keyed_collection_widgets 0.4.4

BottomNavigationBar, IndexedStack, and TabController that use item keys instead of indexes.

These are replacements to BottomNavigationBar, IndexedStack, and TabController that use item keys instead if numeric indexes.

Problem

With traditional widgets, you write something like

const tabFavorites = 0;
const tabSearch = 1;
// ...
if (tabIndex == tabFavorites) {
  // ...
}

If items in your bar can change, you get an error-prone conversion from indexes to meanings. Also with a mature architecture you tend to use enum for your tabs, and even with constant bar items you must write code to convert between enum and int.

Solution

This package provides widgets to be used with any type instead of int. In most cases you will use enum.

Some advantages of enum over indexes:

• No way for the value to fall out of range.
• Easier debugging with IDE tools.
• You will never use magic numbers for indexes.
• No need to synchronize the order of children between different widgets.

String is also a good type to use with these widgets if you have dynamic or potentially unlimited tabs (like in a browser or a document editor) but still want meaningful keys instead of indexes.

KeyedBottomNavigationBar and KeyedStack

This example uses an enum for selectable navigation items:

This example is runnable. Download the repository and open the example project. Then run nav_stack.dart

enum TabEnum { favorites, search }

class _MyScreenState extends State<MyScreen> {
  TabEnum _tab = TabEnum.favorites;

  @override
  Widget build(BuildContext context) {
    // This is a simplified example: IndexedStack and KeyedStack are only
    // meaningful if they contain stateful widgets to preserve state
    // between switches.
    return Scaffold(
      body: KeyedStack<TabEnum>(
        itemKey: _tab,
        children: const {
          TabEnum.favorites: Center(
            key: ValueKey('favorites_pane'),
            child: Text('Favorites'),
          ),
          TabEnum.search: Center(
            key: ValueKey('search_pane'),
            child: Text('Search'),
          ),
        },
      ),
      bottomNavigationBar: KeyedBottomNavigationBar<TabEnum>(
        currentItemKey: _tab,
        items: const {
          TabEnum.favorites: BottomNavigationBarItem(
            icon: Icon(Icons.star),
            label: 'Favorites',
          ),
          TabEnum.search: BottomNavigationBarItem(
            icon: Icon(Icons.search),
            label: 'Search',
          ),
        },
        onTap: (tab) => setState(() {
          _tab = tab;
        }),
      ),
    );
  }
}

KeyedBottomNavigationBar and KeyedStack support all the arguments of their traditional counterparts. The only difference is that current keys are required and do not default to first element.

KeyedTabController, KeyedTabBar, KeyedTabBarView

Minimal Example

This example uses enum for tabs:

This example is runnable. Download the repository and open the example project. Then run tabs.dart

enum MyTab { one, two, three }

class _MyScreenState extends State<MyScreen> with TickerProviderStateMixin {
  late final _tabController = KeyedTabController<TabEnum>(
    initialKey: TabEnum.three,
    keys: [TabEnum.one, TabEnum.two, TabEnum.three],
    vsync: this,
  );

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('${_tabController.currentKey}'),
        bottom: KeyedTabBar(
          tabs: {
            for (final key in _tabController.keys) key: Tab(text: '$key'),
          },
          controller: _tabController,
        ),
      ),
      body: KeyedTabBarView(
        children: {
          for (final key in _tabController.keys)
            key: Center(child: Text('$key content')),
        },
        controller: _tabController,
      ),
    );
  }
}

Redundant Children

The ordinary TabBar and TabBarView must have exactly as many children as their controller is set to. This means that if you need to hide some tabs, there must be three locations in your code to know that:

1. The code that updates the controller.
2. The code that creates TabBar widget with tab headers.
3. The code that creates TabBarView widget with tab contents.

This is extremely error-prone.

With this package, KeyedTabBar and KeyedTabBarView have maps of children, so they can contain more widgets than the controller wants to show.

This means that you can unconditionally pass all children for all possible tabs to them, and the only location in your code that needs to know what tabs to show is the code that updates the controller.

Using KeyedTabController as TabController

KeyedTabController implements TabController and is immediately usable as one. If you ever need to get the tab index or select a tab by index, do it as you normally would.

UnanimatedKeyedTabController

With ordinary TabController, you need TickerProvider to create it. And this limits the usage. You must create TabController in a widget. If you want your BLoC or other business logic code to be aware of tabs or control them, it may be tricky to pass the controller there.

This package provides UnanimatedKeyedTabController which has the logic core for KeyedTabController, but not its animation. You can create this controller anywhere and then add the animation in your widget.

Create it like this:

final unanimatedController = UnanimatedKeyedTabController<TabEnum>(
  keys: [TabEnum.one, TabEnum.two, TabEnum.three],
  initialKey: TabEnum.three,
);

Then create KeyedTabController in your widget:

class _MyScreenState extends State<MyScreen> with TickerProviderStateMixin {
  late final _tabController = KeyedTabController<TabEnum>.fromUnanimated(
    controller: unanimatedController,
    vsync: this,
  );
  // ...

