## When to use Redux Saga

In an application using Redux, when you fire an action something changes in the state of the app.

As this happens, you might need to do something that derives from this state change.

For example you might want to:

• make a HTTP call to a server
• send a WebSocket event
• fetch some data from a GraphQL server
• save something to the cache or browser local storage

…you got the idea.

Those are all things that don’t really relate to the app state, or are async, and you need to move them into a place different than your actions or reducers (while you technically could, it’s not a good way to have a clean codebase).

Enter Redux Saga, a Redux middleware helping you with side effects.

## Basic example of using Redux Saga

To avoid diving into too much theory before showing some actual code, I briefly present how I solved a problem I faced when building a sample app.

In a chat room, when a user writes a message I immediately show the message on the screen, to provide a prompt feedback. This is done through a Redux Action:

const addMessage = (message, author) => ({
message,
author
})


and the state is changed through a reducer:

const messages = (state = [], action) => {
switch (action.type) {
return state.concat([{
message: action.message,
author: action.author
}])
default:
return state
}
}


You initialize Redux Saga by first importing it, then by applying a saga as a middleware to the Redux Store:

//...
import createSagaMiddleware from 'redux-saga'
//...


Then we create a middleware and we apply it to our newly created Redux Store:

const sagaMiddleware = createSagaMiddleware()

const store = createStore(
reducers,
applyMiddleware(sagaMiddleware)
)


The last step is running the saga. We import it and pass it to the run method of the middleware:

import handleNewMessage from './sagas'
//...
sagaMiddleware.run(handleNewMessage)


We just need to write the saga, in ./sagas/index.js:

import { takeEvery } from 'redux-saga/effects'

const handleNewMessage = function* handleNewMessage(params) {
const socket = new WebSocket('ws://localhost:8989')
socket.send(JSON.stringify(action))
})
}

export default handleNewMessage


What this code means is: every time the ADD_MESSAGE action fires, we send a message to the WebSockets server, which responds in this case on localhost:8989.

Notice the use of function*, which is not a normal function, but a generator.

## How it works behind the scenes

Being a Redux Middleware, Redux Saga can intercept Redux Actions, and inject its own functionality.

There are a few concepts to grasp, and here are the main keywords that you’ll want to stick in your head, altogether: saga, generator, middleware, promise, pause, resume, effect, dispatch, action, fulfilled, resolved, yield, yielded.

A saga is some “story” that reacts to an effect that your code is causing. That might contain one of the things we talked before, like an HTTP request or some procedure that saves to the cache.

We create a middleware with a list of sagas to run, which can be one or more, and we connect this middleware to the Redux store.

A saga is a generator function. When a promise is run and yielded, the middleware suspends the saga until the promise is resolved.

Once the promise is resolved the middleware resumes the saga, until the next yield statement is found, and there it is suspended again until its promise resolves.

Inside the saga code, you will generate effects using a few special helper functions provided by the redux-saga package. To start with, we can list:

• takeEvery()
• takeLatest()
• take()
• call()
• put()

When an effect is executed, the saga is paused until the effect is fulfilled.

For example:

import { takeEvery } from 'redux-saga/effects'

const handleNewMessage = function* handleNewMessage(params) {
const socket = new WebSocket('ws://localhost:8989')
socket.send(JSON.stringify(action))
})
}

export default handleNewMessage


When the middleware executes the handleNewMessage saga, it stops at the yield takeEvery instruction and waits (asynchronously, of course) until the ADD_MESSAGE action is dispatched. Then it runs its callback, and the saga can resume.

## Basic Helpers

Helpers are abstractions on top of the low-level saga APIs.

Let’s introduce the most basic helpers you can use to run your effects:

• takeEvery()
• takeLatest()
• take()
• put()
• call()

### takeEvery()

takeEvery(), used in some examples, is one of those helpers.

In the code:

import { takeEvery } from 'redux-saga/effects'

function* watchMessages() {
}


The watchMessages generator pauses until an ADD_MESSAGE action fires, and every time it fires, it’s going to call the postMessageToServer function, infinitely, and concurrently (there is no need for postMessageToServer to terminate its execution before a new once can run)

### takeLatest()

Another popular helper is takeLatest(), which is very similar to takeEvery() but only allows one function handler to run at a time, avoiding concurrency. If another action is fired when the handler is still running, it will cancel it, and run again with the latest data available.

As with takeEvery(), the generator never stops and continues to run the effect when the specified action occurs.

### take()

take() is different in that it only waits a single time. When the action it’s waiting for occurs, the promise resolves and the iterator is resumed, so it can go on to the next instruction set.

### put()

Dispatches an action to the Redux store. Instead of passing in the Redux store or the dispatch action to the saga, you can just use put():

yield put({ type: 'INCREMENT' })
yield put({ type: "USER_FETCH_SUCCEEDED", data: data })


which returns a plain object that you can easily inspect in your tests (more on testing later).

### call()

When you want to call some function in a saga, you can do so by using a yielded plain function call that returns a promise:

delay(1000)


but this does not play nice with tests. Instead, call() allows you to wrap that function call and returns an object that can be easily inspected:

call(delay, 1000)


returns

{ CALL: {fn: delay, args: [1000]}}


## Running effects in parallel

Running effects in parallel is possible using all() and race(), which are very different in what they do.

### all()

If you write

import { call } from 'redux-saga/effects'

const todos = yield call(fetch, '/api/todos')
const user = yield call(fetch, '/api/user')


the second fetch() call won’t be executed until the first one succeeds.

To execute them in parallel, wrap them into all():

import { all, call } from 'redux-saga/effects'

const [todos, user]  = yield all([
call(fetch, '/api/todos'),
call(fetch, '/api/user')
])


all() won’t be resolved until both call() return.

### race()

race() differs from all() by not waiting for all of the helpers calls to return. It just waits for one to return, and it’s done.

It’s a race to see which one finishes first, and then we forget about the other participants.

It’s typically used to cancel a background task that runs forever until something occurs:

import { race, call, take } from 'redux-saga/effects'

while(1) {
//...
}
}

yield race([

when the CANCEL_TASK action is emitted, we stop the other task that would otherwise run forever.