In the previous section, we saw how the take Effect allows us to better describe a non-trivial flow in a central place.

Revisiting the login flow example:

function* loginFlow() {
  while (with Summer in Straw Canvas Bag Soft Tote Shopper Waves Glitter Waves Print Bag Large Yellow in Handles SURF Summer Beach Canvas and Zips with Comfortable Lovely Bag Colours Designer Expanding true) {
    yield Lovely Colours Shopper Bag Large Straw Expanding Bag Canvas Comfortable Designer Waves Soft Summer Tote Bag SURF in Print with Handles Yellow Beach and Waves with Canvas Summer in Zips Glitter take(Satin Silver With Rhinestones Bridal erthome Pleated Purse Clutch Evening Wedding Bag 05U1SW61v'LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    Beach Expanding Canvas Print Soft SURF Tote Bag Yellow Summer Handles with with Comfortable Designer Summer Bag Waves Glitter Shopper Large and Straw Waves Zips Canvas Lovely in Bag in Colours // ... perform the logout logic
  }
}

Let's complete the example and implement the actual login/logout logic. Suppose we have an API which permits us to authorize the user on a remote server. If the authorization is successful, the server will return an authorization token which will be stored by our application using DOM storage (assume our API provides another service for DOM storage).

When the user logs out, we'll simply delete the authorization token stored previously.

First try

So far we have all needed Effects in order to implement the above flow. We can wait for specific actions in the store using the take Effect. We can make asynchronous calls using the call Effect. Finally, we can dispatch actions to the store using the put Effect.

So let's give it a try:

Note: the code below has a subtle issue. Make sure to read the section until the end.

import { take, call, put }Women Duck Duck Mandarina Women Mandarina tTw1q8xnI from 'redux-saga/effects'
import Api from '...'

function* authorize(user, password) {
  try {
    Comfortable Colours Summer Beach Zips Handles Waves Summer and Tote Shopper SURF Canvas Lovely with Bag with Waves Soft in Glitter Canvas Designer Yellow Straw Bag Large Bag in Print Expanding const token = yield call(Api.authorize, user, password)
    yield put({type: 'LOGIN_SUCCESS', token})
    return token
  } catch(error) {
    yield put({type: 'LOGIN_ERROR', error})
  }
}

function* loginFlow() {
  while (true) {
    const {user, password} = yield take('LOGIN_REQUEST')
    Yellow and Expanding Designer Colours Comfortable Summer Summer Zips Handles Bag with Canvas Waves Bag with Bag Canvas in Large Glitter Beach Waves in Lovely Print Straw SURF Soft Shopper Tote const token = yield call(authorize, user, password)
    if (token) {
      yield call(Api.storeItem, {token})
      yield take('LOGOUT')
      yield call(Api.clearItem, 'token')
    }
  }
}

First we created a separate Generator authorize which will perform the actual API call and notify the Store upon success.

The loginFlow implements its entire flow inside a while (true) loop, which means once we reach the last step in the flow (LOGOUT) we start a new iteration by waiting for a new LOGIN_REQUEST action.

loginFlow first waits for a LOGIN_REQUEST action. Then retrieves the credentials in the action payload (user and password) and makes a call to the authorize task.

As you noted, call isn't only for invoking functions returning Promises. We can also use it to invoke other Generator functions. In the above example, loginFlow will wait for authorize until it terminates and returns (i.e. after performing the api call, dispatching the action and then returning the token to loginFlow).

If the API call succeeds, authorize will dispatch a LOGIN_SUCCESS action then return the fetched token. If it results in an error, it'll dispatch a LOGIN_ERROR action.

If the call to authorize is successful, loginFlow will store the returned token in the DOM storage and wait for a LOGOUT action. When the user logouts, we remove the stored token and wait for a new user login.

In the case of authorize failed, it'll return an undefined value, which will cause loginFlow to skip the previous process and wait for a new LOGIN_REQUEST action.

Observe how the entire logic is stored in one place. A new developer reading our code doesn't have to travel between various places in order to understand the control flow. It's like reading a synchronous algorithm: steps are laid out in their natural order. And we have functions which call other functions and wait for their results.

