Small cleanup in CupertinoSliverRefreshControl (#27570)

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

@@ -27,6 +27,7 @@ class _CupertinoSliverRefresh extends SingleChildRenderObjectWidget {
   // The amount of space the indicator should occupy in the sliver in a
   // resting state when in the refreshing mode.
   final double refreshIndicatorLayoutExtent;
+
   // _RenderCupertinoSliverRefresh will paint the child in the available
   // space either way but this instructs the _RenderCupertinoSliverRefresh
   // on whether to also occupy any layoutExtent space or not.
@@ -82,8 +83,8 @@ class _RenderCupertinoSliverRefresh extends RenderSliver
   }
 
   // The child box will be laid out and painted in the available space either
-  // way but this determines whether to also occupy any layoutExtent space or
-  // not.
+  // way but this determines whether to also occupy any
+  // [SliverGeometry.layoutExtent] space or not.
   bool get hasLayoutExtent => _hasLayoutExtent;
   bool _hasLayoutExtent;
   set hasLayoutExtent(bool value) {
@@ -268,7 +269,7 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
   /// Create a new refresh control for inserting into a list of slivers.
   ///
   /// The [refreshTriggerPullDistance] and [refreshIndicatorExtent] arguments
-  /// must not be null.
+  /// must not be null and must be >= 0.
   ///
   /// The [builder] argument may be null, in which case no indicator UI will be
   /// shown but the [onRefresh] will still be invoked. By default, [builder]
@@ -389,8 +390,8 @@ class CupertinoSliverRefreshControl extends StatefulWidget {
 }
 
 class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshControl> {
-  /// Reset the state from done to inactive when only this fraction of the
-  /// original `refreshTriggerPullDistance` is left.
+  // Reset the state from done to inactive when only this fraction of the
+  // original `refreshTriggerPullDistance` is left.
   static const double _inactiveResetOverscrollFraction = 0.1;
 
   RefreshIndicatorMode refreshState;
@@ -399,12 +400,12 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
   // The amount of space available from the inner indicator box's perspective.
   //
   // The value is the sum of the sliver's layout extent and the overscroll
-  // (which partially gets transfered into the layout extent when the refresh
+  // (which partially gets transferred into the layout extent when the refresh
   // triggers).
   //
-  // The value of lastIndicatorExtent doesn't change when the sliver scrolls
+  // The value of latestIndicatorBoxExtent doesn't change when the sliver scrolls
   // away without retracting; it is independent from the sliver's scrollOffset.
-  double lastIndicatorExtent = 0.0;
+  double latestIndicatorBoxExtent = 0.0;
   bool hasSliverLayoutExtent = false;
 
   @override
@@ -433,7 +434,7 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
 
     switch (refreshState) {
       case RefreshIndicatorMode.inactive:
-        if (lastIndicatorExtent <= 0) {
+        if (latestIndicatorBoxExtent <= 0) {
           return RefreshIndicatorMode.inactive;
         } else {
           nextState = RefreshIndicatorMode.drag;
@@ -441,9 +442,9 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
         continue drag;
       drag:
       case RefreshIndicatorMode.drag:
-        if (lastIndicatorExtent == 0) {
+        if (latestIndicatorBoxExtent == 0) {
           return RefreshIndicatorMode.inactive;
-        } else if (lastIndicatorExtent < widget.refreshTriggerPullDistance) {
+        } else if (latestIndicatorBoxExtent < widget.refreshTriggerPullDistance) {
           return RefreshIndicatorMode.drag;
         } else {
           if (widget.onRefresh != null) {
@@ -476,7 +477,7 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
           continue done;
         }
 
-        if (lastIndicatorExtent > widget.refreshIndicatorExtent) {
+        if (latestIndicatorBoxExtent > widget.refreshIndicatorExtent) {
           return RefreshIndicatorMode.armed;
         } else {
           nextState = RefreshIndicatorMode.refresh;
@@ -496,7 +497,7 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
         // to 0.0 since the last bit of the animation can take some time and
         // can feel sluggish if not going all the way back to 0.0 prevented
         // a subsequent pull-to-refresh from starting.
-        if (lastIndicatorExtent >
+        if (latestIndicatorBoxExtent >
             widget.refreshTriggerPullDistance * _inactiveResetOverscrollFraction) {
           return RefreshIndicatorMode.done;
         } else {
@@ -517,13 +518,13 @@ class _CupertinoSliverRefreshControlState extends State<CupertinoSliverRefreshCo
       // its owner to trigger state changes.
       child: LayoutBuilder(
         builder: (BuildContext context, BoxConstraints constraints) {
-          lastIndicatorExtent = constraints.maxHeight;
+          latestIndicatorBoxExtent = constraints.maxHeight;
           refreshState = transitionNextState();
           if (widget.builder != null && refreshState != RefreshIndicatorMode.inactive) {
             return widget.builder(
               context,
               refreshState,
-              lastIndicatorExtent,
+              latestIndicatorBoxExtent,
               widget.refreshTriggerPullDistance,
               widget.refreshIndicatorExtent,
             );