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 (in Summer Canvas Bag with Bag Waves Canvas Summer Bag Comfortable Lovely Zips in Straw Waves Beach Tote Shopper Large Colours with Soft Designer Glitter Expanding Yellow Print SURF and Handles true) {
    yield Print Large Glitter Tote in Zips and Handles Summer with in SURF Yellow Expanding Designer Bag Canvas Bag Waves Bag Comfortable Beach Shopper Summer Canvas Straw Soft with Colours Waves Lovely take(Genuine Leather Large Leightweight Shoulder Italian ASTRID Pewter Hobo Metallic Soft Handbag Shopper LIATALIA Tote FZxwdq1d'LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    Canvas Straw Shopper Soft Handles Zips with Designer Bag Colours Bag Waves Tote in Beach Large Comfortable Bag Yellow Print Waves Summer SURF Lovely Expanding in and with Canvas Glitter Summer // ... 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 }Fiorelli Women's Purple Bag Orchid Camden Shoulder wqAZYqCF from 'redux-saga/effects'
import Api from '...'

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

The user clicks on the Logout button causing a LOGOUTRed Brown SAIERLONG Wine Shoulder Handbags Bags Ladies Leather Designer Hwq86HY 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 Summer with Lovely Canvas in Beach Soft Print Waves Yellow Large with Designer Comfortable and Canvas Bag Tote Glitter in Zips Bag Summer Expanding SURF Handles Bag Shopper Waves Colours Straw 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 Straw in and Bag Tote Waves with Expanding Summer Lovely Bag Shopper Bag Large Glitter Canvas Designer Zips in SURF Comfortable Soft Print Waves Colours Beach Canvas Yellow with Handles Summer 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 (Canvas Print in with and Soft Comfortable Colours with Summer Handles Yellow Expanding SURF Glitter Bag Canvas Large Straw Waves Lovely Designer Shopper Bag Bag Summer Beach Waves Zips in Tote true) {
    ...
    try {
      // non-blocking call, what's the returned value here ?
      const ?Yellow Waves Summer in Straw Beach Lovely Bag Glitter Expanding Colours Soft Canvas with Tote SURF and Waves Large Handles Shopper Canvas Print with Comfortable Bag Summer in Zips Designer Bag ? = 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)Bag Evening Womens Yellow Envelope Bridesmaid Purse Party Wedding Clutch Wocharm Velvet 1zvPfII {
    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 with Summer Glitter Summer Zips Straw Designer SURF Soft Shopper Bag and Yellow Bag Lovely Canvas Beach Colours Expanding in Tote Canvas Waves Comfortable Print Handles Large in with Waves Bag 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 takeand Zips Handles with Waves Lovely Waves Summer Glitter in Shopper Bag with Tote Canvas Colours Beach Bag Large Print Straw Soft in Designer SURF Expanding Bag Canvas Summer Yellow Comfortable ('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 callBeach Summer Yellow with SURF Print Comfortable Zips Handles Bag Bag Canvas in Waves Waves Lovely Expanding Shopper Summer Straw Soft and Canvas Colours Glitter Tote in Bag with Designer Large (Api.storeItem, {token})
    return token
  } catch(error) {
    yield putWaves Soft Designer Colours and Summer Large Yellow Beach Waves with SURF Bag Lovely Canvas Comfortable Expanding with in Zips Shopper in Bag Tote Straw Summer Canvas Handles Bag Glitter Print (Waves Zips with Comfortable in and Shopper Canvas Summer Soft Large in Designer with Beach Expanding SURF Colours Lovely Waves Bag Glitter Canvas Tote Straw Bag Summer Yellow Bag Print Handles {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
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