Readable Store, delayed "set"

Question

Is there a way to define a readable store, with its set function, however delay the calling of that function until certain criteria is met?

I'm thinking of using them for API calls, however I want to decide when to call the the set function, which would then fetch the data and populate the store with the results.

Answer 1

The store contract for readable stores is simply a subscribe function; you can arbitrarily extend the store with additional properties and methods. You could e.g. add an update function that internally uses set:

import { readable } from 'svelte/store';

export function apiStore() {
    const store = readable(null, set => {
        store.update = () => set(new Date());
    });

    return store;
}

Usage:

<script>
    import { apiStore } from './stores';
    const store = apiStore();
</script>
<button on:click={() => store.update()}>Update</button>
<pre>{$store}</pre>

REPL

If you want it to update fully automatically, you should use a derived store instead. They update whenever a store they depend upon changes.