Ejemplo de redux-saga y prueba de un método `createSaga` similar a `createReducer` de redux-duck

saga-example.js

import {
  applyMiddleware,
  createStore,
} from 'redux';

import createSagaMiddleware, {
  takeEvery,
  takeLatest,
} from 'redux-saga';
import { call, put } from 'redux-saga/effects';

// delay as promised (to simulate a async request)
function delay(ms) {
  return new Promise(resolve => {
    setTimeout(resolve, ms);
  });
}

// create middleware
const sagaMiddleware = createSagaMiddleware()

// test reducer
function reducer(state = 0, action) {
  console.log(action);
  switch (action.type) {
    case 'INCREASE': {
      return state + 1;
    }
    case 'DECREASE': {
      return state - 1;
    }
    default: {
      return state;
    }
  }
}

// our createSaga function
function createSaga(cases) {
  return function* saga() {
    yield Object
      .keys(cases)
      .map(function*(caseName) {
        const [
          caseHandler,
          caseSaga,
        ] = cases[caseName];

        if (caseHandler === 'every') {
          yield* takeEvery(caseName, caseSaga);
        }
        if (caseHandler === 'latest') {
          yield* takeLatest(caseName, caseSaga);
        }
      });
  }
}

// our custom saga using createSaga
const saga = createSaga({
  ['MESSAGE_FETCH_REQUESTED']: [
    'latest',
    function* fetchMessage(action) {
      yield call(delay, 500);
      yield put({ type: 'DECREASE' });
    },
  ],
  ['USER_FETCH_REQUESTED']: [
    'every',
    function* fetchUser(action) {
      yield call(delay, 500);
      yield put({ type: 'INCREASE' });
    },
  ],
});

// store instance
const store = createStore(
  reducer,
  applyMiddleware(sagaMiddleware)
);
// run our saga
sagaMiddleware.run(saga);
// listen changes in the store
store.subscribe(() => console.log(store.getState()));
// dispatch test actions
store.dispatch({
  type: 'USER_FETCH_REQUESTED',
});
store.dispatch({
  type: 'USER_FETCH_REQUESTED',
});
store.dispatch({
  type: 'USER_FETCH_REQUESTED',
});
store.dispatch({
  type: 'MESSAGE_FETCH_REQUESTED',
});
store.dispatch({
  type: 'MESSAGE_FETCH_REQUESTED',
});