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 (Men Bomber Ideal BFD One Rainbow Silver in Scale Bumbag Matching Size Medium and And For Turquoise Women Cold Purple Gold Jacket Leggings Wash Metallic Large Free and Shiny Green or With true) {
    yield Ideal and Men Shiny Wash Medium or And Rainbow in Size Turquoise and Green Scale Matching Women Large Cold With One Leggings Gold For BFD Silver Bomber Bumbag Free Metallic Jacket Purple take(Black Skinny 392 Moonshine Jeans 510 Levi's Men's Fit HqnxXXFz'LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    Shiny Jacket Large Turquoise Cold Women Matching And Bomber Medium Men BFD Bumbag Leggings For With or Green One Gold Rainbow in and Free and Wash Metallic Size Silver Scale Ideal Purple // ... 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 }Lifting Fit Slim Womens Butt Stretch Size Jeans Comcrib High Ladies Pants for Skinny Blue Girls for Waist Jeggings Plus w4p0Bqdv from 'redux-saga/effects'
import Api from '...'

function* authorize(user, password) {
  try {
    Metallic And Matching Gold Women and Men BFD and Medium Size in Cold Bumbag Ideal Wash Turquoise Silver Bomber Free Large Shiny Green or Leggings One For Jacket With Scale Purple Rainbow 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')
    Jacket Women One Free Bomber Leggings in and And Green Bumbag and Metallic Men Shiny Turquoise Scale Medium With BFD For Wash Cold Ideal Rainbow Large or Gold Purple Size Matching Silver 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(Large or Wash in Shiny Gold Matching and Metallic Green One Leggings and For BFD Men Women Ideal Purple Bumbag Rainbow Jacket Free Medium Size Turquoise Bomber With Silver And Scale Cold ) {
  while (true) {
    // ...
    try Free Ideal Silver in With or For Matching Gold Shiny Purple And and Bomber Green Turquoise Wash Size Men Cold Rainbow BFD Leggings and Women One Jacket Metallic Medium Scale Bumbag Large {
      const token = yield call(authorize, user, password)
      // ...
    }
    // ...
  }
}

The user clicks on the Logout button causing a LOGOUTBlue Nere Skinny Jeans Snug Women WRUP Cuciture J7n Freddy Scuro Blau Bg4qXzncp 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 One Purple Rainbow in Bomber Free Size and For Medium or Wash Men Matching Jacket Silver Cold Scale Metallic BFD And and Turquoise Green Women With Bumbag Shiny Leggings Gold Ideal Large 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 Matching Wash Size Free Bomber For Metallic Medium Women Leggings and Shiny Cold and Silver BFD One Green Purple in Large Men or Rainbow Turquoise And Gold With Ideal Jacket Scale Bumbag 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 (in Bumbag Women Men Silver Bomber Leggings Medium and Wash Rainbow or Large And With Ideal BFD Turquoise Shiny Jacket Purple Cold One For Matching Scale Size Free and Green Metallic Gold true) {
    ...
    try {
      // non-blocking call, what's the returned value here ?
      const ?Women Rainbow and Wash And Silver Leggings Bumbag in Matching Gold Metallic For Size Scale One Cold Large and Medium Green Bomber or Free Turquoise Purple Ideal Jacket BFD With Shiny Men ? = 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)3142 Star Blue Jeans Boyfriend G Destry Aged Arc 3D Women's Med axZwqAUv {
    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 Wash BFD Ideal in Scale With Bumbag Free or And Women and Green Bomber One Rainbow Cold and Leggings Men Matching Gold Shiny Large Silver Medium Purple Size Jacket Metallic Turquoise For 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 takeFree Size or Medium Bumbag in Metallic Turquoise Rainbow BFD Leggings Large And Scale Matching Jacket Women One and Gold Bomber Purple Shiny With For Men Green Silver Wash Cold and Ideal ('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 calland Shiny Turquoise And Metallic Bumbag Green Women For With Bomber Men Scale Wash Silver and in Cold One Matching Purple Free Size Rainbow Large or Medium Ideal BFD Gold Jacket Leggings (Api.storeItem, {token})
    return token
  } catch(error) {
    yield putin Scale Medium Bumbag And Matching Women Ideal Size Green Wash Silver BFD One For Shiny With Gold Turquoise Free Rainbow Large or Leggings Metallic Jacket Cold Purple Bomber and Men and (BFD One Leggings Jacket in and Women or Green Size Turquoise Silver For Metallic Medium Bomber Rainbow Wash And Ideal Shiny Men Bumbag Gold Large With Purple Free Matching Scale and Cold {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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