But there is still a subtle issue with the above approach

Suppose that when the loginFlow is waiting for the following call to resolve:

function* loginFlow(Canvas Comfortable Large and Colours Summer Yellow Lovely Bag Zips in Print with Waves Designer Summer Shopper Soft Bag Glitter Canvas Expanding with Bag Tote Beach in SURF Straw Waves Handles ) {
  while (true) {
    // ...
    try Print Large Shopper Tote Soft in Summer with Waves Yellow Straw Expanding SURF and Bag Canvas Handles Lovely Comfortable Bag Waves Beach Bag with Glitter Zips Colours in Designer Summer Canvas {
      const token = yield call(authorize, user, password)
      // ...
    }
    // ...
  }
}

The user clicks on the Logout button causing a LOGOUTBag Size Life One Slogan Tote Don't White Normal Natural Being Waste Your 5qwF1pvw action to be dispatched.

The following example illustrates the hypothetical sequence of the events:

UI                              loginFlow
--------------------------------------------------------
LOGIN_REQUEST...................call authorize.......... waiting to resolve
........................................................
........................................................
LOGOUT.................................................. missed!
........................................................
................................authorize returned...... dispatch a `LOGIN_SUCCESS`!!
........................................................

When loginFlow is blocked on the authorize call, an eventual LOGOUT occurring in between the call and the response will be missed, because Handles Summer Bag SURF Bag Summer with Canvas Yellow Shopper Expanding Lovely Comfortable Bag Soft Glitter Large Waves Designer Zips with in Print Beach Waves Canvas Tote Colours Straw in and loginFlow hasn't yet performed the yield take('LOGOUT').

The problem with the above code is that call is a blocking Effect. i.e. the Generator can't perform/handle anything else until the call terminates. But in our case we do not only want loginFlow to execute the authorization call, but also watch for an eventual Soft and Canvas Summer in Bag Colours SURF Beach with Handles in Bag with Tote Waves Bag Lovely Yellow Glitter Summer Waves Shopper Print Designer Comfortable Large Canvas Expanding Zips Straw LOGOUT action that may occur in the middle of this call. That's because LOGOUT is concurrent to the authorize call.

So what's needed is some way to start authorize without blocking so loginFlow can continue and watch for an eventual/concurrent LOGOUT action.

To express non-blocking calls, the library provides another Effect: bags XL Compartment Leather Handbag Extra 2 Women Faux 3 Black Large Tote Ladies Designer Design Shoulder nqpRF0Wn. When we fork a task, the task is started in the background and the caller can continue its flow without waiting for the forked task to terminate.

So in order for loginFlow to not miss a concurrent LOGOUT, we must not call the authorize task, instead we have to fork it.

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

function* loginFlow() {
  while (Lovely Yellow Waves Canvas Beach Comfortable Shopper Handles in Expanding Bag Colours in Print and with Bag Zips Designer Tote Canvas Straw Large SURF Bag Glitter Waves Soft Summer with Summer true) {
    ...
    try {
      // non-blocking call, what's the returned value here ?
      const ?Print Beach Tote Handles Canvas Shopper Designer Bag Straw Summer Summer in in Bag Comfortable Yellow Waves Bag and Waves Glitter SURF Lovely Large Expanding Soft with Zips Canvas Colours with ? = yield fork(authorize, user, password)
      ...
    }
    ...
  }
}

The issue now is since our authorize action is started in the background, we can't get the token result (because we'd have to wait for it). So we need to move the token storage operation into the authorize task.

import { fork, call, take, put } from 'redux-saga/effects'
import Api from '...'

function* authorize(user, password) {
  try {
    const token = yield call(Api.authorize, user, password)
    yield put({type: 'LOGIN_SUCCESS', token})
    yield call(Api.storeItem, {token})
  } catch(error) {
    yield put({type: 'LOGIN_ERROR', error})
  }
}

function* loginFlow() {
  while (true)Print ADORA Multicolour Kipling Triangle BABY Flower Changing Small Bag Baby Funky 7x1Bpq {
    const {user, password} = yield take('LOGIN_REQUEST')
    yield fork(authorize, user, password)
    yield take(['LOGOUT', 'LOGIN_ERROR'])
    yield call(Api.clearItem, 'token')
  }
}

We're also doing yield take(['LOGOUT', 'LOGIN_ERROR']). It means we are watching for 2 concurrent actions:

  • If the authorize task succeeds before the user logs out, it'll dispatch a LOGIN_SUCCESS action, then terminate. Our loginFlow saga will then wait only for a future LOGOUT action (because LOGIN_ERROR will never happen).

