[React] Use the useReducer Hook and Dispatch Actions to Update State (useReducer, useMemo, useEffect)

As an alternate to useState, you could also use the useReducer hook that provides state and a dispatch method for triggering actions. In this lesson, we’ll centralize logic that is spread across a web application and centralize it using the useReducer hook.

NOTE: Since hooks are still a proposal and only pre-released, it’s not currently recommended to use them in production.

useReducer:

  const [todos, dispatch] = useReducer((state, action) => {
    switch (action.type) {
      case "ADD_TODO":
        todoId.current += 1;
        return [
          ...state,
          { id: todoId.current, text: action.text, completed: false }
        ];
      case "DELETE_TODO":
        return state.filter(todo => todo.id !== action.id);
      case "TOGGLE_TODO":
        return state.map(todo =>
          todo.id === action.id ? { ...todo, completed: !todo.completed } : todo
        );
      default:
        return state;
    }
  },initialValue);

useReducer can accept second param as a function, so we can replace 'initialValue' with a function return a 'initialValue', which means we can using cache for the function if the param doesn't change, we always return the cache. To do that we can use 

useMemo:

  const [todos, dispatch] = useReducer((state, action) => {
    switch (action.type) {
      case "ADD_TODO":
        todoId.current += 1;
        return [
          ...state,
          { id: todoId.current, text: action.text, completed: false }
        ];
      case "DELETE_TODO":
        return state.filter(todo => todo.id !== action.id);
      case "TOGGLE_TODO":
        return state.map(todo =>
          todo.id === action.id ? { ...todo, completed: !todo.completed } : todo
        );
      default:
        return state;
    }
  }, useMemo(initialValue, []));

The second parameter of 'useMemo' indicates when the memorized version should change. In our case, we want it to always be the same, so passing an empty array conveys that message.

To handle side effect of action, in our case, is update localstorage, we can use useEffect:

useEffect(
  () => {
    window.localStorage.setItem("todos", JSON.stringify(todos));
  },
  [todos]
);

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