Binding focus state to elements

[Maximinodotpy]

Describe the problem

I hope i'm not missing something but it would be cool if we were able to bin variables to focusable elements abd find out this way if there focused or not, furthermore if we set this bound variable to true it should focus this element and blur in case its turned to false.

Describe the proposed solution

A property like this bind:focused

Alternatives considered

Using the focus and blur functions for focusable elements and the activeElement property of the document.

Importance

nice to have

[brunnerh]

It would be convenient, but you already can get pretty close using actions.

Something along the lines of:

export function focused(node, store) {
    const onChange = () => store.set(document.activeElement == node);
    node.addEventListener('focus', onChange);
    node.addEventListener('blur', onChange);

    const cancel = store.subscribe(shouldFocus => {
        const focused = document.activeElement == node;
        if (focused && shouldFocus == false)
            node.blur();
        else if (focused == false && shouldFocus)
            node.focus();
    });

    return {
        destroy() {
            node.removeEventListener('focus', onChange);
            node.removeEventListener('blur', onChange);
            cancel();
        }
    }
}

Usage:

<script>
  import { writable } from 'svelte/store';
  import { focused } from './actions.js';

  const inputFocused = writable(false);
</script>

<input use:focused={inputFocused} />

REPL

Of course that is more verbose as it requires imports and stores.

---

For convenience the action and store could be combined into one object, e.g.

import { writable } from 'svelte/store';

export function createFocused(initialValue) {
  const store = writable(initialValue);

  function action(node) {
    // [action as above]
  }

  Object.assign(action, store);
  return action;
}

<script>
  import { createFocused } from './actions.js';
  const focused = createFocused(false);
</script>

<input use:focused />
Focused: {$focused}

REPL

[Maximinodotpy]

Ah youre right it could also be done this way but would it also be possible without a store?

[brunnerh]

I do not think so. You need some way to bridge reactivity between component and non-component code.

[Maximinodotpy]

I'm not sure but I imagine that normal let variables internally somewhat behave like stores and other dom things like the width are also possible so I think it would be easiest to make this a built in feature, of course dont take me too seriously because I have no idea of the source code.

[StagnantIce]

Without store you can use callback in this case

export function focused(node, onFocused) {
       const onChange = () => onFocused(document.activeElement == node);
       node.addEventListener('focus', onChange);
       node.addEventListener('blur', onChange);
       ...
}

function onFocused(isFocused: boolean) {

}

<input use:focused={{onFocused}} />

[RaiVaibhav]

Hey @Maximinodotpy there is a onFocus and onBlur exists, if that still doesn't full fill your need, @Rich-Harris I would like to work on this part of the feature.

[trueadm]

Why do we need this? You can just check if something is focused by using document.activeElement.

[RaiVaibhav]

Kindly close the PR if it not needed

[brunnerh]

I'd say the main reason to have this would be the write access: You can focus the element declaratively without using bind:this first or using an action.

[trueadm]

Write access is tricky with focus. If you try to bind focus to two elements by accident, you cause so many issues with accessibility. It's much better to handle focus management with a global state machine instead of on a component-by-component basis. It gets even more complex too when working with elements that can have focus but not be in scroll view – so managing scroll + focus kind of flow together.

[RaiVaibhav]

Few things I came accross while writing a PR, if a single variable bind to two different element, binding multiple Focus on multiple elements, multiple issues related to accessibility like screen reader, scrolling behaviour.

PR cover above scenarios but again focus itself have option like preventScroll, so I am not sure, letting he user bind the focus is a good idea, because if they need something after component mount, they can use, autofocus?

[trueadm]

Sorry, I don't think this is something we want to invest our time into now. Like I said, focus is complicated and easy to mess up and hard to debug as to why.

[trueadm]

Actually, I think this would be best if it was read-only, rather than writable. Then I can see a way forward for the PR too :)

[RaiVaibhav]

Let me know if I can work on this @trueadm, I would be happy to open a new PR

[trueadm]

@RaiVaibhav Sure, go for it :)