ClampOverscrolls Inherited Widget (#3267)

* ClampOverscrolls Inherited Widget

examples/material_gallery/lib/demo/list_demo.dart

@@ -194,7 +194,6 @@ class ListDemoState extends State<ListDemo> {
           child: new MaterialList(
             type: _itemType,
             scrollablePadding: new EdgeInsets.all(_dense ? 4.0 : 8.0),
-            clampOverscrolls: true,
             children: listItems
           )
         )

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

@@ -24,7 +24,6 @@ class MaterialList extends StatefulWidget {
     this.initialScrollOffset,
     this.onScroll,
     this.type: MaterialListType.twoLine,
-    this.clampOverscrolls: false,
     this.children,
     this.scrollablePadding: EdgeInsets.zero,
     this.scrollableKey
@@ -33,7 +32,6 @@ class MaterialList extends StatefulWidget {
   final double initialScrollOffset;
   final ScrollListener onScroll;
   final MaterialListType type;
-  final bool clampOverscrolls;
   final Iterable<Widget> children;
   final EdgeInsets scrollablePadding;
   final Key scrollableKey;
@@ -49,7 +47,6 @@ class _MaterialListState extends State<MaterialList> {
       key: config.scrollableKey,
       initialScrollOffset: config.initialScrollOffset,
       scrollDirection: Axis.vertical,
-      clampOverscrolls: config.clampOverscrolls,
       onScroll: config.onScroll,
       itemExtent: kListItemExtent[config.type],
       padding: const EdgeInsets.symmetric(vertical: 8.0) + config.scrollablePadding,

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

@@ -202,7 +202,10 @@ class _OverscrollIndicatorState extends State<OverscrollIndicator> {
             child: child
           );
         },
-        child: config.child
+        child: new ClampOverscrolls(
+          child: config.child,
+          value: true
+        )
       )
     );
   }

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

@@ -90,7 +90,7 @@ abstract class ExtentScrollBehavior extends ScrollBehavior<double, double> {
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
     description.add('content: ${contentExtent.toStringAsFixed(1)}');
-    description.add('container: ${contentExtent.toStringAsFixed(1)}');
+    description.add('container: ${containerExtent.toStringAsFixed(1)}');
     description.add('range: ${minScrollOffset?.toStringAsFixed(1)} .. ${maxScrollOffset?.toStringAsFixed(1)}');
   }
 }

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

@@ -11,6 +11,40 @@ import 'virtual_viewport.dart';
 
 import 'package:flutter/rendering.dart';
 
+/// If true, the ClampOverscroll's [Scrollable] descendant will clamp its
+/// viewport's scrollOffsets to the [ScrollBehavior]'s min and max values.
+/// In this case the Scrollable's scrollOffset will still over and undershoot
+/// the ScrollBehavior's limits, but the viewport itself will not.
+class ClampOverscrolls extends InheritedWidget {
+  ClampOverscrolls({
+    Key key,
+    this.value,
+    Widget child
+  }) : super(key: key, child: child) {
+    assert(value != null);
+    assert(child != null);
+  }
+
+  /// True if the [Scrollable] descendant should clamp its viewport's scrollOffset
+  /// values when they are less than the [ScrollBehavior]'s minimum or greater than
+  /// its maximum.
+  final bool value;
+
+  static bool of(BuildContext context) {
+    final ClampOverscrolls result = context.inheritFromWidgetOfExactType(ClampOverscrolls);
+    return result?.value ?? false;
+  }
+
+  @override
+  bool updateShouldNotify(ClampOverscrolls old) => value != old.value;
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('value: $value');
+  }
+}
+
 class ScrollableList extends Scrollable {
   ScrollableList({
     Key key,
@@ -21,7 +55,6 @@ class ScrollableList extends Scrollable {
     SnapOffsetCallback snapOffsetCallback,
     this.itemExtent,
     this.itemsWrap: false,
-    this.clampOverscrolls: false,
     this.padding,
     this.children
   }) : super(
@@ -37,7 +70,6 @@ class ScrollableList extends Scrollable {
 
   final double itemExtent;
   final bool itemsWrap;
-  final bool clampOverscrolls;
   final EdgeInsets padding;
   final Iterable<Widget> children;
 
@@ -64,10 +96,11 @@ class _ScrollableListState extends ScrollableState<ScrollableList> {
 
   @override
   Widget buildContent(BuildContext context) {
-    final double listScrollOffset = config.clampOverscrolls
+    final bool clampOverscrolls = ClampOverscrolls.of(context);
+    final double listScrollOffset = clampOverscrolls
       ? scrollOffset.clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset)
       : scrollOffset;
-    return new ListViewport(
+    Widget viewport = new ListViewport(
       onExtentsChanged: _handleExtentsChanged,
       scrollOffset: listScrollOffset,
       mainAxis: config.scrollDirection,
@@ -77,6 +110,9 @@ class _ScrollableListState extends ScrollableState<ScrollableList> {
       padding: config.padding,
       children: config.children
     );
+    if (clampOverscrolls)
+      viewport = new ClampOverscrolls(value: false, child: viewport);
+    return viewport;
   }
 }