Reusable App Bar and Scafold in Flutter (Pseudo code only)

app_bar.dart

class AppAppBar extends StatelessWidget implements PreferredSizeWidget {
  final WidgetBuilder? secondaryAction;
  final bool showBackButton;
  final String title;
  final VoidCallback? onBackPressed;
  final String? popScopeDescription;

  /// Called when the pop scope is invoked and yes pressed after showYesNoDialog
  final VoidCallback? onPopScopeDiscard;

  const AppAppBar({
    super.key,
    this.secondaryAction,
    this.showBackButton = true,
    this.title = '',
    this.onBackPressed,
    this.popScopeDescription,
    this.onPopScopeDiscard,
  });

  @override
  Widget build(BuildContext context) {
    final description = popScopeDescription;
    if (description != null && description.isNotEmpty) {
      return PopScope(
          canPop: false,
          onPopInvoked: (didPop) async {
            if (didPop) {
              return;
            }
            final result = await showYesNoDialog(
              context: context,
              title: 'Discard',
              subTitle: description,
            );
            if (result && context.mounted) {
              onPopScopeDiscard?.call();
              Navigator.pop(context);
            }
          },
          child: _buildWidget(context));
    }

    return _buildWidget(context);
  }

  @override
  Size get preferredSize => const Size.fromHeight(kToolbarHeight);

  Widget _buildWidget(BuildContext context) {
    return Padding(
      padding: EdgeInsets.only(top: MediaQuery.of(context).padding.top + 16, left: 16, right: 16),
      child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: [
          Row(
            children: [
              Visibility(
                visible: showBackButton,
                maintainAnimation: title.isEmpty,
                maintainSize: title.isEmpty,
                maintainState: title.isEmpty,
                child: TransParentRoundIconButton(
                  Icons.arrow_back,
                  onPressed: () {
                    final tempOnBackPressed = onBackPressed;
                    tempOnBackPressed != null ? tempOnBackPressed.call() : Navigator.maybePop(context);
                  },
                ),
              ),
              if (title.isNotEmpty)
                Padding(
                  padding: const EdgeInsets.only(left: 8),
                  child: Text(title),
                ),
            ],
          ),
          if (secondaryAction != null) secondaryAction!(context),
        ],
      ),
    );
  }
}

app_scaffold.dart

class AppScaffold extends StatelessWidget {
  final Widget? background;
  final Widget? body;
  final WidgetBuilder? secondaryAction;
  final bool showAppBar;
  final bool showBackground;
  final Widget? bottomNavigationBar;
  final bool showBackButton;
  final String title;
  final VoidCallback? onBackPressed;
  final String? popScopeDescription;
  final VoidCallback? onPopScopeDiscard;
  final bool? extendBody;

  const AppScaffold({
    super.key,
    this.background,
    this.body,
    this.secondaryAction,
    this.showAppBar = true,
    this.showBackground = true,
    this.showBackButton = true,
    this.bottomNavigationBar,
    this.title = '',
    this.onBackPressed,
    this.popScopeDescription,
    this.onPopScopeDiscard,
    this.extendBody,
  });

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: showAppBar
          ? AppAppBar(
              secondaryAction: secondaryAction,
              showBackButton: showBackButton,
              title: title,
              onBackPressed: onBackPressed,
              popScopeDescription: popScopeDescription,
              onPopScopeDiscard: onPopScopeDiscard,
            )
          : null,
      bottomNavigationBar: bottomNavigationBar,
      extendBody: extendBody ?? true,
      extendBodyBehindAppBar: true,
      backgroundColor: Colors.transparent,
      body: Stack(
        children: [
          if (showBackground) (background != null) ? background! : const TopGradient(),
          if (body != null) body!,
        ],
      ),
    );
  }
}

usage.dart

//The `secondaryAction` has a builder pattern so that we can use bloc builder only for this button state updates
AppScaffold(
  secondaryAction: (context) => BlocBuilder<SomeBloc, BlocState>(
      buildWhen: (previous, current) => current is BlocSuccessState,
      builder: (context, state) {
        return Visibility(
          visible: context.read<SomeBloc>().data.isNotEmpty ?? false,
          child: TransParentRoundIconButton(
            Icons.check,
            onPressed: () => _onConfirm(context),
          ),
        );
  }),);