How to `ref`er to an element? Missing docs?

[Hi-Angel]

Was first brought up on the discourse forum.

It's often useful to refer to one DOM element from another. So for example you may have a modal window Component/JSX, and you have a button that would call showModal on it. So you want the button to refer to the window.

That's provided by both Halogen and the original React.

Now, React Hooks presumably provides something similar… This is hinted by the existence of ref field in each JSX, and by useRef presence. But whether and how it works is completely unknown.

The useRef returns a data UseRef and Ref, but the former is an opaque data with zero functions for it, and the latter is just a container for an arbitrary value you'd pass over to the call. So I've no idea what it could be used for.

Similarly, the ref fields are supposed to contain some Node, but again that requires for a function JSX -> Node to exist, which it doesn't.

So what's the status of this feature, and what these ref and useRef are for anyway?

[pete-murphy]

Here's an example using useRef: https://github.com/pete-murphy/demo-foreign-import-hooks/blob/main/src/App.purs#L20

and the latter is just a container for an arbitrary value you'd pass over to the call

Sorry, I don't understand what this is saying. Binding useRef gives you a Ref a, and the ref property of JSX elements takes a Ref (Nullable Node).

[pete-murphy]

and the ref property of JSX elements takes a Ref (Nullable Node).

In regular React, the ref property is overloaded to also take a function (and at one point it also could take a string I think, but I think that's been deprecated). The overloading is not implemented in this library: ref must be Ref (Nullable Node). I don't know if that clears up the confusion.

[pete-murphy]

So for example you may have a modal window Component/JSX, and you have a button that would call showModal on it. So you want the button to refer to the window.

Here's a minimal example of opening a dialog with showModal. The Try PureScript playground doesn't support FFI code, but if you were to do this for real, you'd want to have something like

foreign import showModal :: Node -> Effect Unit

instead of using unsafeCoerce.

https://try.purescript.org/?code=LYewJgrgNgpgBAIQgF2SAdgZwEowIYDGyAEiCANaYB0AsngJbpwDuAFjAE4wBQ39wABxAdkcAApcoEMDz6DhogCJ5keWngCeAI3gAKOtpi6qVAJSm5QkXGWqqAOWhQ8W2HF3onF-ldEBRADMAmCJ3QOCib3lrcJDkKj8ADwIYAWR6DHdkVg4QZijfOFxCeIQ8THoCKkUAeRo4cqLLBSL8IioyiqramioAYSh6GHRRXQIuFRhsMmQAGjgudBkOaZBkApbi9s7KqlIKTHc%2BkHl0Ybm4AhOhM5H5iEwpmAD5ibBcAIMdDest0vLdvtKA1Dn9mr82v8ugkAG7nQ6NPxwkaYcGiADqMC01TqDgwflgwHOYjwi2Q9nAegA5jBkASYESRggNABJMA-DFYvYAFRoABl3MxGGA8hy4JjscReXyefzFCACBBGaM0BT0PTlSSyRSZGKJbKZejhXl3CLFcq9VyelQWSNOOg8FAqNyNAIYIddDqYGLYu0AKroRUcDhDMDuDgQdVBOIAMXQAEYLGi4AHMHhgv0QJwUu5I2ngsds97eABaEutR1QDRwbL0Q7MEDQMM6OA%2BQnnGBhxhwGMxlncTCsPI0cCOuAALnHcC9cBLAD44L7RAH6MgB0PmCOwGP0JS4ABeBaRpdx%2BO5rDpmCFjg53e6qiD4ejqBwSOr3jABhMSeL6OhFdrp%2B3aHiK3BwHAZpwAAPOWZpKucB5QVBLDGswYELDM0HljSdLtkyrJhgARLkayEXAAAkNYgGqGrEqS5wzma6EEOU8AkaIIABOh4EUrW6BUrOC7ZLkzBwIRxxNnAu6iAEwoYWsVCEdxcAAFIPKIVwjF%2BnCCRBIDKeBExEKsogwZcEzIFMmGaaojCcAZDQCAIWFwMA5AAIJOQ5izLCZCyQn5uh4E5cAAN4AL5Jm5nnOT%2BxynAhEXcNFIUgfp4FXAlIxidy7rIGRAA6AD6ul-FQoHgYZzwuQ8TwBFJTjKcpsBrpVlUYAMlTkAei7Isg1CsHgSywBwJUVW1lVvB8XxsdVDltXOc6HixjxwCVnHzRNamYKIYD0I6IACfOm0TXAj6bs%2BEH7VAh0nZVvGsIwR1zndbVEoIyAaMpAgQFwFEOdg5X0DCRWbQA2kUVBaCgaBMHdoVwB1gwEOQd3zAQj1QGAixThDgOWYkoiETUbpMHtB1UmRAC6d3hZt8yA%2BTN1UvD-lcadbXo5j2PDLjkME0TijXYd1O05tNNAA

[Hi-Angel]

Oh, thank you! So, am I correct in my understanding that this line initializes ref no basically nothing:

    ref <- useRef null

and then the actual initialization happens here?

        , R.dialog
          { ref
          , children: [ R.text "Dialog" ]
          }

So, like, the ref : ref initialization isn't actually initialization of the field, but instead it is initialization of the variable ref to make it point to the JSX? If so, I think it definitely deserves some documentation, because I would've never figured it out 😅

[Hi-Angel]

Although, no, even with unsafe magic I don't think you can make ref to get called as a function to perform effect during the field initialization. So no, I'm clearly misunderstanding how it works. Do you maybe create a ref, and then assign it to the field, so later it can be used from a button? That makes sense. But then why null is there?

