fromStore not reactive in a class?

[dusty-phillips]

I'm trying to migrate a dexie-based app to svelte 5, and I'm experiencing a lot more headache than happiness.

For context, Dexie's reactive liveQuery syntax spits out RxJS observables, which interface neatly with Svelte stores. I was expecting the fromStore function to allow this to interface fairly tidily with runes, although it's going to pollute my life with .current everywhere. But it doesn't work in global files or classes. :-(

Here's a not-too-trivial example, which is currently in a themeStore.ts file:

import { derived, fromStore, writable, type Readable } from 'svelte/store'
import { loadSettings } from '../queries/settings'
import { dexieStore } from './dexieStore'

const darkQuery = window.matchMedia('(prefers-color-scheme: dark)')
const osMediaStore = writable<'dark' | 'light'>(darkQuery.matches ? 'dark' : 'light')
darkQuery.addEventListener('change', (event) => {
  osMediaStore.set(event.matches ? 'dark' : 'light')
})

export function deriveTheme(): Readable<'dark' | 'light'> {
  const settings = dexieStore(loadSettings)
  return derived([settings, osMediaStore], ([settings, osMedia]) => {
    if (settings?.theme === 'system') {
      return osMedia
    } else {
      return settings?.theme ?? 'light'
    }
  })
}

This code listens to changes on a matchMedia query to update a store whenever the operating system theme changes. The user can choose to override the OS theme by setting a Dexie value to system, light, or dark. The deriveTheme function derives the two stores and decides what the theme value should be.

The easiest way to migrate this code to runes mode is to add a single global at the end that wraps deriveTheme() in a fromStore:

export const themeState = fromStore(deriveTheme())

Inside my components, I can import themeState and use a simple $derived rune to get my expression.

  const theme = $derived(themeState.current)

This works fine, so yay!

But I hear Svelte 5 is amazing because we can use runes in global .svelte.ts files. I know my dexie store isn't going away, since rxjs is the reactive protocol Dexie conforms to, but surely I can replace the osMediaStore with a $state rune, right?

In fact, I haven't found any way to create derived state that depends on a osMediaState $state rune and a store inside a global file. After getting extremely confused by the fact that neither $derived nor $effect work in global files (something that needs to be better documented), I realized that I probably need to use reactive classes, which allow $derived runes on fields.

This is the closest I've gotten:

import { fromStore } from 'svelte/store'
import { loadSettings } from '../queries/settings'
import { dexieStore } from '../stores/dexieStore'

class ThemeState {
  #darkQuery = window.matchMedia('(prefers-color-scheme: dark)')
  #osTheme: 'light' | 'dark' = $state(this.#darkQuery.matches ? 'dark' : 'light')
  #settingsQuery = fromStore(dexieStore(loadSettings))
  #settings = $derived(this.#settingsQuery.current)

  current = $derived(
    this.#settings?.theme === 'system' ? this.#osTheme : (this.#settings?.theme ?? this.#osTheme),
  )

  constructor() {
    this.#darkQuery.addEventListener('change', (event) => {
      this.#osTheme = event.matches ? 'dark' : 'light'
    })
  }
}

export const themeState = new ThemeState()

(The dexieStore function just wraps dexie's liveQuery function with return liveQuery(querier) as unknown as Readable<T> to satisfy typescript.)

My testing indicates that the #osTheme $state is reacting the way that I want to, but the #settingsQuery field, based on fromStore is not reactive.

I think either:

• I'm doing something trivially wrong
• I haven't identified the best patterns for mixing stores and $state
• There's a bug in the svelte compiler

The only way I've been able to mix the osTheme rune and the dexie loadStettings query such that it works is to subscribe to the rxjs observable in the constructor:

import { loadSettings } from '../queries/settings'
import { dexieStore } from '../stores/dexieStore'

class ThemeState {
  #darkQuery = window.matchMedia('(prefers-color-scheme: dark)')
  #osTheme: 'light' | 'dark' = $state(this.#darkQuery.matches ? 'dark' : 'light')
  #settingsQuery = dexieStore(loadSettings)
  #userTheme: 'light' | 'dark' | 'system' = $state('system')

  current = $derived(this.#userTheme === 'system' ? this.#osTheme : this.#userTheme)

  constructor() {
    this.#darkQuery.addEventListener('change', (event) => {
      this.#osTheme = event.matches ? 'dark' : 'light'
    })
    this.#settingsQuery.subscribe((value) => {
      console.log(value)
      this.#userTheme = value.theme
    })
  }
}

export const themeState = new ThemeState()

While this is work, it smells. The original pure-store version is actually easier to read, and that subscribe call not having a later unsubscribe call tells me something is amiss. Is there a recommended (or planned) way to integrate stores and runes beyond fromStore?

[brunnerh]

Looks like a bug, opened an issue:

• fromStore() in combination with $derived seems broken #14002

[brunnerh]

A fix should be in the next release, but note that there is still another issue around object mutations within stores:

• Reactivity problem with fromStore() and objects #13077

[brunnerh]

Fix was released in v5.1.4.