Async producers & createDraft / finishDraft

egghead.io lesson 11: Creating async producers (and why you shouldn’t)

It is allowed to return Promise objects from recipes. Or, in other words, to use async / await. This can be pretty useful for long running processes, that only produce the new object once the promise chain resolves. Note that produce itself (even in the curried form) will return a promise if the producer is async. Example:

import produce from "immer"

const user = {
    name: "michel",
    todos: []
}

const loadedUser = await produce(user, async function(draft) {
    draft.todos = await (await window.fetch("http://host/" + draft.name)).json()
})

Warning: please note that the draft shouldn't be 'leaked' from the async process and stored else where. The draft will still be revoked as soon as the async process completes.

`createDraft` and `finishDraft`#

createDraft and finishDraft are two low-level functions that are mostly useful for libraries that build abstractions on top of immer. It avoids the need to always create a function in order to work with drafts. Instead, one can create a draft, modify it, and at some time in the future finish the draft, in which case the next immutable state will be produced. We could for example rewrite our above example as:

import {createDraft, finishDraft} from "immer"

const user = {
    name: "michel",
    todos: []
}

const draft = createDraft(user)
draft.todos = await (await window.fetch("http://host/" + draft.name)).json()
const loadedUser = finishDraft(draft)

Note: finishDraft takes a patchListener as second argument, which can be used to record the patches, similarly to produce.

Warning: in general, we recommend to use produce instead of the createDraft / finishDraft combo, produce is less error prone in usage, and more clearly separates the concepts of mutability and immutability in your code base.

createDraft and finishDraft#

`createDraft` and `finishDraft`#