[Hi-Angel]

Do you maybe create a ref, and then assign it to the field, so later it can be used from a button? That makes sense. But then why null is there?

However, if that is correct, that means that the readRefMaybe ref inside the handler doesn't actually just "reads the ref", but instead it performs a search over the DOM trying to find the element ref refers to. Which is quite possible seeing how it returns Effect …, but kinda contradicts to what's written in the name.

[Hi-Angel]

Another thing I don't understand is why readRefMaybe returns Effect (Maybe a) instead of Effect (Maybe Node). Had it been the latter, that would've provided at least some possibility to guess how one can use the refs.

In regular React, the ref property is overloaded to also take a function (and at one point it also could take a string I think, but I think that's been deprecated).

FWIW, I've never worked with React before starting to use this library (I wasn't even a web-programmer for that matter), so I'm coming to this library with completely clean mind 😊

[pete-murphy]

Another thing I don't understand is why readRefMaybe returns Effect (Maybe a) instead of Effect (Maybe Node)

A Ref can be any value, it doesn't need to be Node. A Ref in React is just a mutable value that is scoped to the component that initializes it (the component where useRef is called). A very common use case is to reference DOM nodes, but something like this is also valid

mkCounter :: Component {}
mkCounter = do
  component "Counter" \_ -> React.do
    ref <- useRef 0
    
    let
      handleClick = Events.handler_ do
        current <- readRef ref
        writeRef ref (current + 1)
        next <- readRef ref
        Console.log ("You clicked " <> show next <> " times!")
    
    pure $
      R.div_
        [ R.button 
          { onClick: handleClick
          , children: [ R.text "Click me!" ]
          }
        ]

That would be equivalent to this JS

function Counter() {
  let ref = useRef(0);

  function handleClick() {
    ref.current = ref.current + 1;
    console.log('You clicked ' + ref.current + ' times!');
  }

  return (
    <button onClick={handleClick}>
      Click me!
    </button>
  );
}

The official React documentation goes into greater detail about useRef: https://react.dev/reference/react/useRef

[Hi-Angel]

So, suppose I used a

ref <- useRef 0

and then assigned ref to a JSX

        , R.dialog
          { ref
          , children: [ R.text "Dialog" ]
          }

Does that imply the ref now contains two values at the same time:

1. The 0 that can be read by readRef
2. The Maybe Node that can be read by readRefMaybe

?

[pete-murphy]

Does that imply the ref now contains two values at the same time:

No, that example wouldn't compile. In

ref <- useRef 0

ref has type Ref Int. In

        , R.dialog
          { ref
          , children: [ R.text "Dialog" ]
          }

it has type Ref (Nullable Node). You should get a type unification error if you try to compile that.

[pete-murphy]

Reassigning ref will do just that: reassign it. The ref won't "contain two values at the same time". In

ref <- useRef null

-- ...
        , R.dialog
          { ref
          , children: [ R.text "Dialog" ]
          }

The ref is being initialized as null before the JSX is rendered, and then assigned to the dialog DOM node during render. Again, the official React docs are a good reference if you haven't read through them yet.

[Hi-Angel]

Okay, I see, thank you. Part of my confusion was due to the fact that null wraps some type a, but somehow useRef doesn't require specifying that type, and then during a JSX creation a becomes a Node. I am still confused how could that possibly work on the type-level, but I am presuming it's just an FFI magic (is that correct?).

So, how about adding to the library a helper function useRefNode that doesn't requires passing null and can be used exactly to create a reference to a JSX element? That would help immensely (along with some description of course), because right now the biggest problem IMO is not even lack of docs but that the types has no mention of Node anywhere (and in languages like Haskell and PureScript types help a lot in reading the code).

[Hi-Angel]

And yeah, I can send a PR

[pete-murphy]

Part of my confusion was due to the fact that null wraps some type a, but somehow useRef doesn't require specifying that type, and then during a JSX creation a becomes a Node. I am still confused how could that possibly work on the type-level, but I am presuming it's just an FFI magic (is that correct?).

This is not really anything to do with FFI, or with React, this is just how type inference works in PureScript. Maybe it helps to think of a simpler example:

x = Nothing

foo :: Maybe Int -> String
foo = case _ of
  Just 1 -> "one"
  _ -> "something else"

bar = foo x

The type of Nothing is forall a. Maybe a, but because foo takes Maybe Int, the compiler can infer that x has type Maybe Int. Similarly, because the type of ref here is Ref (Nullable Node), the compiler can infer the type of ref in

ref <- useRef null

-- ...
        , R.dialog
          { ref
          , children: [ R.text "Dialog" ]
          }

[Hi-Angel]

So, how about adding to the library a helper function useRefNode that doesn't requires passing null and can be used exactly to create a reference to a JSX element? That would help immensely (along with some description of course), because right now the biggest problem IMO is not even lack of docs but that the types has no mention of Node anywhere (and in languages like Haskell and PureScript types help a lot in reading the code).

And yeah, I can send a PR

While at it, I think it might also be useful to add an alias type RefNode = Ref (Nullable Node), because the full type is to large to type. Additionally, Node is a Web.DOM import whereas the alias would be just the React Hooks import. So yeah, I think it should be useful as well.