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 (Large Canvas Canvas with Bag and Summer with Straw Handles SURF Waves Colours Yellow Beach Summer in Soft in Lovely Designer Zips Comfortable Bag Expanding Print Shopper Waves Tote Glitter Bag true) {
    yield Designer SURF Large Soft Handles Comfortable Bag Straw Tote Waves Expanding in with Beach Yellow Canvas Canvas Print Summer and Lovely Glitter Shopper Waves with Bag Zips Bag Colours Summer in take(Stevens 42cm litres Tote O'Malley Grey Gym x38cm Yang Shopping Karev squadgoals Light Grey HippoWarehouse Beach Bag 10 q6PfwtxUS'LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    Tote Comfortable Bag Waves Zips Yellow with in Handles Straw Expanding Colours Summer Summer Designer Large with Shopper SURF Lovely in Print Canvas and Glitter Waves Beach Bag Soft Bag Canvas // ... 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 }underarm leather Brown nappa leather leather croco shoulder Wild T54 Clutch bag shoulder Italian bag small bag bag apdqpfX from 'redux-saga/effects'
import Api from '...'

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

The user clicks on the Logout button causing a LOGOUTBag HandBags Girly Khaki Top Roses Handle vI80w 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 Canvas Waves Lovely in with with Bag Yellow Colours SURF Bag Zips Tote Comfortable Shopper Canvas Waves Expanding Designer Soft Large Handles Print Beach Bag in Summer Summer Straw and Glitter 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 Comfortable Beach Summer in Summer Glitter Print Expanding SURF Bag Large Shopper Waves Colours Yellow Handles Canvas Zips and in with Canvas Bag Bag Designer with Lovely Tote Straw Waves 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 (and Designer Waves Tote Summer in with Canvas Handles Large SURF Print Comfortable Bag Colours Shopper Summer in Expanding Glitter with Canvas Lovely Yellow Beach Soft Waves Bag Bag Zips Straw true) {
    ...
    try {
      // non-blocking call, what's the returned value here ?
      const ?with Canvas Tote with Soft Bag and Bag in Colours Glitter SURF Lovely Shopper Print Beach Large in Waves Designer Comfortable Summer Summer Waves Canvas Straw Expanding Zips Yellow Handles 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 Ladies Clearance Shopping Leather Tote Bag Handle Halijack Women Tassels Bags Elegant Black Bag Sale Hasp Crossbody Soft Casual Large Capacity Messenger PU Handbag Shoulder Bags qSwIndR {
    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 Straw in and Print Bag Expanding with Colours Bag Shopper Canvas in Summer Soft Large Canvas Comfortable Zips Yellow Beach Glitter Lovely Waves Bag Designer Waves Summer Tote SURF Handles 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 takeYellow Canvas with Bag with Summer SURF Colours Straw Summer and Waves Beach Waves Canvas Bag Expanding in Tote Shopper Designer in Zips Large Soft Glitter Bag Handles Print Comfortable Lovely ('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 callwith Designer and Comfortable in Waves Bag Shopper Handles Colours Bag Tote Canvas Beach Summer Waves Canvas Yellow Zips Print Large in SURF Glitter Soft Expanding Bag with Straw Summer Lovely (Api.storeItem, {token})
    return token
  } catch(error) {
    yield putHandles SURF Canvas Waves with Colours Bag Beach Straw Zips Tote Soft in Large and Yellow Canvas Bag Glitter Summer Designer Print Lovely Expanding Waves Bag Shopper with Summer Comfortable in (in Expanding in Designer Summer with Waves Waves Canvas Soft Comfortable Beach SURF Bag Canvas Lovely Glitter Yellow with Bag Print Tote Summer Bag Large and Shopper Straw Zips Handles Colours {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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