Feature refresh indicator padding (#72052)

Add ability to control RefreshProgressIndicator's distance from edges.

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

@@ -113,6 +113,7 @@ class RefreshIndicator extends StatefulWidget {
     Key? key,
     required this.child,
     this.displacement = 40.0,
+    this.edgeOffset = 0.0,
     required this.onRefresh,
     this.color,
     this.backgroundColor,
@@ -136,11 +137,31 @@ class RefreshIndicator extends StatefulWidget {
   /// Typically a [ListView] or [CustomScrollView].
   final Widget child;
 
-  /// The distance from the child's top or bottom edge to where the refresh
-  /// indicator will settle. During the drag that exposes the refresh indicator,
-  /// its actual displacement may significantly exceed this value.
+  /// The distance from the child's top or bottom [edgeOffset] where
+  /// the refresh indicator will settle. During the drag that exposes the refresh
+  /// indicator, its actual displacement may significantly exceed this value.
+  ///
+  /// In most cases, [displacement] distance starts counting from the parent's
+  /// edges. However, if [edgeOffset] is larger than zero then the [displacement]
+  /// value is calculated from that offset instead of the parent's edge.
   final double displacement;
 
+  /// The offset where [RefreshProgressIndicator] starts to appear on drag start.
+  ///
+  /// Depending whether the indicator is showing on the top or bottom, the value
+  /// of this variable controls how far from the parent's edge the progress
+  /// indicator starts to appear. This may come in handy when, for example, the
+  /// UI contains a top [Widget] which covers the parent's edge where the progress
+  /// indicator would otherwise appear.
+  ///
+  /// By default, the edge offset is set to 0.
+  ///
+  /// See also:
+  ///
+  ///  * [displacement], can be used to change the distance from the edge that
+  ///    the indicator settles.
+  final double edgeOffset;
+
   /// A function that's called when the user has dragged the refresh indicator
   /// far enough to demonstrate that they want the app to refresh. The returned
   /// [Future] must complete when the refresh operation is finished.
@@ -500,8 +521,8 @@ class RefreshIndicatorState extends State<RefreshIndicator> with TickerProviderS
       children: <Widget>[
         child,
         if (_mode != null) Positioned(
-          top: _isIndicatorAtTop! ? 0.0 : null,
-          bottom: !_isIndicatorAtTop! ? 0.0 : null,
+          top: _isIndicatorAtTop! ? widget.edgeOffset : null,
+          bottom: !_isIndicatorAtTop! ? widget.edgeOffset : null,
           left: 0.0,
           right: 0.0,
           child: SizeTransition(

packages/flutter/test/material/refresh_indicator_test.dart

@@ -500,6 +500,83 @@ void main() {
     );
   });
 
+  testWidgets('RefreshIndicator responds to edgeOffset', (WidgetTester tester) async {
+    await tester.pumpWidget(
+        MaterialApp(
+          home: RefreshIndicator(
+            onRefresh: () async {},
+            child: ListView(
+              physics: const AlwaysScrollableScrollPhysics(),
+              children: <String>['A', 'B', 'C', 'D', 'E', 'F'].map<Widget>((String item) {
+                return SizedBox(
+                  height: 200.0,
+                  child: Text(item),
+                );
+              }).toList(),
+            ),
+          ),
+        )
+    );
+
+    //By default the value of edgeOffset is 0.0
+    expect(
+        tester.widget<RefreshIndicator>(find.byType(RefreshIndicator)).edgeOffset,
+        0.0,
+    );
+
+    await tester.pumpWidget(
+        MaterialApp(
+          home: RefreshIndicator(
+            onRefresh: () async {},
+            edgeOffset: kToolbarHeight,
+            child: ListView(
+              physics: const AlwaysScrollableScrollPhysics(),
+              children: <String>['A', 'B', 'C', 'D', 'E', 'F'].map<Widget>((String item) {
+                return SizedBox(
+                  height: 200.0,
+                  child: Text(item),
+                );
+              }).toList(),
+            ),
+          ),
+        )
+    );
+
+    expect(
+        tester.widget<RefreshIndicator>(find.byType(RefreshIndicator)).edgeOffset,
+        kToolbarHeight,
+    );
+  });
+
+  testWidgets('RefreshIndicator appears at edgeOffset', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: RefreshIndicator(
+        edgeOffset: kToolbarHeight,
+        displacement: kToolbarHeight,
+        onRefresh: () async {
+          await Future<void>.delayed(const Duration(seconds: 1), () { });
+        },
+        child: ListView(
+          physics: const AlwaysScrollableScrollPhysics(),
+          children: <String>['A', 'B', 'C', 'D', 'E', 'F'].map<Widget>((String item) {
+            return SizedBox(
+              height: 200.0,
+              child: Text(item),
+            );
+          }).toList(),
+        ),
+      ),
+    ));
+
+    await tester.fling(find.byType(ListView), const Offset(0.0, 2.0 * kToolbarHeight), 1000.0);
+    await tester.pump(const Duration(seconds: 2));
+
+    expect(
+      tester.getTopLeft(find.byType(RefreshProgressIndicator)).dy,
+      greaterThanOrEqualTo(2.0 * kToolbarHeight),
+    );
+  });
+
   testWidgets('Top RefreshIndicator(anywhere mode) should be shown when dragging from non-zero scroll position', (WidgetTester tester) async {
     refreshCalled = false;
     final ScrollController scrollController = ScrollController();