
-          case ScrollDirection.idle:
-            adjustedUserScrollDirection = ScrollDirection.idle;
-            break;
-        }
-        break;
-    }
+    final ScrollDirection adjustedUserScrollDirection =
+        applyGrowthDirecitonToScrollDirection(offset.userScrollDirection, growthDirection);
     assert(adjustedUserScrollDirection != null);
     double maxPaintOffset = layoutOffset;
-    double initialLayoutOffset = layoutOffset;
+    final double initialLayoutOffset = layoutOffset;
     while (child != null) {
       assert(scrollOffset >= 0.0);
       child.layout(new SliverConstraints(

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -33,6 +33,23 @@ enum ScrollDirection {
   reverse,
 }
 
+/// Returns the opposite of the given [ScrollDirection].
+///
+/// Specifically, returns [AxisDirection.reverse] for [AxisDirection.forward]
+/// (and vice versa) and returns [ScrollDirection.idle] for
+/// [ScrollDirection.idle].
+ScrollDirection flipScrollDirection(ScrollDirection direction) {
+  switch (direction) {
+    case ScrollDirection.idle:
+      return ScrollDirection.idle;
+    case ScrollDirection.forward:
+      return ScrollDirection.reverse;
+    case ScrollDirection.reverse:
+      return ScrollDirection.forward;
+  }
+  return null;
+}
+
 abstract class ViewportOffset extends ChangeNotifier {
   ViewportOffset();
   factory ViewportOffset.fixed(double value) = _FixedViewportOffset;