Concurrency

In the basics section, we saw how to use the helper effects takeEvery and takeLatest in order to manage concurrency between Effects.

In this section we'll see how those helpers could be implemented using the low-level Effects.

takeEvery

function* takeEvery(pattern, saga, ...args) {
  const task = yield fork(function* () {
    while (true) {
      const action = yield take(pattern)
      yield fork(saga, ...args.concat(action))
    }
  })
  return task
}

takeEvery allows multiple saga tasks to be forked concurrently.

takeLatest

function* takeLatest(pattern, saga, ...args) {
  const task = yield fork(function* () {
    let lastTask
    while (true) {
      const action = yield take(pattern)
      if (lastTask)
        yield cancel(lastTask) // cancel is no-op if the task has already terminated

      lastTask = yield fork(saga, ...args.concat(action))
    }
  })
  return task
}

takeLatest doesn't allow multiple Saga tasks to be fired concurrently. As soon as it gets a new dispatched action, it cancels any previously-forked task (if still running).

takeLatest can be useful to handle AJAX requests where we want to only have the response to the latest request.