Pulling future actions

Until now we've used the helper effect takeEvery in order to spawn a new task on each incoming action. This mimics somewhat the behavior of redux-thunk: each time a Component, for example, invokes a fetchProducts Action Creator, the Action Creator will dispatch a thunk to execute the control flow.

In reality, takeEvery is just a wrapper effect for internal helper function built on top of the lower level and more powerful API. In this section we'll see a new Effect, take, which makes it possible to build complex control flow by allowing total control of the action observation process.

A simple logger

Let's take a simple example of a Saga that watches all actions dispatched to the store and logs them to the console.

Using takeEvery('*') (with the wildcard * pattern) we can catch all dispatched actions regardless of their types.

import { select, takeEvery } from 'redux-saga/effects'

function* watchAndLog() {
  yield takeEvery('*', function* logger(action) {
    const state = yield select()

    console.log('action', action)
    console.log('state after', state)
  })
}

Now let's see how to use the take Effect to implement the same flow as above

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

function* watchAndLog() {
  while (true) {
    const action = yield take('*')
    const state = yield select()

    console.log('action', action)
    console.log('state after', state)
  }
}

The take is just like call and put we saw earlier. It creates another command object that tells the middleware to wait for a specific action. The resulting behavior of the call Effect is the same as when the middleware suspends the Generator until a Promise resolves. In the take case it'll suspend the Generator until a matching action is dispatched. In the above example watchAndLog is suspended until any action is dispatched.

Note how we're running an endless loop while (true). Remember this is a Generator function, which doesn't have a run-to-completion behavior. Our Generator will block on each iteration waiting for an action to happen.

Using take has a subtle impact on how we write our code. In the case of takeEvery the invoked tasks have no control on when they'll be called. They will be invoked again and again on each matching action. They also have no control on when to stop the observation.

In the case of take the control is inverted. Instead of the actions being pushed to the handler tasks, the Saga is pulling the action by itself. It looks as if the Saga is performing a normal function call action = getNextAction() which will resolve when the action is dispatched.

This inversion of control allows us to implement control flows that are non-trivial to do with the traditional push approach.

As a simple example, suppose that in our Todo application, we want to watch user actions and show a congratulation message after the user has created his first three todos.

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

function* watchFirstThreeTodosCreation() {
  for (let i = 0; i < 3; i++) {
    const action = yield take('TODO_CREATED')
  }
  yield put({type: 'SHOW_CONGRATULATION'})
}

Instead of a while (true) we're running a for loop which will iterate only three times. After taking the first three TODO_CREATED actions, watchFirstThreeTodosCreation will cause the application to display a congratulation message then terminate. This means the Generator will be garbage collected and no more observation will take place.

Another benefit of the pull approach is that we can describe our control flow using a familiar synchronous style. For example, suppose we want to implement a login flow with two actions LOGIN and LOGOUT. Using takeEvery (or redux-thunk) we'll have to write two separate tasks (or thunks): one for LOGIN and the other for LOGOUT.

The result is that our logic is now spread in two places. In order for someone reading our code to understand it, they would have to read the source of the two handlers and make the link between the logic in both in their head. In other words, it means they would have to rebuild the model of the flow in their head by rearranging mentally the logic placed in various places of the code in the correct order.

Using the pull model we can write our flow in the same place instead of handling the same action repeatedly.

function* loginFlow() {
  while (true) {
    yield take('LOGIN')
    // ... perform the login logic
    yield take('LOGOUT')
    // ... perform the logout logic
  }
}

The loginFlow Saga more clearly conveys the expected action sequence. It knows that the LOGIN action should always be followed by a LOGOUT action and that LOGOUT is always followed by a LOGIN (a good UI should always enforce a consistent order of the actions, by hiding or disabling unexpected action).

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