Make FlexibleSpaceBar title padding configurable (#27623)

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

@@ -47,6 +47,7 @@ class FlexibleSpaceBar extends StatefulWidget {
     this.title,
     this.background,
     this.centerTitle,
+    this.titlePadding,
     this.collapseMode = CollapseMode.parallax
   }) : assert(collapseMode != null),
        super(key: key);
@@ -63,7 +64,8 @@ class FlexibleSpaceBar extends StatefulWidget {
 
   /// Whether the title should be centered.
   ///
-  /// Defaults to being adapted to the current [TargetPlatform].
+  /// By default this property is true if the current target platform
+  /// is [TargetPlatform.iOS], false otherwise.
   final bool centerTitle;
 
   /// Collapse effect while scrolling.
@@ -71,6 +73,18 @@ class FlexibleSpaceBar extends StatefulWidget {
   /// Defaults to [CollapseMode.parallax].
   final CollapseMode collapseMode;
 
+  /// Defines how far the [title] is inset from either the widget's
+  /// bottom-left or its center.
+  ///
+  /// Typically this property is used to adjust how far the title is
+  /// is inset from the bottom-left and it is specified along with
+  /// [centerTitle] false.
+  ///
+  /// By default the value of this property is
+  /// `EdgeInsetsDirectional.only(start: 72, bottom: 16)` if the title is
+  /// not centered, `EdgeInsetsDirectional.only(start 0, bottom: 16)` otherwise.
+  final EdgeInsetsGeometry titlePadding;
+
   /// Wraps a widget that contains an [AppBar] to convey sizing information down
   /// to the [FlexibleSpaceBar].
   ///
@@ -206,15 +220,17 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
           color: titleStyle.color.withOpacity(opacity)
         );
         final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
+        final EdgeInsetsGeometry padding = widget.titlePadding ??
+          EdgeInsetsDirectional.only(
+            start: effectiveCenterTitle ? 0.0 : 72.0,
+            bottom: 16.0
+          );
         final double scaleValue = Tween<double>(begin: 1.5, end: 1.0).transform(t);
         final Matrix4 scaleTransform = Matrix4.identity()
           ..scale(scaleValue, scaleValue, 1.0);
         final Alignment titleAlignment = _getTitleAlignment(effectiveCenterTitle);
         children.add(Container(
-          padding: EdgeInsetsDirectional.only(
+          padding: padding,
-            start: effectiveCenterTitle ? 0.0 : 72.0,
-            bottom: 16.0
-          ),
           child: Transform(
             alignment: titleAlignment,
             transform: scaleTransform,

packages/flutter/test/material/flexible_space_bar_test.dart

@@ -114,6 +114,88 @@ void main() {
 
     expect(clipRect.size.height, minExtent);
   });
+
+  testWidgets('FlexibleSpaceBar test titlePadding defaults', (WidgetTester tester) async {
+    Widget buildFrame(TargetPlatform platform, bool centerTitle) {
+      return MaterialApp(
+        theme: ThemeData(platform: platform),
+        home: Scaffold(
+          appBar: AppBar(
+            flexibleSpace: FlexibleSpaceBar(
+              centerTitle: centerTitle,
+              title: const Text('X'),
+            ),
+          ),
+        ),
+      );
+    }
+
+    final Finder title = find.text('X');
+    final Finder flexibleSpaceBar = find.byType(FlexibleSpaceBar);
+    Offset getTitleBottomLeft() {
+      return Offset(
+        tester.getTopLeft(title).dx,
+        tester.getBottomRight(flexibleSpaceBar).dy - tester.getBottomRight(title).dy,
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.android, null));
+    expect(getTitleBottomLeft(), const Offset(72.0, 16.0));
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.android, true));
+    expect(getTitleBottomLeft(), const Offset(390.0, 16.0));
+
+    // Clear the widget tree to avoid animating between Android and iOS.
+    await tester.pumpWidget(Container(key: UniqueKey()));
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.iOS, null));
+    expect(getTitleBottomLeft(), const Offset(390.0, 16.0));
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.iOS, false));
+    expect(getTitleBottomLeft(), const Offset(72.0, 16.0));
+
+  });
+
+  testWidgets('FlexibleSpaceBar test titlePadding override', (WidgetTester tester) async {
+    Widget buildFrame(TargetPlatform platform, bool centerTitle) {
+      return MaterialApp(
+        theme: ThemeData(platform: platform),
+        home: Scaffold(
+          appBar: AppBar(
+            flexibleSpace: FlexibleSpaceBar(
+              titlePadding: EdgeInsets.zero,
+              centerTitle: centerTitle,
+              title: const Text('X'),
+            ),
+          ),
+        ),
+      );
+    }
+
+    final Finder title = find.text('X');
+    final Finder flexibleSpaceBar = find.byType(FlexibleSpaceBar);
+    Offset getTitleBottomLeft() {
+      return Offset(
+        tester.getTopLeft(title).dx,
+        tester.getBottomRight(flexibleSpaceBar).dy - tester.getBottomRight(title).dy,
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.android, null));
+    expect(getTitleBottomLeft(), Offset.zero);
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.android, true));
+    expect(getTitleBottomLeft(), const Offset(390.0, 0.0));
+
+    // Clear the widget tree to avoid animating between Android and iOS.
+    await tester.pumpWidget(Container(key: UniqueKey()));
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.iOS, null));
+    expect(getTitleBottomLeft(), const Offset(390.0, 0.0));
+
+    await tester.pumpWidget(buildFrame(TargetPlatform.iOS, false));
+    expect(getTitleBottomLeft(), Offset.zero);
+  });
 }
 
 class TestDelegate extends SliverPersistentHeaderDelegate {