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Flutter Hooks

A Flutter implementation of React hooks: https://medium.com/@dan_abramov/making-sense-of-react-hooks-fdbde8803889

Hooks are a new kind of object that manage the life-cycle of a Widget. They exist for one reason: increase the code-sharing between widgets by removing duplicates.

Motivation

StatefulWidget suffers from a big problem: it is very difficult to reuse the logic of say initState or dispose. An obvious example is AnimationController:

class Example extends StatefulWidget {
  final Duration duration;

  const Example({Key? key, required this.duration})
      : super(key: key);

  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> with SingleTickerProviderStateMixin {
  AnimationController? _controller;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(vsync: this, duration: widget.duration);
  }

  @override
  void didUpdateWidget(Example oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.duration != oldWidget.duration) {
      _controller!.duration = widget.duration;
    }
  }

  @override
  void dispose() {
    _controller!.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

All widgets that desire to use an AnimationController will have to reimplement almost all of this logic from scratch, which is of course undesired.

Dart mixins can partially solve this issue, but they suffer from other problems:

• A given mixin can only be used once per class.
• Mixins and the class share the same object.
  This means that if two mixins define a variable under the same name, the result may vary between compilation fails to unknown behavior.

---

This library proposes a third solution:

class Example extends HookWidget {
  const Example({Key? key, required this.duration})
      : super(key: key);

  final Duration duration;

  @override
  Widget build(BuildContext context) {
    final controller = useAnimationController(duration: duration);
    return Container();
  }
}

This code is functionally equivalent to the previous example. It still disposes the AnimationController and still updates its duration when Example.duration changes. But you're probably thinking:

Where did all the logic go?

That logic has been moved into useAnimationController, a function included directly in this library (see Existing hooks) - It is what we call a Hook.

Hooks are a new kind of object with some specificities:

• They can only be used in the build method of a widget that mix-in Hooks.

• The same hook can be reused arbitrarily many times. The following code defines two independent AnimationController, and they are correctly preserved when the widget rebuild.

  Widget build(BuildContext context) {
    final controller = useAnimationController();
    final controller2 = useAnimationController();
    return Container();
  }

• Hooks are entirely independent of each other and from the widget.
  This means that they can easily be extracted into a package and published on pub for others to use.

Principle

Similar to State, hooks are stored in the Element of a Widget. However, instead of having one State, the Element stores a List<Hook>. Then in order to use a Hook, one must call Hook.use.

The hook returned by use is based on the number of times it has been called. The first call returns the first hook; the second call returns the second hook, the third call returns the third hook and so on.

If this idea is still unclear, a naive implementation of hooks could look as follows:

class HookElement extends Element {
  List<HookState> _hooks;
  int _hookIndex;

  T use<T>(Hook<T> hook) => _hooks[_hookIndex++].build(this);

  @override
  performRebuild() {
    _hookIndex = 0;
    super.performRebuild();
  }
}

For more explanation of how hooks are implemented, here's a great article about how is was done in React: https://medium.com/@ryardley/react-hooks-not-magic-just-arrays-cd4f1857236e

Rules

Due to hooks being obtained from their index, some rules must be respected:

DO always prefix your hooks with use:

Widget build(BuildContext context) {
  // starts with `use`, good name
  useMyHook();
  // doesn't start with `use`, could confuse people into thinking that this isn't a hook
  myHook();
  // ....
}

DO call hooks unconditionally

Widget build(BuildContext context) {
  useMyHook();
  // ....
}

DON'T wrap use into a condition

Widget build(BuildContext context) {
  if (condition) {
    useMyHook();
  }
  // ....
}

---

About hot-reload

Since hooks are obtained from their index, one may think that hot-reloads while refactoring will break the application.

But worry not, a HookWidget overrides the default hot-reload behavior to work with hooks. Still, there are some situations in which the state of a Hook may be reset.

Consider the following list of hooks:

useA();
useB(0);
useC();

Then consider that we edited the parameter of HookB after performing a hot-reload:

useA();
useB(42);
useC();

Here everything works fine and all hooks maintain their state.

Now consider that we removed HookB. We now have:

useA();
useC();

In this situation, HookA maintains its state but HookC gets hard reset. This happens because, when a hot-reload is performed after refactoring, all hooks after the first line impacted are disposed of. So, since HookC was placed after HookB, it will be disposed.

