Implement reactive page (#9)

app/lib/components/app_page.dart

@@ -0,0 +1,94 @@
+import 'dart:io';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+
+class AppPage extends StatelessWidget {
+  final String? title;
+  final Widget? titleWidget;
+  final List<Widget>? actions;
+  final Widget? leading;
+  final Widget? floatingActionButton;
+  final Widget? body;
+  final bool automaticallyImplyLeading;
+  final bool? resizeToAvoidBottomInset;
+
+  const AppPage({
+    super.key,
+    this.title,
+    this.titleWidget,
+    this.actions,
+    this.leading,
+    this.body,
+    this.floatingActionButton,
+    this.resizeToAvoidBottomInset,
+    this.automaticallyImplyLeading = true,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    if (Platform.isIOS || Platform.isMacOS) {
+      return _buildCupertinoScaffold(context);
+    } else {
+      return _buildMaterialScaffold(context);
+    }
+  }
+
+  Widget _buildCupertinoScaffold(BuildContext context) => CupertinoPageScaffold(
+        navigationBar: (title == null && titleWidget == null) &&
+                actions == null &&
+                leading == null
+            ? null
+            : CupertinoNavigationBar(
+                leading: leading,
+                middle: titleWidget ?? _title(context),
+                border: null,
+                trailing: actions == null
+                    ? null
+                    : actions!.length == 1
+                        ? actions!.first
+                        : Row(
+                            mainAxisSize: MainAxisSize.min,
+                            children: actions!,
+                          ),
+                automaticallyImplyLeading: automaticallyImplyLeading,
+                previousPageTitle: automaticallyImplyLeading
+                    ? MaterialLocalizations.of(context).backButtonTooltip
+                    : null,
+              ),
+        resizeToAvoidBottomInset: resizeToAvoidBottomInset ?? true,
+        child: Stack(
+          alignment: Alignment.bottomRight,
+          children: [
+            body ?? const SizedBox(),
+            SafeArea(
+              child: Padding(
+                padding: const EdgeInsets.only(right: 16, bottom: 16),
+                child: floatingActionButton ?? const SizedBox(),
+              ),
+            ),
+          ],
+        ),
+      );
+
+  Widget _buildMaterialScaffold(BuildContext context) => Scaffold(
+        appBar: (title == null && titleWidget == null) &&
+                actions == null &&
+                leading == null
+            ? null
+            : AppBar(
+                title: titleWidget ?? _title(context),
+                actions: actions,
+                leading: leading,
+                automaticallyImplyLeading: automaticallyImplyLeading,
+              ),
+        body: body,
+        floatingActionButton: floatingActionButton,
+        resizeToAvoidBottomInset: resizeToAvoidBottomInset,
+      );
+
+  Widget _title(BuildContext context) => Text(
+        title ?? '',
+        maxLines: 1,
+        overflow: TextOverflow.ellipsis,
+      );
+}

style/lib/animations/parallex_effect.dart

@@ -0,0 +1,62 @@
+import 'package:flutter/material.dart';
+
+class ParallaxFlowDelegate extends FlowDelegate {
+  ParallaxFlowDelegate({
+    required this.scrollable,
+    required this.listItemContext,
+    required this.backgroundImageKey,
+  }) : super(repaint: scrollable.position);
+
+  final ScrollableState scrollable;
+  final BuildContext listItemContext;
+  final GlobalKey backgroundImageKey;
+
+  @override
+  BoxConstraints getConstraintsForChild(int i, BoxConstraints constraints) {
+    return BoxConstraints.tightFor(
+      width: constraints.maxWidth,
+    );
+  }
+
+  @override
+  void paintChildren(FlowPaintingContext context) {
+    // Ensure necessary objects are not null before proceeding
+    final backgroundImageContext = backgroundImageKey.currentContext;
+    if (backgroundImageContext == null) {
+      return;
+    }
+
+    // Calculate the position of this list item within the viewport.
+    final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
+    final listItemBox = listItemContext.findRenderObject() as RenderBox;
+    final listItemOffset = listItemBox.localToGlobal(
+      listItemBox.size.centerLeft(Offset.zero),
+      ancestor: scrollableBox,
+    );
+
+    // Determine the percent position of this list item within the scrollable area.
+    final viewportDimension = scrollable.position.viewportDimension;
+    final scrollFraction = (listItemOffset.dy / viewportDimension).clamp(0.0, 1.0);
+
+    // Calculate the vertical alignment of the background based on the scroll percent.
+    final verticalAlignment = Alignment(0.0, scrollFraction * 2 - 1);
+
+    // Convert the background alignment into a pixel offset for painting purposes.
+    final backgroundSize = (backgroundImageContext.findRenderObject() as RenderBox).size;
+    final listItemSize = context.size;
+    final childRect = verticalAlignment.inscribe(backgroundSize, Offset.zero & listItemSize);
+
+    // Paint the background.
+    context.paintChild(
+      0,
+      transform: Transform.translate(offset: Offset(0.0, childRect.top)).transform,
+    );
+  }
+
+  @override
+  bool shouldRepaint(ParallaxFlowDelegate oldDelegate) {
+    return scrollable != oldDelegate.scrollable ||
+        listItemContext != oldDelegate.listItemContext ||
+        backgroundImageKey != oldDelegate.backgroundImageKey;
+  }
+}