Fix excessive rebuilds of DSS (flutter#69724)

Author: dnfield

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

@@ -288,17 +288,17 @@ class _DraggableSheetExtent {
 class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
   late _DraggableScrollableSheetScrollController _scrollController;
   late _DraggableSheetExtent _extent;
+  // The child only gets rebuilt when dependencies or the widget change.
+  // Otherwise, excessive rebuilds of the child are triggered every time the
+  // scroll extent changes, which is very expensive and does not provide any
+  // helpful information to the child. If the child needs to rebuild whenever
+  // the scroll position changes, they can always subscribe to it.
+  Widget? _child;
 
   @override
   void initState() {
     super.initState();
-    _extent = _DraggableSheetExtent(
-      minExtent: widget.minChildSize,
-      maxExtent: widget.maxChildSize,
-      initialExtent: widget.initialChildSize,
-      listener: _setExtent,
-    );
-    _scrollController = _DraggableScrollableSheetScrollController(extent: _extent);
+    _updateExtent();
   }
 
   @override
@@ -317,13 +317,33 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
       }
       _extent._currentExtent.value = _extent.initialExtent;
     }
+    _child = widget.builder(context, _scrollController);
+  }
+
+  @override
+  void didUpdateWidget(DraggableScrollableSheet oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    _updateExtent();
+    // Call this unconditionally - the closure may not change even though it
+    // refers to things outside of its identity, e.g. a tearoff from state that
+    // has an `if (stateVariable)`.
+    _child = widget.builder(context, _scrollController);
+  }
+
+  void _updateExtent() {
+    _extent = _DraggableSheetExtent(
+      minExtent: widget.minChildSize,
+      maxExtent: widget.maxChildSize,
+      initialExtent: widget.initialChildSize,
+      listener: _setExtent,
+    );
+    _scrollController = _DraggableScrollableSheetScrollController(extent: _extent);
   }
 
   void _setExtent() {
     setState(() {
       // _extent has been updated when this is called.
     });
-
   }
 
   @override
@@ -333,7 +353,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
         _extent.availablePixels = widget.maxChildSize * constraints.biggest.height;
         final Widget sheet = FractionallySizedBox(
           heightFactor: _extent.currentExtent,
-          child: widget.builder(context, _scrollController),
+          child: _child,
           alignment: Alignment.bottomCenter,
         );
         return widget.expand ? SizedBox.expand(child: sheet) : sheet;
@@ -344,6 +364,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
   @override
   void dispose() {
     _scrollController.dispose();
+    _child = null;
     super.dispose();
   }
 }

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -309,4 +309,149 @@ void main() {
       expect(notificationTypes, types);
     });
   }
+
+  testWidgets('Builder is not called excessively', (WidgetTester tester) async {
+    int buildCount = 0;
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: Stack(
+        children: <Widget>[
+          DraggableScrollableSheet(
+            builder: (BuildContext context, ScrollController scrollController) {
+              buildCount += 1;
+              return Container(
+                color: const Color(0xFFABCDEF),
+                child: ListView.builder(
+                  controller: scrollController,
+                  itemExtent: 100,
+                  itemCount: 100,
+                  itemBuilder: (BuildContext context, int index) => Text('Item $index'),
+                ),
+              );
+            },
+          ),
+        ],
+      ),
+    ));
+    expect(buildCount, 1);
+    await tester.flingFrom(const Offset(0, 325), const Offset(0, -325), 200);
+    expect(buildCount, 1);
+    await tester.pumpAndSettle();
+    expect(buildCount, 1);
+  });
+
+  testWidgets('Builder is called if widget updates', (WidgetTester tester) async {
+    int buildCount = 0;
+    final GlobalKey key = GlobalKey();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: Stack(
+        children: <Widget>[
+          DraggableScrollableSheet(
+            key: key,
+            builder: (BuildContext context, ScrollController scrollController) {
+              buildCount += 1;
+              return Container(
+                color: const Color(0xFFABCDEF),
+                child: ListView.builder(
+                  controller: scrollController,
+                  itemExtent: 100,
+                  itemCount: 100,
+                  itemBuilder: (BuildContext context, int index) => Text('Item $index'),
+                ),
+              );
+            },
+          ),
+        ],
+      ),
+    ));
+    expect(buildCount, 1);
+    expect(find.text('Item 1'), findsOneWidget);
+
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: Stack(
+        children: <Widget>[
+          DraggableScrollableSheet(
+            key: key,
+            builder: (BuildContext context, ScrollController scrollController) {
+              buildCount += 1;
+              return Container(
+                color: const Color(0xFFFEDCBA),
+                child: ListView.builder(
+                  controller: scrollController,
+                  itemExtent: 50,
+                  itemCount: 100,
+                  itemBuilder: (BuildContext context, int index) => Text('New Item $index'),
+                ),
+              );
+            },
+          ),
+        ],
+      ),
+    ));
+    expect(buildCount, 2);
+    expect(find.text('Item 1'), findsNothing);
+    expect(find.text('New Item 1'), findsOneWidget);
+  });
+
+  testWidgets('Changes to min/max/initial child size are respected', (WidgetTester tester) async {
+    final GlobalKey key = GlobalKey();
+    final Key childKey = UniqueKey();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: Stack(
+        children: <Widget>[
+          DraggableScrollableSheet(
+            key: key,
+            minChildSize: .25,
+            maxChildSize: 1.0,
+            initialChildSize: .5,
+            builder: (BuildContext context, ScrollController scrollController) {
+              return Container(
+                key: childKey,
+                color: const Color(0xFFABCDEF),
+                child: ListView.builder(
+                  controller: scrollController,
+                  itemExtent: 100,
+                  itemCount: 100,
+                  itemBuilder: (BuildContext context, int index) => Text('Item $index'),
+                ),
+              );
+            },
+          ),
+        ],
+      ),
+    ));
+    expect(find.text('Item 1'), findsOneWidget);
+    expect(tester.getRect(find.byKey(childKey)), const Rect.fromLTRB(0, 300, 800, 600));
+
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: Stack(
+        children: <Widget>[
+          DraggableScrollableSheet(
+            key: key,
+            minChildSize: .5,
+            maxChildSize: .75,
+            initialChildSize: .6,
+            builder: (BuildContext context, ScrollController scrollController) {
+              return Container(
+                key: childKey,
+                color: const Color(0xFFFEDCBA),
+                child: ListView.builder(
+                  controller: scrollController,
+                  itemExtent: 50,
+                  itemCount: 100,
+                  itemBuilder: (BuildContext context, int index) => Text('New Item $index'),
+                ),
+              );
+            },
+          ),
+        ],
+      ),
+    ));
+    expect(find.text('New Item 1'), findsOneWidget);
+    expect(tester.getRect(find.byKey(childKey)), const Rect.fromLTRB(0, 240, 800, 600));
+  });
 }