This binds the two controllers. If you change the tab via UnanimatedKeyedTabController, then KeyedTabController gets updated, and the tab change is animated in the UI.

And if the user changes the tab by interacting with it, both controllers get updated.

Mutable Tabs and Animation Duration

Ordinary TabController has a fixed length and animationDuration. If you need to change them, you must create a new controller and replace it everywhere.

KeyedTabController has these mutable.

You can change the tabs at any time by setting KeyedTabController.keys property. If the currently selected tab also exists in the new set, its selection is preserved, otherwise the first new tab gets selected.

This is possible because KeyedTabController does not extend but contains TabController and so it can re-create its internal TabController with different parameters without disturbing its own listeners.

TickerProviderStateMixin vs SingleTickerProviderStateMixin

In Flutter's tab examples, you often see the widget's State created with SingleTickerProviderStateMixin. This only allows one TabController to be created in it. However, KeyedTabController re-creates its TabController if you change keys or animationDuration, so it will break if created with SingleTickerProviderStateMixin.

You should use TickerProviderStateMixin for your widgets instead. It allows many TabController objects to be created with it.

DefaultKeyedTabController

Flutter provides DefaultTabController widget which accepts the number of tabs, creates a TabController and provides it to all tab-related widgets under it.

It has the following advantages:

1. It is declarative. You don't have to manage your TabController and dispose it.
2. It allows to change the number of tabs. You just change the length passed to this widget, and it re-creates the TabController, and all widgets under it are updated for the new number of tabs.

This is matched by DefaultKeyedTabController. Although tabs get mutable with this package, the advantage #1 still stands.

This widget comes in two forms:

fromKeys

DefaultKeyedTabController.fromKeys(
  keys: [TabEnum.one, TabEnum.two],
  child: ...
),

Use this when you know the keys to show in your widget.

fromUnanimated

DefaultKeyedTabController.fromUnanimated(
  controller: unanimatedController,
  child: ...
),

Use this if you use UnanimatedKeyedTabController.

Creating Widgets for DefaultKeyedTabController

In Flutter, both TabBar and TabBarView widgets can be created without the controller argument. In this case, they rely on DefaultTabController widget present in the tree above them and break if it is missing.

This is error-prone because the controller argument may simply be forgotten, and this cannot be detected at compile time.

In this package, the controller argument to KeyedTabBar and KeyedTabBarView is required. To rely on the DefaultKeyedTabController, use .withDefaultController static methods of those widgets instead of their default constructors.

There is still no check at compile time that the default controller is present in the tree, but at least you must explicitly declare that you want it and not just have forgotten to pass the controller.

When to use DefaultKeyedTabController

All things equal, prefer DefaultKeyedTabController over manual KeyedTabController creation. This is because that widget will dispose the controller for you when it is not needed anymore.

Advanced Example

This example shows:

• Changing the set of tabs without re-creating a controller.
• Programmatically activating a tab by its key.
• Programmatically activating a tab by its index.
• Changing the animation duration without re-creating a controller.
• DefaultKeyedTabController widgets.
• UnanimatedKeyedTabController.
• Replacing the UnanimatedKeyedTabController that a KeyedTabController is linked to.

This example is runnable. Download the repository and open the example project. Then run nav_stack_tabs.dart

enum_map

Although enum enhances type safety for tabs, it is still not absolute. In widgets, you may still forget to use all keys in children map and only know that at runtime.

You can make this compile-time safe by using enum_map package that generates maps that are guaranteed to have all keys at compile time (see that package's README for more info):

@unmodifiableEnumMap                                    // CHANGED
enum TabEnum { one, two, three }

class _MyScreenState extends State<MyScreen> with TickerProviderStateMixin {
  late final _tabController = KeyedTabController<TabEnum>(
    initialKey: TabEnum.three,
    keys: [TabEnum.one, TabEnum.two, TabEnum.three],
    vsync: this,
  );

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('${_tabController.currentKey}'),
        bottom: KeyedTabBar(
          controller: _tabController,
          tabs: const UnmodifiableTabEnumMap(           // CHANGED
            one: Tab(text: 'One'),                      // CHANGED
            two: Tab(text: 'Two'),                      // CHANGED
            three: Tab(text: 'Three'),                  // CHANGED
          ),                                            // CHANGED
        ),
      ),
      body: KeyedTabBarView(
        controller: _tabController,
        children: const UnmodifiableTabEnumMap(         // CHANGED
          one: Center(child: Text('One content')),      // CHANGED
          two: Center(child: Text('Two content')),      // CHANGED
          three: Center(child: Text('Three content')),  // CHANGED
        ),                                              // CHANGED
      ),
    );
  }
}

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