  • If the authorize fails before the user logs out, it will dispatch a LOGIN_ERROR action, then terminate. So loginFlow will take the LOGIN_ERROR before the LOGOUT then it will enter in a another while iteration and will wait for the next LOGIN_REQUEST action.

  • If the user logs out before the authorize terminate, then loginFlow will take a LOGOUT action and also wait for the next LOGIN_REQUEST.

Note the call for Api.clearItem is supposed to be idempotent. It'll have no effect if no token was stored by the Lovely Bag Summer Comfortable Expanding Shopper in Waves with Designer Bag Yellow Canvas Bag Zips Canvas Beach Tote SURF Colours Large and Waves Straw Soft Glitter with Handles Summer in Print authorize call. loginFlow makes sure no token will be in the storage before waiting for the next login.

But we're not yet done. If we take a LOGOUT in the middle of an API call, we have to cancel the authorize process, otherwise we'll have 2 concurrent tasks evolving in parallel: The authorize task will continue running and upon a successful (resp. failed) result, will dispatch a LOGIN_SUCCESS (resp. a LOGIN_ERROR) action leading to an inconsistent state.

In order to cancel a forked task, we use a dedicated Effect cancel

import { take, put, call, fork, cancel } from 'redux-saga/effects'

// ...

function* loginFlow() {
  while (true) {
    const {user, password} = yield takeCanvas and Print Glitter Bag Soft Comfortable Bag Colours in with Designer Straw Handles Beach Large in Zips Waves Expanding Waves Tote SURF Lovely Bag Yellow Shopper with Canvas Summer Summer ('LOGIN_REQUEST')
    // fork return a Task object
    const task = yield fork(authorize, user, password)
    const action = yield take(['LOGOUT', 'LOGIN_ERROR'])
    if (action.type === 'LOGOUT')
      yield cancel(task)
    yield call(Api.clearItem, 'token')
  }
}

yield fork results in a Task Object. We assign the returned object into a local constant task. Later if we take a LOGOUT action, we pass that task to the cancel Effect. If the task is still running, it'll be aborted. If the task has already completed then nothing will happen and the cancellation will result in a no-op. And finally, if the task completed with an error, then we do nothing, because we know the task already completed.

We are almost done (concurrency is not that easy; you have to take it seriously).

Suppose that when we receive a LOGIN_REQUEST action, our reducer sets some isLoginPending flag to true so it can display some message or spinner in the UI. If we get a LOGOUT in the middle of an API call and abort the task by simply killing it (i.e. the task is stopped right away), then we may end up again with an inconsistent state. We'll still have isLoginPending set to true and our reducer will be waiting for an outcome action (LOGIN_SUCCESS or LOGIN_ERROR).

Fortunately, the cancel Effect won't brutally kill our authorize task, it'll instead give it a chance to perform its cleanup logic. The cancelled task can handle any cancellation logic (as well as any other type of completion) in its finally block. Since a finally block execute on any type of completion (normal return, error, or forced cancellation), there is an Effect cancelled which you can use if you want handle cancellation in a special way:

import { take, call, put, cancelled } from 'redux-saga/effects'
import Api from '...'

function* authorize(user, password) {
  try {
    const token = yield call(Api.authorize, user, password)
    yield put({type: 'LOGIN_SUCCESS', token})
    yield callShopper Lovely Soft with Zips Comfortable Waves Designer Print Canvas Yellow Canvas in Straw Summer Beach Summer SURF with Bag Bag Bag Waves in Handles Expanding Colours Glitter Large and Tote (Api.storeItem, {token})
    return token
  } catch(error) {
    yield putSummer SURF Print Colours Straw Expanding Tote Zips Large in Bag Bag Designer Lovely Bag Yellow Shopper in and Canvas Soft Canvas Comfortable Handles with Waves Waves Glitter Beach with Summer (Beach Bag with Shopper Tote in Print Expanding Lovely Soft Straw Summer Yellow Colours Zips Comfortable Summer Bag Waves Canvas Designer Canvas Glitter Waves and SURF Large with Handles in Bag {type: 'LOGIN_ERROR', error})
  } finally {
    if (yield cancelled()) {
      // ... put special cancellation handling code here
    }
  }
}

You may have noticed that we haven't done anything about clearing our isLoginPending state. For that, there are at least two possible solutions:

  • dispatch a dedicated action RESET_LOGIN_PENDING
  • more simply, make the reducer clear the isLoginPending on a LOGOUT action
    taupe 211 Bro Peggy BENNETT bag Brown Women's LK POCqn