redux-saga
is a library that aims to make side effects (i.e. asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
The mental model is that a saga is like a separate thread in your application that's solely responsible for side effects. redux-saga
is a redux middleware, which means this thread can be started, paused and cancelled from the main application with normal redux actions, it has access to the full redux application state and it can dispatch redux actions as well.
It uses an ES6 feature called Generators to make those asynchronous flows easy to read, write and test. (if you're not familiar with them here are some introductory links) By doing so, these asynchronous flows look like your standard synchronous JavaScript code. (kind of like async
/await
, but generators have a few more awesome features we need)
You might've used redux-thunk
before to handle your data fetching. Contrary to redux thunk, you don't end up in callback hell, you can test your asynchronous flows easily and your actions stay pure.
$ npm install --save redux-saga
or
$ yarn add redux-saga
Alternatively, you may use the provided UMD builds directly in the <script>
tag of an HTML page. See this section.
Suppose we have an UI to fetch some user data from a remote server when a button is clicked. (For brevity, we'll just show the action triggering code.)
class UserComponent extends React.Component {
...
onSomeButtonClicked() {
const { userId, dispatch } = this.props
dispatch({type: 'USER_FETCH_REQUESTED', payload: {userId}})
}
...
}
The Component dispatches a plain Object action to the Store. We'll create a Saga that watches for all USER_FETCH_REQUESTED
actions and triggers an API call to fetch the user data.
import { call, put, takeEvery, takeLatest } from 'redux-saga/effects'
import Api from '...'
// worker Saga: will be fired on USER_FETCH_REQUESTED actions
function* fetchUser(action) {
try {
const user = yield call(Api.fetchUser, action.payload.userId);
yield put({type: "USER_FETCH_SUCCEEDED", user: user});
} catch (e) {
yield put({type: "USER_FETCH_FAILED", message: e.message});
}
}
/*
Starts fetchUser on each dispatched `USER_FETCH_REQUESTED` action.
Allows concurrent fetches of user.
*/
function* mySaga() {
yield takeEvery("USER_FETCH_REQUESTED", fetchUser);
}
/*
Alternatively you may use takeLatest.
Does not allow concurrent fetches of user. If "USER_FETCH_REQUESTED" gets
dispatched while a fetch is already pending, that pending fetch is cancelled
and only the latest one will be run.
*/
function* mySaga() {
yield takeLatest("USER_FETCH_REQUESTED", fetchUser);
}
export default mySaga;
To run our Saga, we'll have to connect it to the Redux Store using the redux-saga
middleware.
import { createStore, applyMiddleware } from 'redux'
import createSagaMiddleware from 'redux-saga'
import reducer from './reducers'
import mySaga from './sagas'
// create the saga middleware
const sagaMiddleware = createSagaMiddleware()
// mount it on the Store
const store = createStore(
reducer,
applyMiddleware(sagaMiddleware)
)
// then run the saga
sagaMiddleware.run(mySaga)
// render the application
- Introduction
- Basic Concepts
- Advanced Concepts
- Recipes
- External Resources
- Troubleshooting
- Glossary
- API Reference
There is also a umd build of redux-saga
available in the dist/
folder. When using the umd build redux-saga
is available as ReduxSaga
in the window object. This enables you to create Saga middleware without using ES6 import
sytnax like this:
var sagaMiddleware = ReduxSaga.default()
The umd version is useful if you don't use Webpack or Browserify. You can access it directly from unpkg.
The following builds are available:
Important! If the browser you are targeting doesn't support ES2015 generators, you must provide a valid polyfill, such as the one provided by babel
. The polyfill must be imported before redux-saga:
import 'babel-polyfill'
// then
import sagaMiddleware from 'redux-saga'
$ git clone https://github.com/redux-saga/redux-saga.git
$ cd redux-saga
$ npm install
$ npm test
Below are the examples ported (so far) from the Redux repos.
There are three counter examples.
Demo using vanilla JavaScript and UMD builds. All source is inlined in index.html
.
To launch the example, just open index.html
in your browser.
Important: your browser must support Generators. Latest versions of Chrome/Firefox/Edge are suitable.
Demo using webpack
and high-level API takeEvery
.
$ npm run counter
# test sample for the generator
$ npm run test-counter
Demo using low-level API to demonstrate task cancellation.
$ npm run cancellable-counter
$ npm run shop
# test sample for the generator
$ npm run test-shop
$ npm run async
# test sample for the generators
$ npm run test-async
$ npm run real-world
# sorry, no tests yet
You can find the official Redux-Saga logo with different flavors in the logo directory.
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