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Troubleshooting.md

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Troubleshooting

App freezes after adding a saga

Make sure that you yield the effects from the generator function.

Consider this example:

import { take } from 'redux-saga/effects'

function* logActions() {
  while (true) {
    const action = take() // wrong
    console.log(action)
  }
}

It will put the application into an infinite loop because take() only creates a description of the effect. Unless you yield it for the middleware to execute, the while loop will behave like a regular while loop, and freeze your application.

Adding yield will pause the generator and return control to the Redux Saga middleware which will execute the effect. In case of take(), Redux Saga will wait for the next action matching the pattern, and only then will resume the generator.

To fix the example above, simply yield the effect returned by take():

import { take } from 'redux-saga/effects'

function* logActions() {
  while (true) {
    const action = yield take() // correct
    console.log(action)
  }
}

My Saga is missing dispatched actions

Make sure the Saga is not blocked on some effect. When a Saga is waiting for an Effect to resolve, it will not be able to take dispatched actions until the Effect is resolved.

For example, consider this example

function* watchRequestActions() {
  while (true) {
    const {url, params} = yield take('REQUEST')
    yield call(handleRequestAction, url, params) // The Saga will block here
  }
}

function* handleRequestAction(url, params) {
  const response = yield call(someRemoteApi, url, params)
  yield put(someAction(response))
}

When watchRequestActions performs yield call(handleRequestAction, url, params), it'll wait for handleRequestAction until it terminates an returns before continuing on the next yield take. For example suppose we have this sequence of events

UI                     watchRequestActions             handleRequestAction
-----------------------------------------------------------------------------
.......................take('REQUEST').......................................
dispatch(REQUEST)......call(handleRequestAction).......call(someRemoteApi)... Wait server resp.
.............................................................................
.............................................................................
dispatch(REQUEST)............................................................ Action missed!!
.............................................................................
.............................................................................
.......................................................put(someAction).......
.......................take('REQUEST')....................................... saga is resumed

As illustrated above, when a Saga is blocked on a blocking call then it will miss all the actions dispatched in-between.

To avoid blocking the Saga, you can use a non-blocking call using fork instead of call

function* watchRequestActions() {
  while (true) {
    const {url, params} = yield take('REQUEST')
    yield fork(handleRequestAction, url, params) // The Saga will resume immediately
  }
}