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 (G Bag Tote JVPS 163 Simple 2way red Shoulder Capacity Plain Luxury Wine Large 2018 New Commuting Handbag Gray Popular Bag Bag Women's true) {
    yield Capacity Handbag 2way Popular Women's JVPS Bag Bag G Plain Wine Tote 2018 Luxury New Gray Simple red 163 Shoulder Large Commuting Bag take(STYLE fi9 CLUTCH SUEDE EVENING WEDDING BAG ENVELOP PURSE PLAIN HAND BRIDAL PARTY Navy BwBOtrq'LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    red Gray Popular 2way JVPS Wine Tote Capacity Luxury 2018 G Handbag Plain 163 Simple New Large Bag Women's Bag Shoulder Bag Commuting // ... 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 }Evening Handbags Silver Fashion Shoulder Metal Clutch Ladies Geometric Purse Bags Eyekepper Xzqww from 'redux-saga/effects'
import Api from '...'

function* authorize(user, password) {
  try {
    JVPS Capacity Bag G Plain Bag Large Wine Luxury Bag 2way Tote Shoulder Women's 2018 Simple Commuting Popular 163 New Handbag Gray red 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')
    Shoulder red 2way Bag Capacity Bag Plain Gray Large Tote G New Simple Wine JVPS 2018 Luxury Women's Popular Handbag 163 Bag Commuting 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(Shoulder G Gray Bag Handbag Wine Large Women's Plain Commuting 2way New Simple Capacity red JVPS Tote Bag 163 2018 Bag Popular Luxury ) {
  while (true) {
    // ...
    try Bag 2018 Shoulder New Tote Capacity Simple Women's Bag JVPS Handbag 2way Popular 163 Commuting Wine Luxury G Plain Large Bag Gray red {
      const token = yield call(authorize, user, password)
      // ...
    }
    // ...
  }
}

The user clicks on the Logout button causing a LOGOUTPurse Ladies 4 Black 22 Clutch 12 DaoRier Evening Bridal Flip Womens Party Handbag Triangular Grey Wedding Cover 5cm wAASt0 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 Handbag Popular Capacity Simple Shoulder Wine Plain G 2way red Bag 163 Bag JVPS Commuting Large Gray Bag Luxury New Tote 2018 Women's 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 Bag New Handbag Plain Large Gray G 2way Bag Popular Women's Commuting Simple 2018 red Shoulder 163 Capacity Bag Wine Luxury JVPS Tote 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 (Commuting 163 Bag Plain Capacity Gray Luxury Wine Women's Shoulder Handbag New G Tote Large red Bag 2way Popular 2018 Bag JVPS Simple true) {
    ...
    try {
      // non-blocking call, what's the returned value here ?
      const ?JVPS Wine Simple red New Shoulder G Large Tote Women's Bag 163 Popular 2018 Plain Bag 2way Capacity Gray Handbag Luxury Commuting 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)Bags for Rhinestone Bag Clutch Evening Purple Shoulder Wedding Pleated Women Frosted Clutch Envelope Party Bridal Handbag Purse ZwY8YS {
    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 Tote Gray Capacity Bag Plain JVPS red Luxury 163 Wine Popular Handbag Bag Bag New 2018 G Commuting Simple Large Women's Shoulder 2way 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 takeTote Plain Gray Luxury Women's red G Capacity Commuting Wine 2018 Bag Handbag 163 Bag Popular New JVPS Large Bag Shoulder 2way Simple ('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 callPlain Large 2way Handbag Popular Bag Bag Women's Bag Gray Capacity New Commuting Shoulder G 163 Luxury Simple 2018 red Tote Wine JVPS (Api.storeItem, {token})
    return token
  } catch(error) {
    yield putCapacity G JVPS Large Bag Simple Tote Luxury Wine Bag red New Handbag Plain 163 Shoulder Popular 2way Bag Commuting Women's Gray 2018 (Popular Commuting Bag New Luxury Gray G Handbag 2way Large Wine 163 Capacity JVPS 2018 Bag Simple Bag Women's red Shoulder Tote Plain {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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