[idreyn]

Hooks let you colocate logic that, in class components, would need to be split across multiple lifecycle methods. In practice you need to remove the event listener when the component unmounts. You can get reasonably close to a hooksy API for doing this here:

    class MyComponent {
        componentWillMount() {
            this.stopWatchingViewport = watchViewPort((x, y) => {
                this.setState({ something: x + y });
            });
        }

        componentWillUnmount() {
            this.stopWatchingViewport();
        }
    }

    watchViewPort(callback) {
        const onResize = (event) => {
            // get x and y
            callback(x, y);
        };
        addEventListener("resize", onResize);
        return () => removeEventListener("resize", onResize);
    }

But being able to slice up logic by functionality, rather than by lifecycle event, gets gradually nicer as you have more of it.

[spion]

Thats purely a React API limitation.

The hook API could be class based:

  class ViewportHook {
    // API on use
    constructor(component) {
      this.viewportState = component.addState(this, initialValue)
      const unsubscribe = watchViewport((x, y) => this.viewportState.set({something: x + y}))
      component.addOnUnmount(unsubscribe);

      // you can also use another hook - hook composition works
      this.otherHook = component.use(SubHook);
      // use the other hook's api
    }

    // API to expose (in render)
    value() {
      return this.viewportState.get()
      // use this.otherHook too if you like
    }
  }

You would be able to use it in a component like this

  class MyComponent
    constructor() { 
      this.viewport = this.use(ViewportHook);
    }

    render() {
      const viewportSize = this.viewport.value();
      // use in render
    }
  }

Boring, and a bit less weird.

[orangepanda]

How would passing a value from one hook to another look like, for example, Subhook requiring viewport value to set up some subscription?

[spion]

Good question.

  component.use(SubHook, param)

could be used, that would pass the param as a second argument to the constructor.

The main reason why this isn't the case, I think, is concurrent mode. Hooks force certain values to be retreived and stay stable during render (i.e. you can only get a component state value during render function) and this is important if there are multiple setup and teardowns going on.

(Concurrent mode is IMO a bit of unfortunate React complexity that a lot of users of React don't really need, and many others can avoid)

[rimunroe]

This is a hook version of the same code as near as I could guess it.

  function useViewport(initialValue) {
    const [state, setState] = useState(initialValue);
    useEffect(() => {
      return watchViewport((x, y) => setState(x + y))
    }, [])
    return state.get();
  }
  
  // usage
  
  function MyComponent() {
    const viewportSize = useViewport()
    // use in render
  }

I made a couple of assumptions here. From usage, I assume watchViewport is supposed to both subscribe and return a teardown function. I also assume that the viewportState.set/get are functions for getting and setting the tracked value in the component's state.

In my opinion, there are many advantages to the hook version and several major disadvantages to the class version:

1. There's no need for hooks to have a value method or React.Components to grow the use or addState methods because the hook is just a function call which returns a value. How it produces that value is up to the code inside the function--which does call out to React--but the fact the function will always be called when MyComponent is called (absent something throwing earlier in the function body of course). The value you see being returned by the hook will always match the value you get when you call useViewport() in your component. These two things are guaranteed to have the exact same behavior as any other JavaScript function call and thus you can reason about the "registration" and value passing without needing to learn any framework-specific APIs.

2. There's no need for an addOnUnmount method on the component object passed to the ViewportHook constructor, because the hook function can use the function component's equivalent to componentWillUnmount (the return value of a useEffect) in the exact same way as a function component can, but without need for external registration.

3. In order to implement the class API, you'd need to either (A) pass the component's actual instance to the hook, or (B) create a new type of value to represent the instance of the component the hook is registering against. Option (B) is yet another API to learn, as you have to learn a new type of object to deal with in a React application. Option (A) would mean figuring out a way to prevent people from calling those methods after construction, OR introducing the possibility of registering a new slice of state partway through. The latter might be possible, and maybe that's even what you intend, but I'd want to know what the expected impact on methods like shouldComponentUpdate or getDerivedStateFromProps would be. Speaking of those...

4. I can't think of any obvious way you could pass previous versions of hook-related instance properties to lifecycle methods in the same way that you can pass prevProps and prevState.

5. Concision: the hook version has a dramatic reduction in the amount of code you have to read

6. Bundle size: because the hook version relies on functions rather than class properties, it can be minified trivially and thus reduce bundle size even more than the obvious reduction in character count would imply

[spion]

The class version is

  class ViewportHook {
    constructor(component) {
      this.viewportState = component.addState(this, initialValue)
      component.addEffect(() => {
        return watchViewport((x, y) => this.viewportState.set({something: x + y})
      })
    }

    value() { return this.viewportState.get() }
  }

The hook version is

  function useViewport() {
    const [state, setState] = useState(initialValue);
    useEffect(() => {
      return watchViewport((x, y) => setState(x + y))
    }, [])
    return state;
  }

I changed the API to be more similar to the existing useEffect where you can return the unsubscribe function. That makes the difference in the size of code non-significant.

Again, its more about API design rather than classes or functions.

Another crucial benefit of the class version: you can use hooks in conditions or loops, in any order. This is because they're not called on every render. I also used a unique `key` to pass to `addState`, but its not really a requirement, since the hook constructors only get called once.

I don't understand points 3 and 4, can you elaborate? I'm assuming that when i call `component.use(HookClass)` the component creates a new instance of that hookclass. Regarding learning, I don't think the whole concept of (functional) hooks and their idiosyncracies are any easier to learn than learning one new type of class.

> Bundle size: because the hook version relies on functions rather than class properties, it can be minified trivially and thus reduce bundle size even more than the obvious reduction in character count would imply

I don't think this will make any meaningful difference. The full component methods API (addState, addEffect) is likely to be in use in any nontrivial project, so that can't be minified away. For 3rd party hook classes, they would be small and independend through `use` and easy to minify / dead-code-eliminate if not in use.

