reaction

Sometimes you need to react to changes to state. Typically you want to run some imperative logic related to something in the state changing.

reaction(
// Access and return some state to react to
(state) => state.foo,
// Do something with the returned value
(foo) => {},
{
// If you return an object or array from the state you can set this to true.
// The reaction will run when any nested changes occur as well
nested: false,
// Runs the reaction immediately
immediate: false
}
)

There are two points of setting up reactions in Overmind.

onInitialize

The onInitialize hook is where you set up reactions that lives throughout your application lifetime. The reaction function returns a function to dispose it. That means you can give effects the possibility to create and dispose of reactions in any action.

overmind/onInitialize.ts
overmind/onInitialize.ts
import { OnInitialize } from 'overmind'
export const onInitialize: OnInitialize = ({ effects }, instance) => {
instance.reaction(
({ todos }) => todos,
(todos) => effects.storage.saveTodos(todos),
{
nested: true
}
)
}

components

With components you typically use reactions to manipulate DOM elements or other UI related imperative libraries.

React
Angular
Vue
React
import * as React from 'react'
import { useOvermind } from '../overmind'
const App: React.FC = () => {
const { reaction } = useOvermind()
React.useEffect(() => reaction(
({ currentPage }) => currentPage,
() => {
document.querySelector('#page').scrollTop = 0
}
))
return <div id="page"></div>
}
export default App
Angular
import { Component } from '@angular/core';
import { Store } from '../overmind'
@Component({
selector: 'app-root',
template: `
<div id="page"></div>
`
})
export class App {
constructor(private store: Store) {}
ngOnInit() {
this.store.reaction(
({ currentPage }) => currentPage,
() => {
document.querySelector('#page').scrollTop = 0
}
)
}
}
Vue
<template>
<div id="page"></div>
</template>
<script>
export default {
mounted() {
this.overmind.reaction(
({ currentPage }) => currentPage,
() => {
document.querySelector('#page').scrollTop = 0
}
)
}
}
</script>