create-async-saga

For those that use both redux-toolkit and redux-saga, it is like createAsyncThunk, but using generator instead of asynchronous callback.

createAsyncSaga accepts a Redux action type string and generator function. It generates lifecycle action types based on the action type prefix that you pass in, and returns an object that allow to

• dispatch an action to trigger your function
• update your state using a slice

Getting started

!! You must have redux-toolkit & redux-saga installed and configured

You want to load a user using an api:

1interface User {
2  userId: number,
3  name: string;
4}
5
6const fetchUserApi = (userId: number): User => ({ userId, name: "foo" });

Call createAsyncSaga and provide a generator that call the API:

1export const fetchUser = createAsyncSaga(
2  'users/fetch',
3  function* (userId: number) {
4    return yield call(fetchUserApi, userId)
5  }
6); 

fetchUser object contains action and it's type, the action to execute the saga. It has to be watched and dispatched

• to watch:

1export function* watchFetchUser() {
2  yield takeEvery(fetchUser.actionType, fetchUser.asyncSaga);
3}
4// Don't forget to add this to the saga middleware: sagaMiddleware.run(watchFetchUser) 

• to dispatch:

1dispatch(fetchUser.action(1))

• you can watch you own action:

1export function* watchFetchUser() {
2  yield takeEvery(myOwnAction, fetchUser.asyncSaga);
3}

CreateAsyncSaga dispatch a pending action when the saga start, a fulfilled action when the saga is sucessfull, and a rejected action when saga failed. Like with createAsyncThunk, you write your own reducer logic using those actions:

1createSlice({
2  name: 'user',
3  initialState: { ... },
4  reducers: { ... },
5  extraReducers(builder) {
6    builder.addCase(
7      fetchUser.pending,
8      (state) => { 
9        state.fetchStatus = "pending"; 
10      }
11    );
12    builder.addCase(
13      fetchUser.fulfilled,
14      (state, action) => {
15        state.fetchStatus = "fulfilled";
16        state.user = action.payload;
17      }
18    );
19    builder.addCase(
20      fetchUser.rejected,
21      (state) => {
22        state.fetchStatus = "rejected"; 
23        // action.payload contains error information
24      }
25    );
26  }
27});

---

API

Release 2.0.0 includes requestId in actions. This implies a modification in actions creator interface, which is a breaking change. This impact only code that creates action by itself (which is probably test including full saga if any).

1function createAsyncSaga<Returned, Arg>(typePrefix: string, payloadCreator: PayloadCreator<Returned, Arg>, options?: AsyncSagaOptions<Arg>)
2
3type PayloadCreator<Returned, Arg> = (arg: Arg) => Generator<any, Returned, any>;
4
5interface AsyncSagaOptions<Arg> {
6  condition?: Condition<Arg>,
7  dispatchConditionRejection?: boolean,
8}
9
10type Condition<Arg> = (arg: Arg) => Generator<any, boolean, any>;

• Returned is the return type of the playlod generator and Arg is the arguments type of the payload creator.

• typePrefix: prefix for generated Redux action type constants.

  When typePrefix is users/fetch actions types are users/fetch, users/fetch/pending, users/fetch/fulfilled and users/fetch/rejected.

• payloadCreator: a generator that returns the payload to be put in fulfilled action. It can yield any effects it needs.

  1  function* (userId: string) {
  2    const user: User = yield call(fetchUser, userId); // yield an effect
  3    return user; // returns the result
  4  },

  A payloadCreator can receive an arg. If you need to pass in multiple values, pass them together in an object when you dispatch the action.

• options: an object with the following optional fields:

  • condition: a generator that can be used to skip execution of the payload creator. It can yield any effects but it returns a boolean. It receives arg payload creator as argument.
  • dispatchConditionRejection: by default if condition returns false, nothing is dispatched. When dispatchConditionRejection is set to true a "rejected" action is dispatched with meta.condition set to true.

Actions

Actions are made of action itself, with is dispatched to trigger the async execution, and of createAsyncThunk's lifecycle actions: pending, fulfilled and rejected.

• action:

  1{
  2  type: `typePrefix`, // the typePrefix is given to createAsyncSaga
  3  payload: arg, // object that contains the paylod creator arguments 
  4  meta: {
  5    requestId: string // a uniqe id generatted for each execution
  6  }
  7}

• pending:

  1{
  2  type: `typePrefix`/pending, // the typePrefix is given to createAsyncSaga
  3  payload: undefined, // object that contains the paylod creator arguments 
  4  meta: {
  5    arg, // object that contains the paylod creator arguments
  6    requestId // id generated 
  7  }
  8}

• fulfilled:

  1{
  2  type: `typePrefix`/fulfilled, // the typePrefix is given to createAsyncSaga
  3  payload: returned, // payload creator returned value 
  4  meta: {
  5    arg, // object that contains the paylod creator arguments
  6    requestId // id generated 
  7  }
  8}

• rejected:

  1{
  2  type: `typePrefix`/rejected, // the typePrefix is given to createAsyncSaga
  3  payload: error, // a SerializedError made from what the payload creator thrown 
  4  meta: {
  5    arg, // object that contains the paylod creator arguments
  6    requestId // id generated
  7    condition // true if rejected has been dispatched because condition has return false
  8  }
  9}

---

Testing

One can test the payload generator only or the saga returned by createAsyncSaga, although the full saga requires more stuff. In both case, test can be written using any Saga tests receipes.

A generator payload test (using redux-saga-test-plan):

1it('Test payload generator', () => {
2  const user: User = {
3    id: '123',
4    name: "John doe",
5  };
6  testSaga(fetchUser, user.id)
7    .next()
8    .call(fetchUser, user.id)
9    .next(user)
10    .returns(user);
11});

A full saga test (still using redux-saga-test-plan):

1const fetchUserSaga = createAsyncSaga(
2  'users/fetch',
3  fetchUser
4);
5
6it('Test full saga', () => {
7  const user: User = {
8    id: '123',
9    name: "John doe",
10  };
11  const action = fetchUserSaga.action(user.id);
12  const requestId = action.meta.requestId;
13  testSaga(fetchUserSaga.asyncSaga, action)
14    .next()
15    .put(fetchUserSaga.pending(user.id, requestId))
16    .next()
17    .call(fetchUser, user.id)
18    .next(user)
19    .put(fetchUserSaga.fulfilled(user.id, requestId, user))
20    .next()
21    .isDone();
22});

---

Known limitations & issues

This lib is new, and is missing some advanced functionalities (they will be added in the coming releases):

• dispatchConditionRejection is missing in options Fixed in release 2.1.0
• When the async saga is cancelled a rejected action with meta.aborted===true should be thrown
• requestId is missing in meta Fixed in release 2.0.0
• error may not be a real SerializedError Fixed in release 1.0.2