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324 lines
8.3 KiB
Markdown
324 lines
8.3 KiB
Markdown
Redux Thunk
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=============
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Thunk [middleware](https://redux.js.org/advanced/middleware) for Redux.
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[](https://travis-ci.org/reduxjs/redux-thunk)
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[](https://www.npmjs.com/package/redux-thunk)
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[](https://www.npmjs.com/package/redux-thunk)
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```js
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npm install --save redux-thunk
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```
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## Note on 2.x Update
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Most tutorials today assume Redux Thunk 1.x so you might run into an issue when running their code with 2.x.
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**If you use Redux Thunk 2.x in CommonJS environment, [don’t forget to add `.default` to your import](https://github.com/reduxjs/redux-thunk/releases/tag/v2.0.0):**
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```diff
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- var ReduxThunk = require('redux-thunk')
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+ var ReduxThunk = require('redux-thunk').default
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```
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If you used ES modules, you’re already all good:
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```js
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import ReduxThunk from 'redux-thunk' // no changes here 😀
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```
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Additionally, since 2.x, we also support a [UMD build](https://unpkg.com/redux-thunk/dist/redux-thunk.min.js):
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```js
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var ReduxThunk = window.ReduxThunk.default
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```
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As you can see, it also requires `.default` at the end.
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## Why Do I Need This?
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If you’re not sure whether you need it, you probably don’t.
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**[Read this for an in-depth introduction to thunks in Redux.](http://stackoverflow.com/questions/35411423/how-to-dispatch-a-redux-action-with-a-timeout/35415559#35415559)**
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## Motivation
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Redux Thunk [middleware](https://github.com/reactjs/redux/blob/master/docs/advanced/Middleware.md) allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods `dispatch` and `getState` as parameters.
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An action creator that returns a function to perform asynchronous dispatch:
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```js
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const INCREMENT_COUNTER = 'INCREMENT_COUNTER';
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function increment() {
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return {
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type: INCREMENT_COUNTER
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};
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}
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function incrementAsync() {
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return dispatch => {
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setTimeout(() => {
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// Yay! Can invoke sync or async actions with `dispatch`
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dispatch(increment());
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}, 1000);
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};
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}
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```
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An action creator that returns a function to perform conditional dispatch:
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```js
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function incrementIfOdd() {
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return (dispatch, getState) => {
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const { counter } = getState();
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if (counter % 2 === 0) {
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return;
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}
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dispatch(increment());
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};
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}
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```
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## What’s a thunk?!
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A [thunk](https://en.wikipedia.org/wiki/Thunk) is a function that wraps an expression to delay its evaluation.
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```js
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// calculation of 1 + 2 is immediate
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// x === 3
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let x = 1 + 2;
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// calculation of 1 + 2 is delayed
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// foo can be called later to perform the calculation
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// foo is a thunk!
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let foo = () => 1 + 2;
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```
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The term [originated](https://en.wikipedia.org/wiki/Thunk#cite_note-1) as a humorous past-tense version of "think".
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## Installation
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```
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npm install --save redux-thunk
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```
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Then, to enable Redux Thunk, use [`applyMiddleware()`](https://redux.js.org/api-reference/applymiddleware):
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```js
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import { createStore, applyMiddleware } from 'redux';
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import thunk from 'redux-thunk';
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import rootReducer from './reducers/index';
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// Note: this API requires redux@>=3.1.0
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const store = createStore(
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rootReducer,
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applyMiddleware(thunk)
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);
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```
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## Composition
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Any return value from the inner function will be available as the return value of `dispatch` itself. This is convenient for orchestrating an asynchronous control flow with thunk action creators dispatching each other and returning Promises to wait for each other’s completion:
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```js
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import { createStore, applyMiddleware } from 'redux';
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import thunk from 'redux-thunk';
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import rootReducer from './reducers';
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// Note: this API requires redux@>=3.1.0
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const store = createStore(
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rootReducer,
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applyMiddleware(thunk)
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);
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function fetchSecretSauce() {
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return fetch('https://www.google.com/search?q=secret+sauce');
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}
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// These are the normal action creators you have seen so far.
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// The actions they return can be dispatched without any middleware.
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// However, they only express “facts” and not the “async flow”.
