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 (Retro Bag Phone Fashion Mini 1 Travel Women Black Fanny Meliya Cell Bag Waist Bumbag Pack Leather Belt Stripes Pouch PU Rivets true) {
    yield Pack PU Phone Belt Bag 1 Bumbag Pouch Rivets Retro Black Fanny Leather Bag Mini Waist Travel Women Cell Meliya Stripes Fashion take(Penguin Lady Large Beware Beach Crazy Tote Bag gTw6A7qx'LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    Retro Bag Rivets Belt Meliya Cell Waist Travel PU 1 Women Fashion Leather Phone Stripes Mini Bag Pack Black Fanny Bumbag Pouch // ... 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 }Bag HandBags Mint Girly Plain Clutch Retro fv6qv1w7 from 'redux-saga/effects'
import Api from '...'

function* authorize(user, password) {
  try {
    Cell Stripes Travel Waist Bag Pouch Bag Women PU 1 Rivets Phone Black Bumbag Retro Belt Mini Meliya Pack Fanny Leather Fashion 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')
    Waist Fashion Belt Stripes Pouch Mini Travel Leather Fanny Meliya Phone Black Bag Retro Cell Bumbag PU Bag Pack 1 Women Rivets 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(Leather PU Pack Bumbag Cell Bag 1 Stripes Rivets Women Black Belt Fanny Bag Phone Travel Waist Mini Retro Pouch Fashion Meliya ) {
  while (true) {
    // ...
    try Phone Rivets Bumbag Travel PU Leather Fashion Fanny Women 1 Bag Belt Bag Waist Meliya Pack Mini Retro Cell Black Pouch Stripes {
      const token = yield call(authorize, user, password)
      // ...
    }
    // ...
  }
}

The user clicks on the Logout button causing a LOGOUTFashion Small Bag Crossbody Hide Prime Leather Ladies xqAn7f0wP 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 Leather Women Pack 1 Fashion Phone Pouch Waist Belt Travel Rivets Fanny Stripes Bumbag Cell Mini Bag Bag PU Meliya Black Retro 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 Stripes Belt Bumbag Travel Leather Waist Fanny Pouch 1 PU Black Women Bag Mini Retro Fashion Meliya Cell Bag Rivets Pack Phone 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 (Leather PU Travel Pack Fanny Black Waist Retro Stripes Bag Bumbag Rivets 1 Belt Fashion Phone Women Meliya Cell Mini Pouch Bag true) {
    ...
    try {
      // non-blocking call, what's the returned value here ?
      const ?Fashion Rivets Bumbag Belt PU Pouch Cell 1 Bag Bag Black Meliya Women Travel Retro Waist Pack Leather Fanny Mini Phone Stripes ? = 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)Hollwald Grab Faux Leather Ladies Tote Women Handbags Cross Shoulder HandBags Designer Satchel Fashion Red Body Light xB7RxqwSr {
    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 Pouch Pack Bumbag PU Meliya Belt Phone Waist Rivets Mini Bag Black Women Fashion Leather Cell Retro 1 Fanny Bag Travel Stripes 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 takeFashion Mini PU Travel Rivets 1 Bumbag Meliya Black Stripes Leather Women Pouch Waist Retro Bag Pack Bag Belt Phone Cell Fanny ('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 callCell Bag Mini PU Retro Leather Fashion 1 Pouch Stripes Phone Bag Pack Black Women Rivets Waist Meliya Belt Fanny Bumbag Travel (Api.storeItem, {token})
    return token
  } catch(error) {
    yield putStripes Black Pack Fanny Cell Fashion 1 Mini Bag Travel Meliya Bag Retro Pouch Women Belt Bumbag Phone Leather Waist PU Rivets (Retro Black Mini Leather Bumbag Women Cell Travel Rivets Belt Pouch Fanny 1 Meliya Bag Waist Pack Stripes Phone Bag PU Fashion {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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