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Event

Event is an intention to change state. Let's imagine life situation, you come to a shop and on etiquette you should say "hello" - intention, then you say "hello" - event.

Event Methods

watch(watcher)

It is a function which allows you to watch or to create side-effects.

Arguments

  1. watcher (Function): A function that receives payload.

Returns

(Subscription): Unsubscribe function.

Example

import {createEvent} from 'effector'

const sayHi = createEvent()
const unwatch = sayHi.watch(name => console.log(`${name}, hi there!`))

sayHi('Peter') // => Peter, hi there!
unwatch()

sayHi('Drew') // => nothing happened

map(fn)

Сreates a new event, which will be called after the original event is called, applying the result of a fn as a payload. It is special function which allows you to decompose dataflow, extract or transform data.

Arguments

  1. fn (Function): A function that receives payload, must be pure.

Returns

(Event): New event.

Example

import {createEvent} from 'effector'

const userUpdated = createEvent()
const userNameUpdated = userUpdated.map(({name}) => name) // you may decompose dataflow with .map() method
const userRoleUpdated = userUpdated.map(({role}) => role.toUpperCase()) // either way you can transform data

userNameUpdated.watch(name => console.log(`User's name is [${name}] now`))
userRoleUpdated.watch(name => console.log(`User's role is [${name}] now`))

userUpdated({name: 'john', role: 'admin'})
// => User's name is [john] now
// => User's role is [ADMIN] now

filter({fn})

Сreates a new event, which will be called after the original event is called, if fn returns true.

Arguments

  1. fn (Function): A function that receives payload, must be pure.

Returns

(Event): New event.

Example

import {createEvent, createStore} from 'effector'

const numbers = createEvent('event with {x: number}')

const positiveNumbers = numbers.filter({
  fn: ({x}) => x > 0,
})

const lastPositive = createStore(0).on(positiveNumbers, (n, {x}) => x)

filterMap(fn)

Сreates a new event, which will be called after the original event is called, if fn returns not undefined.

Arguments

  1. fn (Function): A function that receives payload, must be pure.

Returns

(Event): New event.

Example

import React from 'react'

import {createEvent, createStore} from 'effector'

const openModal = createEvent('open that modal')

const openModalUnboxed = openModal.filterMap(ref => {
  if (ref.current) return ref.current
})

openModalUnboxed.watch(modal => modal.showModal())

const closeModal = createEvent('close that modal')

closeModal
  .filter(ref => {
    if (ref.current) return ref.current
  })
  .watch(modal => modal.close())

const modalRef = React.createRef()

const App = () => (
  <>
    <dialog ref={modalRef}>
      <form method="dialog">
        <fieldset>
          <legend>Modal</legend>
          Tap to close
          <button type="submit" onSubmit={() => closeModal(modalRef)}>
            ❌
          </button>
        </fieldset>
      </form>
    </dialog>

    <button onClick={() => openModal(modalRef)}>Open modal</button>
  </>
)

prepend(fn)

Creates an event, upon trigger it does send transformed data into source event. Works kind of like reverse .map. In the case of .prepend data transforms before the original event occurs and in the case of .map, data transforms after original event occurred.

Arguments

  1. fn (Function): A function that receives payload, must be pure.

Returns

(Event): New event.

Example

import {createEvent} from 'effector'

const nameChanged = createEvent()
nameChanged.watch(name => console.log(`Current name is: ${name}`))

const inputChanged = nameChanged.prepend(e => e.target.value) // event, which will be bound to DOM element
const input = document.createElement('input')
input.onchange = inputChanged

document.body.appendChild(input)
// input something in input, and press Enter
// => Current name is: something
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