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function makeASandwich(forPerson, secretSauce) {
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return {
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type: 'MAKE_SANDWICH',
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forPerson,
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secretSauce
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};
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}
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function apologize(fromPerson, toPerson, error) {
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return {
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type: 'APOLOGIZE',
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fromPerson,
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toPerson,
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error
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};
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}
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function withdrawMoney(amount) {
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return {
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type: 'WITHDRAW',
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amount
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};
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}
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// Even without middleware, you can dispatch an action:
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store.dispatch(withdrawMoney(100));
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// But what do you do when you need to start an asynchronous action,
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// such as an API call, or a router transition?
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// Meet thunks.
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// A thunk is a function that returns a function.
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// This is a thunk.
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function makeASandwichWithSecretSauce(forPerson) {
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// Invert control!
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// Return a function that accepts `dispatch` so we can dispatch later.
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// Thunk middleware knows how to turn thunk async actions into actions.
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return function (dispatch) {
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return fetchSecretSauce().then(
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sauce => dispatch(makeASandwich(forPerson, sauce)),
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error => dispatch(apologize('The Sandwich Shop', forPerson, error))
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);
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};
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}
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// Thunk middleware lets me dispatch thunk async actions
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// as if they were actions!
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store.dispatch(
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makeASandwichWithSecretSauce('Me')
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);
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// It even takes care to return the thunk’s return value
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// from the dispatch, so I can chain Promises as long as I return them.
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store.dispatch(
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makeASandwichWithSecretSauce('My wife')
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).then(() => {
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console.log('Done!');
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});
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// In fact I can write action creators that dispatch
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// actions and async actions from other action creators,
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// and I can build my control flow with Promises.
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function makeSandwichesForEverybody() {
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return function (dispatch, getState) {
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if (!getState().sandwiches.isShopOpen) {
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// You don’t have to return Promises, but it’s a handy convention
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// so the caller can always call .then() on async dispatch result.
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return Promise.resolve();
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}
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// We can dispatch both plain object actions and other thunks,
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// which lets us compose the asynchronous actions in a single flow.
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return dispatch(
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makeASandwichWithSecretSauce('My Grandma')
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).then(() =>
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Promise.all([
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dispatch(makeASandwichWithSecretSauce('Me')),
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dispatch(makeASandwichWithSecretSauce('My wife'))
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])
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).then(() =>
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dispatch(makeASandwichWithSecretSauce('Our kids'))
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).then(() =>
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dispatch(getState().myMoney > 42 ?
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withdrawMoney(42) :
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apologize('Me', 'The Sandwich Shop')
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)
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);
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};
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}
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// This is very useful for server side rendering, because I can wait
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// until data is available, then synchronously render the app.
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store.dispatch(
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makeSandwichesForEverybody()
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).then(() =>
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response.send(ReactDOMServer.renderToString(<MyApp store={store} />))
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);
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// I can also dispatch a thunk async action from a component
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// any time its props change to load the missing data.
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import { connect } from 'react-redux';
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import { Component } from 'react';
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class SandwichShop extends Component {
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componentDidMount() {
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this.props.dispatch(
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makeASandwichWithSecretSauce(this.props.forPerson)
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);
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}
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componentDidUpdate(prevProps) {
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if (prevProps.forPerson !== this.props.forPerson) {
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this.props.dispatch(
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makeASandwichWithSecretSauce(this.props.forPerson)
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);
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}
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}
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render() {
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return <p>{this.props.sandwiches.join('mustard')}</p>
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}
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}
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export default connect(
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state => ({
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sandwiches: state.sandwiches
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})
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)(SandwichShop);
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```
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## Injecting a Custom Argument
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Since 2.1.0, Redux Thunk supports injecting a custom argument using the `withExtraArgument` function:
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```js
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const store = createStore(
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reducer,
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applyMiddleware(thunk.withExtraArgument(api))
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)
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// later
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function fetchUser(id) {
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return (dispatch, getState, api) => {
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// you can use api here
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}
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}
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```
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To pass multiple things, just wrap them in a single object and use destructuring:
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```js
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const store = createStore(
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reducer,
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applyMiddleware(thunk.withExtraArgument({ api, whatever }))
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)
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// later
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function fetchUser(id) {
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return (dispatch, getState, { api, whatever }) => {
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// you can use api and something else here
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}
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}
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```
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## License
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MIT
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