Thunk middleware for Redux.
npm install --save redux-thunkA thunk is a function that wraps an expression to delay its evaluation.
// calculation of 1 + 2 is immediate
// x === 3
let x = 1 + 2;
// calculation of 1 + 2 is delayed
// foo can be called later to perform the calculation
// foo is a thunk!
let foo = () => 1 + 2;Redux Thunk middleware 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.
An action creator that returns a function to perform asynchronous dispatch:
const INCREMENT_COUNTER = 'INCREMENT_COUNTER';
function increment() {
return {
type: INCREMENT_COUNTER
};
}
function incrementAsync() {
return dispatch => {
setTimeout(() => {
// Yay! Can invoke sync or async actions with `dispatch`
dispatch(increment());
}, 1000);
};
}An action creator that returns a function to perform conditional dispatch:
function incrementIfOdd() {
return (dispatch, getState) => {
const { counter } = getState();
if (counter % 2 === 0) {
return;
}
dispatch(increment());
};
}npm install --save redux-thunk
Then, to enable Redux Thunk, use applyMiddleware():
import { createStore, applyMiddleware } from 'redux';
import thunk from 'redux-thunk';
import rootReducer from './reducers/index';
// create a store that has redux-thunk middleware enabled
const createStoreWithMiddleware = applyMiddleware(
thunk
)(createStore);
const store = createStoreWithMiddleware(rootReducer);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:
import { createStore, applyMiddleware } from 'redux';
import thunk from 'redux-thunk';
import rootReducer from './reducers';
// applyMiddleware supercharges createStore with middleware:
let createStoreWithMiddleware = applyMiddleware(thunk)(createStore);
// We can use it exactly like “vanilla” createStore.
let store = createStoreWithMiddleware(rootReducer);
function fetchSecretSauce() {
return fetch('https://www.google.com/search?q=secret+sauce');
}
// These are the normal action creators you have seen so far.
// The actions they return can be dispatched without any middleware.
// However, they only express “facts” and not the “async flow”.
function makeASandwich(forPerson, secretSauce) {
return {
type: 'MAKE_SANDWICH',
forPerson,
secretSauce
};
}
function apologize(fromPerson, toPerson, error) {
return {
type: 'APOLOGIZE',
fromPerson,
toPerson,
error
};
}
function withdrawMoney(amount) {
return {
type: 'WITHDRAW',
amount
};
}
// Even without middleware, you can dispatch an action:
store.dispatch(withdrawMoney(100));
// But what do you do when you need to start an asynchronous action,
// such as an API call, or a router transition?
// Meet thunks.
// A thunk is a function that returns a function.
// This is a thunk.
function makeASandwichWithSecretSauce(forPerson) {
// Invert control!
// Return a function that accepts `dispatch` so we can dispatch later.
// Thunk middleware knows how to turn thunk async actions into actions.
return function (dispatch) {
return fetchSecretSauce().then(
sauce => dispatch(makeASandwich(forPerson, sauce)),
error => dispatch(apologize('The Sandwich Shop', forPerson, error))
);
};
}
// Thunk middleware lets me dispatch thunk async actions
// as if they were actions!
store.dispatch(
makeASandwichWithSecretSauce('Me')
);
// It even takes care to return the thunk’s return value
// from the dispatch, so I can chain Promises as long as I return them.
store.dispatch(
makeASandwichWithSecretSauce('My wife')
).then(() => {
console.log('Done!');
});
// In fact I can write action creators that dispatch
// actions and async actions from other action creators,
// and I can build my control flow with Promises.
function makeSandwichesForEverybody() {
return function (dispatch, getState) {
if (!getState().sandwiches.isShopOpen) {
// You don’t have to return Promises, but it’s a handy convention
// so the caller can always call .then() on async dispatch result.
return Promise.resolve();
}
// We can dispatch both plain object actions and other thunks,
// which lets us compose the asynchronous actions in a single flow.
return dispatch(
makeASandwichWithSecretSauce('My Grandma')
).then(() =>
Promise.all([
dispatch(makeASandwichWithSecretSauce('Me')),
dispatch(makeASandwichWithSecretSauce('My wife'))
])
).then(() =>
dispatch(makeASandwichWithSecretSauce('Our kids'))
).then(() =>
dispatch(getState().myMoney > 42 ?
withdrawMoney(42) :
apologize('Me', 'The Sandwich Shop')
)
);
};
}
// This is very useful for server side rendering, because I can wait
// until data is available, then synchronously render the app.
store.dispatch(
makeSandwichesForEverybody()
).then(() =>
response.send(React.renderToString(<MyApp store={store} />))
);
// I can also dispatch a thunk async action from a component
// any time its props change to load the missing data.
import { connect } from 'react-redux';
import { Component } from 'react';
class SandwichShop extends Component {
componentDidMount() {
this.props.dispatch(
makeASandwichWithSecretSauce(this.props.forPerson)
);
}
componentWillReceiveProps(nextProps) {
if (nextProps.forPerson !== this.props.forPerson) {
this.props.dispatch(
makeASandwichWithSecretSauce(nextProps.forPerson)
);
}
}
render() {
return <p>{this.props.sandwiches.join('mustard')}</p>
}
}
export default connect(
SandwichShop,
state => ({
sandwiches: state.sandwiches
})
);MIT