[rimunroe]

> I don't understand points 3 and 4, can you elaborate? I'm assuming that when i call `component.use(HookClass)` the component creates a new instance of that hookclass.

Sure! You show ViewportHook's constructor receiving a "component" prop. Since, in the calling component, you call this.use(ViewportHook), the `use` method is supposed to pass some sort of reference of the calling component into ViewportHook's constructor. My question was about what the type of that parameter is. Internally, is `use` something like `use(Hook) { return new Hook(this); }`? If so, you're passing a direct reference to the class instance. I had thought that maybe the framework could pass a more limited delegate for the instance to the constructor to prevent people from doing something silly with it, but that would prevent you from assigning a property safely anyway.

> I don't think this will make any meaningful difference. The full component methods API (addState, addEffect) is likely to be in use in any nontrivial project, so that can't be minified away. For 3rd party hook classes, they would be small and independend through `use` and easy to minify / dead-code-eliminate if not in use.

Object properties can't be safely minified. Any user-defined component using lifecycles will still need to have "constructor", "render", etc in the generated source, whereas identifiers (like "MyComponent" in `const MyComponent = ...`) aren't accessed by being looked up on an object, and thus can safely be minified to one or two character names. This was one of the motivations of the design of hooks. I believe it's called out in the original talk from React Conf 2018.

[Edit]

And regarding point 4: componentDidUpdate receives prevProps and prevState, and shouldComponentUpdate receives nextProps and nextState. How would you provide information about the previous or next version of that hook-based state if it's being tracked as an instance property rather than in the component's state object?

[spion]

> I had thought that maybe the framework could pass a more limited delegate for the instance to the constructor to prevent people from doing something silly with it, but that would prevent you from assigning a property safely anyway.

That seems like a good idea. I'll admit my illustration is not a fully fleshed out design, it was meant more to be a sketch of how it would be possible to make a class based design much more capable than the original one was.

> Any user-defined component using lifecycles will still need to have "constructor", "render", etc in the generated source

Ahh I got it - this is not about DCE but about minified names. Not sure why my mind went with dead code elimination.

My first gut feeling is that this wouldn't be as important for everyday users if they already have code splitting / DCE / gzip. I'll look up the video, would be interesting to see some numbers - I'm hoping thats part of the presentation.

> componentDidUpdate receives prevProps and prevState, and shouldComponentUpdate receives nextProps and nextState. How would you provide information about the previous or next version of that hook-based state if it's being tracked as an instance property rather than in the component's state object?

I'm giving up having a single component state with the new API - this class based hook API also allows you to have multiple states. So I'm going to focus on the props part.

In the hook constructor, you would be able to use a listener for componentUpdate:

  component.onUpdate((prevProps, props) => { run code });

You could also do something like

  component.watch(() => [otherHook.someValue(), this.someState.get()], () => {
    // callback
  })

to get render-time change detection (equivalent to `useEffect(fn, [otherHookValue, someStateValue])`).

Its all a bit wordier, no doubt, but I feel that most of the capabilities can be implemented.

[rimunroe]

I imagine most of them could be, but this is an API which would require a lot of additions to the existing React.Component API, and would end up duplicating a bunch of already-existing functionality for something which in the end would probably need to conform to the same rules hooks do, but with what certainly feel to me like clunkier ergonomics. You'd also still be left with the problems of class components like poor minifiability and unreliability for hot module replacement.

I have to go to bed, but if you'd like to see a much better explanation about why hooks were adopted, I highly recommend the [checks notes] 1,401 comment-long discussion[1] on the PR for adopting the hooks RFC back in 2018, as this sort of design was brought up frequently. Especially worthwhile is Sebastian Markbåge's ending summary about why the team was going with hooks[2].

[1] https://github.com/reactjs/rfcs/pull/68 [2] https://github.com/reactjs/rfcs/pull/68#issuecomment-4393148...

[theteapot]

> Hooks let you colocate logic that, in class components, would need to be split across multiple lifecycle methods.

I think the obvious OO alternative to hooks would have been this:

    class MyComponent {
        constructor() {
            this.attachBehaviour(new MyBehaviour(this));
            this.attachBehaviour(new MyOtherBehaviour(this));
        }

        render() { /* ... */ }
    }
    
    class MyBehaviour implements ComponentLifeCycleHooks {
        componentDidMount() { /* ... */ }
        componentWillUnmount() { /* ... */ }
        componentDidUpdate() { /* ... */ }
    }

Weird React didn't even seem to consider when they went to hooks. Would be possible to implement yourself though.

I'm not saying this is better than hooks / "composable" / "functional" API (I quite like Vue's composable API) but it's less of a departure from class based components.

[codeptualize]

Departure from class based components was one of the motivations behind hooks, those are:

1. It’s hard to reuse stateful logic between components

2. Complex components become hard to understand

3. Classes confuse both people and machines

See https://legacy.reactjs.org/docs/hooks-intro.html#motivation for details. Agree or not, but it is a very intentional and well motivated direction.

[theteapot]

Above was OO an based solution to #1, and arguably #2. Many would argue hooks did nothing to solve #2 and may have made things worse.

> Agree or not, but it is a very intentional and well motivated direction.

Agree with problem exists (roughly), disagree on solution.

[mattgperry]

Interesting you assume they didn't consider this. They probably did.

How do this two behaviours compose together/interact with each other?

[theteapot]

I made no such assumption. I've just never seen it discussed, where as, for example, I've seen "mixins" discussed and dismissed (justifiably) as an alt.

> How do this two behaviours compose together/interact with each other?

What?