How to create a hook

There are two ways to create a hook:

• A function

  Functions are by far the most common way to write hooks. Thanks to hooks being composable by nature, a function will be able to combine other hooks to create a more complex custom hook. By convention, these functions will be prefixed by use.

  The following code defines a custom hook that creates a variable and logs its value to the console whenever the value changes:

  ValueNotifier<T> useLoggedState<T>([T initialData]) {
    final result = useState<T>(initialData);
    useValueChanged(result.value, (_, __) {
      print(result.value);
    });
    return result;
  }

• A class

  When a hook becomes too complex, it is possible to convert it into a class that extends Hook - which can then be used using Hook.use.
  As a class, the hook will look very similar to a State class and have access to widget life-cycle and methods such as initHook, dispose and setState.

  It is usually good practice to hide the class under a function as such:

  Result useMyHook() {
    return use(const _TimeAlive());
  }

  The following code defines a hook that prints the total time a State has been alive on its dispose.

  class _TimeAlive extends Hook<void> {
    const _TimeAlive();
  
    @override
    _TimeAliveState createState() => _TimeAliveState();
  }
  
  class _TimeAliveState extends HookState<void, _TimeAlive> {
    DateTime start;
  
    @override
    void initHook() {
      super.initHook();
      start = DateTime.now();
    }
  
    @override
    void build(BuildContext context) {}
  
    @override
    void dispose() {
      print(DateTime.now().difference(start));
      super.dispose();
    }
  }

Existing hooks

Flutter_Hooks already comes with a list of reusable hooks which are divided into different kinds:

Primitives

A set of low-level hooks that interact with the different life-cycles of a widget

Name	Description
useEffect	Useful for side-effects and optionally canceling them.
useState	Creates a variable and subscribes to it.
useMemoized	Caches the instance of a complex object.
useRef	Creates an object that contains a single mutable property.
useCallback	Caches a function instance.
useContext	Obtains the BuildContext of the building HookWidget.
useValueChanged	Watches a value and triggers a callback whenever its value changed.

Object-binding

This category of hooks the manipulation of existing Flutter/Dart objects with hooks. They will take care of creating/updating/disposing an object.

dart:async related hooks:

Name	Description
useStream	Subscribes to a Stream and returns its current state as an AsyncSnapshot.
useStreamController	Creates a StreamController which will automatically be disposed.
useFuture	Subscribes to a Future and returns its current state as an AsyncSnapshot.

Animation related hooks:

Name	Description
useSingleTickerProvider	Creates a single usage TickerProvider.
useAnimationController	Creates an AnimationController which will be automatically disposed.
useAnimation	Subscribes to an Animation and returns its value.

Listenable related hooks:

Name	Description
useListenable	Subscribes to a Listenable and marks the widget as needing build whenever the listener is called.
useListenableSelector	Similar to useListenable, but allows filtering UI rebuilds
useValueNotifier	Creates a ValueNotifier which will be automatically disposed.
useValueListenable	Subscribes to a ValueListenable and return its value.

Misc hooks:

A series of hooks with no particular theme.

Name	Description
useReducer	An alternative to useState for more complex states.
usePrevious	Returns the previous argument called to [usePrevious].
useTextEditingController	Creates a TextEditingController.
useFocusNode	Creates a FocusNode.
useTabController	Creates and disposes a TabController.
useScrollController	Creates and disposes a ScrollController.
usePageController	Creates and disposes a PageController.
useAppLifecycleState	Returns the current AppLifecycleState and rebuilds the widget on change.
useOnAppLifecycleStateChange	Listens to AppLifecycleState changes and triggers a callback on change.
useTransformationController	Creates and disposes a TransformationController.
useIsMounted	An equivalent to State.mounted for hooks.
useAutomaticKeepAlive	An equivalent to the AutomaticKeepAlive widget for hooks.
useOnPlatformBrightnessChange	Listens to platform Brightness changes and triggers a callback on change.

Contributions

Contributions are welcomed!

If you feel that a hook is missing, feel free to open a pull-request.

For a custom-hook to be merged, you will need to do the following:

• Describe the use-case.

  Open an issue explaining why we need this hook, how to use it, ... This is important as a hook will not get merged if the hook doesn't appeal to a large number of people.

  If your hook is rejected, don't worry! A rejection doesn't mean that it won't be merged later in the future if more people show interest in it. In the mean-time, feel free to publish your hook as a package on https://pub.dev.

• Write tests for your hook

  A hook will not be merged unless fully tested to avoid inadvertently breaking it in the future.

• Add it to the README and write documentation for it.

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