Add SheetTheme (#93)

Closes #91.

package/lib/smooth_sheets.dart

@@ -14,6 +14,7 @@ export 'src/foundation/sheet_content_scaffold.dart';
 export 'src/foundation/sheet_controller.dart' hide SheetControllerScope;
 export 'src/foundation/sheet_extent.dart';
 export 'src/foundation/sheet_physics.dart';
+export 'src/foundation/theme.dart';
 export 'src/modal/cupertino.dart';
 export 'src/modal/modal_sheet.dart';
 export 'src/navigation/navigation_route.dart';

package/lib/src/foundation/sized_content_sheet.dart

@@ -4,6 +4,7 @@ import 'package:smooth_sheets/src/foundation/keyboard_dismissible.dart';
 import 'package:smooth_sheets/src/foundation/sheet_controller.dart';
 import 'package:smooth_sheets/src/foundation/sheet_extent.dart';
 import 'package:smooth_sheets/src/foundation/sheet_physics.dart';
+import 'package:smooth_sheets/src/foundation/theme.dart';
 
 /// [SheetExtent] for a [SizedContentSheet].
 abstract class SizedContentSheetExtent extends SheetExtent {
@@ -118,15 +119,19 @@ abstract class SizedContentSheetState<T extends SizedContentSheet>
 
   @override
   Widget build(BuildContext context) {
+    final theme = SheetTheme.maybeOf(context);
+    final keyboardDismissBehavior =
+        widget.keyboardDismissBehavior ?? theme?.keyboardDismissBehavior;
+
     Widget result = SheetContainer(
       factory: factory,
       controller: widget.controller,
       child: buildContent(context),
     );
 
-    if (widget.keyboardDismissBehavior != null) {
+    if (keyboardDismissBehavior != null) {
       result = SheetKeyboardDismissible(
-        dismissBehavior: widget.keyboardDismissBehavior!,
+        dismissBehavior: keyboardDismissBehavior,
         child: result,
       );
     }

package/lib/src/foundation/theme.dart

@@ -0,0 +1,78 @@
+import 'package:flutter/widgets.dart';
+import 'package:smooth_sheets/src/foundation/keyboard_dismissible.dart';
+
+/// A theme for descendant sheets.
+///
+/// The [SheetTheme] is used to configure the default appearance and behavior of
+/// descendant sheets. The current theme's [SheetThemeData] object can be
+/// obtained by calling [SheetTheme.maybeOf] or [SheetTheme.of].
+class SheetTheme extends InheritedWidget {
+  /// Creates a heme for descendant sheets.
+  const SheetTheme({
+    super.key,
+    required this.data,
+    required super.child,
+  });
+
+  /// The default appearance and behavior for descendant sheets.
+  final SheetThemeData data;
+
+  @override
+  bool updateShouldNotify(SheetTheme oldWidget) => data != oldWidget.data;
+
+  /// Obtains the closest [SheetThemeData] object from the given [context]
+  /// if it exists.
+  static SheetThemeData? maybeOf(BuildContext context) {
+    return context.dependOnInheritedWidgetOfExactType<SheetTheme>()?.data;
+  }
+
+  /// Obtains the closest [SheetThemeData] object from the given [context].
+  static SheetThemeData of(BuildContext context) {
+    final themeData = maybeOf(context);
+    assert(
+      themeData != null,
+      'SheetTheme.of() called with a context that'
+      'does not contain a SheetTheme.',
+    );
+
+    return themeData!;
+  }
+}
+
+/// A set of properties that describe the appearance and behavior of a sheet.
+///
+/// Typically, this object is exposed by a [SheetTheme] to
+/// the descendant widgets.
+class SheetThemeData {
+  /// Creates a set of properties that describe the appearance and
+  /// behavior of a sheet.
+  const SheetThemeData({
+    this.keyboardDismissBehavior,
+  });
+
+  /// Determines when the on-screen keyboard should be dismissed.
+  final SheetKeyboardDismissBehavior? keyboardDismissBehavior;
+
+  /// Creates a copy of this object but with the given fields replaced with
+  /// the new values.
+  SheetThemeData copyWith({
+    SheetKeyboardDismissBehavior? keyboardDismissBehavior,
+  }) =>
+      SheetThemeData(
+        keyboardDismissBehavior:
+            keyboardDismissBehavior ?? this.keyboardDismissBehavior,
+      );
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is SheetThemeData &&
+          runtimeType == other.runtimeType &&
+          keyboardDismissBehavior == other.keyboardDismissBehavior;
+
+  @override
+  int get hashCode => Object.hash(
+        runtimeType,
+        keyboardDismissBehavior,
+      );
+}

package/lib/src/navigation/navigation_sheet.dart

@@ -9,6 +9,7 @@ import 'package:smooth_sheets/src/foundation/sheet_controller.dart';
 import 'package:smooth_sheets/src/foundation/sheet_extent.dart';
 import 'package:smooth_sheets/src/foundation/sheet_physics.dart';
 import 'package:smooth_sheets/src/foundation/sheet_status.dart';
+import 'package:smooth_sheets/src/foundation/theme.dart';
 import 'package:smooth_sheets/src/internal/transition_observer.dart';
 
 typedef NavigationSheetTransitionObserver = TransitionObserver;
@@ -95,6 +96,10 @@ class NavigationSheetState extends State<NavigationSheet>
 
   @override
   Widget build(BuildContext context) {
+    final theme = SheetTheme.maybeOf(context);
+    final keyboardDismissBehavior =
+        widget.keyboardDismissBehavior ?? theme?.keyboardDismissBehavior;
+
     Widget result = SheetContainer(
       factory: const _NavigationSheetExtentFactory(),
       controller: widget.controller,
@@ -104,9 +109,9 @@ class NavigationSheetState extends State<NavigationSheet>
       child: widget.child,
     );
 
-    if (widget.keyboardDismissBehavior != null) {
+    if (keyboardDismissBehavior != null) {
       result = SheetKeyboardDismissible(
-        dismissBehavior: widget.keyboardDismissBehavior!,
+        dismissBehavior: keyboardDismissBehavior,
         child: result,
       );
     }