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 manages a Widget life-cycles. 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})
      : assert(duration != null),
        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 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 shares 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})
      : assert(duration != null),
        super(key: key);

  final Duration duration;

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

This code is strictly 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 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 objects with some specificities:

They can only be used in the build method of a widget that mix-in Hooks.
The same hook is reusable an infinite number of 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 they can easily be extracted into a package and published on pub for others to use.

Principle

Similarly to State, hooks are stored on the Element of a Widget. But instead of having one State, the Element stores a List<Hook>. Then 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 returns the third hook, ...

If this is still unclear, a naive implementation of hooks is the following:

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 they are implemented, here's a great article about how they did it 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-reload while refactoring will break the application.

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

Consider the following list of hooks:

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

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

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

Here everything works fine; all hooks keep their states.

Now consider that we removed HookB. We now have:

useA();
useC();

In this situation, HookA keeps its state but HookC gets a hard reset. This happens because when a refactoring is done, all hooks after the first line impacted are disposed of. Since HookC was placed after HookB, it got disposed of.

How to use

There are two ways to create a hook:

A function

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

The following defines a custom hook that creates a variable and logs its value on the console whenever the value changes:
```
ValueNotifier<T> useLoggedState<T>(BuildContext context, [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 and have access to life-cycles and methods such as initHook, dispose and setState It is usually a good practice to hide the class under a function as such:

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

The following defines a hook that prints the time a State has been alive.

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 comes with a list of reusable hooks already provided.

They are divided into different kinds:

Primitives

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

name	description
useEffect	Useful for side-effects and optionally canceling them.
useState	Create variable and subscribes to it.
useMemoized	Cache the instance of a complex object.
useContext	Obtain the `BuildContext` of the building `HookWidget`.
useValueChanged	Watches a value and calls a callback whenever the value changed.

Object binding

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

dart:async related:

name	description
useStream	Subscribes to a `Stream` and return its current state in an `AsyncSnapshot`.
useStreamController	Creates a `StreamController` automatically disposed.
useFuture	Subscribes to a `Future` and return its current state in an `AsyncSnapshot`.

Animation related:

name	description
useSingleTickerProvider	Creates a single usage `TickerProvider`.
useAnimationController	Creates an `AnimationController` automatically disposed.
useAnimation	Subscribes to an `Animation` and return its value.

Listenable related:

name	description
useListenable	Subscribes to a `Listenable` and mark the widget as needing build whenever the listener is called.
useValueNotifier	Creates a `ValueNotifier` automatically disposed.
useValueListenable	Subscribes to a `ValueListenable` and return its value.

Misc

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	Create a `TextEditingController`
useFocusNode	Create a `FocusNode`
useTabController	Creates and disposes a `TabController`.
useScrollController	Creates and disposes a `ScrollController`.
usePageController	Creates and disposes a `PageController`.
useIsMounted	An equivalent to `State.mounted` for hooks

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 doens'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 shows an 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 unles fully tested, to avoid breaking it inadvertendly in the future.
Add it to the Readme & write documentation for it.

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