useImperativeHandle behaves as if inputs are [] by default

[AhmedElywa]

useImperativeHandle have bug in 16.8.1 if you want to access any state will get the initial state not current state but in 16.8.0-alpha.1 it work good

You can see here https://codesandbox.io/s/xjl8znwpz open console when you change input value will get the currently value in console but if you clicked button get value from ref will get initial value (e)

If you try same here https://codesandbox.io/s/qxkll6po0j with 16.8.0-alpha.1 when clicked button get currently value

[danielkcz]

That's a pretty common "bug" caused by Javascript closures. React apparently is not recreating handle on every render so it will always see only the value from the first render.

You can work around that by using ref: https://codesandbox.io/s/8nr4x36zy9

Personally, I would recommend you to avoid useImperativeHandle. It should be used sparsely for a reason. There are definitely better ways for a data flow.

[danielkcz]

I think this kind of problems should get its own section in the docs. Many people will get burned by this (me included) because it's just not that apparent without knowing this advanced JS behavior.

[AhmedElywa]

But why work in alpha version?

[AhmedElywa]

@FredyC I’m not use it to transfer value this just example
I use here https://github.com/oahtech/rnsc/blob/master/src/components/Accordion/index.js

To control the state of expanded or collapsed

[danielkcz]

I think this is a mindset issue, possibly coming from how jQuery does the things. You shouldn't be controlling components by calling methods on them. Instead, you should pass props that will change that state. I don't have a capacity to update that example of yours right now. Just try to fiddle with it and I am sure you will figure a way without imperative access.

[MicahChalmer]

There's a third parameter to useImperativeHandle called inputs which is intended to be used in the same way as similar parameters to useEffect and useMemo--if you pass an array to it, it will re-create the handle whenever any of the inputs have changed since the last call. I find that if I pass in what the closure I'm passing depends on, it works as expected.

The bug here (and I do think it's a bug) is this: unlike the other hooks that have an inputs parameter (and unlike the previous behavior from the alphas, which I still think is the correct behavior,) if you don't pass inputs it defaults to ignoring the new closure you passed in. It defaults to behaving as if you had passed [] to inputs. The others default to always calling it, which I believe is the right thing to do, for precisely the reason that it avoids this gotcha around values saved from previous renders. The current behavior is "memoized by default" and that's wrong for the same reasons explained in other contexts.

This does mean there's an easy workaround for @AhmedElywa or anyone else hitting this issue: just pass something to inputs that always changes--[Symbol()] would do the trick. Or look at what you're passing and list what the real inputs are, like you would to optimize a useMemo call.

[MicahChalmer]

Here's the first example as updated with workaround: https://codesandbox.io/s/nnrlpxx1qm - note line 13 of Test.js. With that it shows the current value as expected.

[hamlim]

The third argument to useImperativeHandle is a dependency array much like useEffect, useMemo and useCallback's second argument. In this case you need to provide [value] as the third argument to have the second argument (i.e. ref assignment function) be called again when the value changes:

useImperativeHandle(
  ref,
  () => ({
    getValue: () => {
      console.log(value);
    }
  }),
  [value]
);

Working sandbox here: https://codesandbox.io/s/zl8zy0l6jx

[AhmedElywa]

@MicahChalmer and @hamlim thanks for explain

[danielkcz]

@hamlim I think that @MicahChalmer was pointing out correctly, that unlike the useEffect which is executed on each render when no inputs are passed, the useImperativeHandle on the contrary does not update without inputs passed it. It's a slight inconsistency in the API and would worth explaining in the docs at least.

[hamlim]

Oh wow I must have completely skipped over @MicahChalmer's comments above mine 🤦‍♂️ sorry about that!

[gaearon]

It defaults to behaving as if you had passed [] to inputs.

Sounds like a bug.

[gaearon]

#14801

[gaearon]

Canary 0.0.0-0e4135e8c should have the fix. This will make it into 16.8.2.

[gaearon]

Fixed in 16.8.2.
https://codesandbox.io/s/7k7x17rkpj