What is wrong with this approach? Why would I want to use Redux Thunk or Redux Promise, as the documentation suggests?
There is nothing wrong with this approach. It’s just inconvenient in a large application because you’ll have different components performing the same actions, you might want to debounce some actions, or keep some local state like auto-incrementing IDs close to action creators, etc. So it is just easier from the maintenance point of view to extract action creators into separate functions.
You can read my answer to “How to dispatch a Redux action with a timeout” for a more detailed walkthrough.
Middleware like Redux Thunk or Redux Promise just gives you “syntax sugar” for dispatching thunks or promises, but you don’t have to use it.
So, without any middleware, your action creator might look like
// action creator
function loadData(dispatch, userId) { // needs to dispatch, so it is first argument
return fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_DATA_FAILURE', err })
);
}
// component
componentWillMount() {
loadData(this.props.dispatch, this.props.userId); // don't forget to pass dispatch
}
But with Thunk Middleware you can write it like this:
// action creator
function loadData(userId) {
return dispatch => fetch(`http://data.com/${userId}`) // Redux Thunk handles these
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_DATA_FAILURE', err })
);
}
// component
componentWillMount() {
this.props.dispatch(loadData(this.props.userId)); // dispatch like you usually do
}
So there is no huge difference. One thing I like about the latter approach is that the component doesn’t care that the action creator is async. It just calls dispatch
normally, it can also use mapDispatchToProps
to bind such action creator with a short syntax, etc. The components don’t know how action creators are implemented, and you can switch between different async approaches (Redux Thunk, Redux Promise, Redux Saga) without changing the components. On the other hand, with the former, explicit approach, your components know exactly that a specific call is async, and needs dispatch
to be passed by some convention (for example, as a sync parameter).
Also think about how this code will change. Say we want to have a second data loading function, and to combine them in a single action creator.
With the first approach we need to be mindful of what kind of action creator we are calling:
// action creators
function loadSomeData(dispatch, userId) {
return fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
);
}
function loadOtherData(dispatch, userId) {
return fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_OTHER_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_OTHER_DATA_FAILURE', err })
);
}
function loadAllData(dispatch, userId) {
return Promise.all(
loadSomeData(dispatch, userId), // pass dispatch first: it's async
loadOtherData(dispatch, userId) // pass dispatch first: it's async
);
}
// component
componentWillMount() {
loadAllData(this.props.dispatch, this.props.userId); // pass dispatch first
}
With Redux Thunk action creators can dispatch
the result of other action creators and not even think whether those are synchronous or asynchronous:
// action creators
function loadSomeData(userId) {
return dispatch => fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
);
}
function loadOtherData(userId) {
return dispatch => fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_OTHER_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_OTHER_DATA_FAILURE', err })
);
}
function loadAllData(userId) {
return dispatch => Promise.all(
dispatch(loadSomeData(userId)), // just dispatch normally!
dispatch(loadOtherData(userId)) // just dispatch normally!
);
}
// component
componentWillMount() {
this.props.dispatch(loadAllData(this.props.userId)); // just dispatch normally!
}
With this approach, if you later want your action creators to look into current Redux state, you can just use the second getState
argument passed to the thunks without modifying the calling code at all:
function loadSomeData(userId) {
// Thanks to Redux Thunk I can use getState() here without changing callers
return (dispatch, getState) => {
if (getState().data[userId].isLoaded) {
return Promise.resolve();
}
fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
);
}
}
If you need to change it to be synchronous, you can also do this without changing any calling code:
// I can change it to be a regular action creator without touching callers
function loadSomeData(userId) {
return {
type: 'LOAD_SOME_DATA_SUCCESS',
data: localStorage.getItem('my-data')
}
}
So the benefit of using middleware like Redux Thunk or Redux Promise is that components aren’t aware of how action creators are implemented, and whether they care about Redux state, whether they are synchronous or asynchronous, and whether or not they call other action creators. The downside is a little bit of indirection, but we believe it’s worth it in real applications.
Finally, Redux Thunk and friends is just one possible approach to asynchronous requests in Redux apps. Another interesting approach is Redux Saga which lets you define long-running daemons (“sagas”) that take actions as they come, and transform or perform requests before outputting actions. This moves the logic from action creators into sagas. You might want to check it out, and later pick what suits you the most.
I searched the Redux repo for clues, and found that Action Creators were required to be pure functions in the past.
This is incorrect. The docs said this, but the docs were wrong.
Action creators were never required to be pure functions.
We fixed the docs to reflect that.
In redux-saga, the equivalent of the above example would be
export function* loginSaga() {
while(true) {
const { user, pass } = yield take(LOGIN_REQUEST)
try {
let { data } = yield call(request.post, '/login', { user, pass });
yield fork(loadUserData, data.uid);
yield put({ type: LOGIN_SUCCESS, data });
} catch(error) {
yield put({ type: LOGIN_ERROR, error });
}
}
}
export function* loadUserData(uid) {
try {
yield put({ type: USERDATA_REQUEST });
let { data } = yield call(request.get, `/users/${uid}`);
yield put({ type: USERDATA_SUCCESS, data });
} catch(error) {
yield put({ type: USERDATA_ERROR, error });
}
}
The first thing to notice is that we're calling the api functions using the form yield call(func, ...args)
. call
doesn't execute the effect, it just creates a plain object like {type: 'CALL', func, args}
. The execution is delegated to the redux-saga middleware which takes care of executing the function and resuming the generator with its result.
The main advantage is that you can test the generator outside of Redux using simple equality checks
const iterator = loginSaga()
assert.deepEqual(iterator.next().value, take(LOGIN_REQUEST))
// resume the generator with some dummy action
const mockAction = {user: '...', pass: '...'}
assert.deepEqual(
iterator.next(mockAction).value,
call(request.post, '/login', mockAction)
)
// simulate an error result
const mockError = 'invalid user/password'
assert.deepEqual(
iterator.throw(mockError).value,
put({ type: LOGIN_ERROR, error: mockError })
)
Note we're mocking the api call result by simply injecting the mocked data into the next
method of the iterator. Mocking data is way simpler than mocking functions.
The second thing to notice is the call to yield take(ACTION)
. Thunks are called by the action creator on each new action (e.g. LOGIN_REQUEST
). i.e. actions are continually pushed to thunks, and thunks have no control on when to stop handling those actions.
In redux-saga, generators pull the next action. i.e. they have control when to listen for some action, and when to not. In the above example the flow instructions are placed inside a while(true)
loop, so it'll listen for each incoming action, which somewhat mimics the thunk pushing behavior.
The pull approach allows implementing complex control flows. Suppose for example we want to add the following requirements
Handle LOGOUT user action
upon the first successful login, the server returns a token which expires in some delay stored in a expires_in
field. We'll have to refresh the authorization in the background on each expires_in
milliseconds
Take into account that when waiting for the result of api calls (either initial login or refresh) the user may logout in-between.
How would you implement that with thunks; while also providing full test coverage for the entire flow? Here is how it may look with Sagas:
function* authorize(credentials) {
const token = yield call(api.authorize, credentials)
yield put( login.success(token) )
return token
}
function* authAndRefreshTokenOnExpiry(name, password) {
let token = yield call(authorize, {name, password})
while(true) {
yield call(delay, token.expires_in)
token = yield call(authorize, {token})
}
}
function* watchAuth() {
while(true) {
try {
const {name, password} = yield take(LOGIN_REQUEST)
yield race([
take(LOGOUT),
call(authAndRefreshTokenOnExpiry, name, password)
])
// user logged out, next while iteration will wait for the
// next LOGIN_REQUEST action
} catch(error) {
yield put( login.error(error) )
}
}
}
In the above example, we're expressing our concurrency requirement using race
. If take(LOGOUT)
wins the race (i.e. user clicked on a Logout Button). The race will automatically cancel the authAndRefreshTokenOnExpiry
background task. And if the authAndRefreshTokenOnExpiry
was blocked in middle of a call(authorize, {token})
call it'll also be cancelled. Cancellation propagates downward automatically.
You can find a runnable demo of the above flow
Best Answer
This feels like an XY Problem. You shouldn't be "waiting" inside a component's lifecycle function / event handler at any point, but rather make use of the current state of the store.
If I understand correctly, this is your current flow:
What you should now add is the proper reducer/selector to handle your
CREATE_USER_ORDER_SUCCEEDED
:This
CREATE_USER_ORDER_SUCCEEDED
action should be handled by your reducer to create a new state where some "orders" property in your state is populated. This can be connected directly to your component via a selector, at which point your component will be re-rendered andthis.props.userOrders
is populated.Example:
component
reducer
If you really do need side-effects, then add those side-effects to your saga, or create a new saga that takes the SUCCESS action.