useReducer 钩子函数

1、useReducer 钩子函数简要说明

useReducer 钩子函数的作用跟 useState 一样，即对数据进行管理，但是 useReducer 对状态数据的管理是通过配套的 dispatch 方法来管理，这也是和 useState 不同的地方。

2、useReducer 官方例子

const ACTION = {
  increment: "increment",
  decrement: "decrement",
};

const UseReducerDemo1: React.FC = () => {
  const initialState = { count: 0 };
  const [state, dispatch] = useReducer(reducer, initialState);

  function reducer(state: { count: number }, action: { type: string }) {
    switch (action.type) {
      case ACTION.increment:
        return { count: state.count + 1 };
      case ACTION.decrement:
        return { count: state.count - 1 };
      default:
        throw new Error();
    }
  }

  return (
    <div>
      <button onClick={() => dispatch({ type: ACTION.decrement })}>-</button>
      <p>{state.count}</p>
      <button onClick={() => dispatch({ type: ACTION.increment })}>+</button>
    </div>
  );
};

3、useReducer TODO 例子

const ACTIONS = {
  ADD_TODO: "add_todo",
  SWITCH_TODO: "switch_todo",
  DELETE_TODO: "delete_todo",
};

type todoItem = { id: number; name: string; isFinish: boolean };

const UseReducerDemo2: React.FC = () => {
  const [todoName, setTodoName] = useState<string>("");
  const [todos, dispatch] = useReducer(reducer, []);

  function reducer(todos: any[], action: any) {
    switch (action.type) {
      case ACTIONS.ADD_TODO:
        return [...todos, newTodo(todoName)];
      case ACTIONS.SWITCH_TODO:
        return todos.map((item: todoItem) => {
          if (action.item.id === item.id) {
            return { ...item, isFinish: !item.isFinish };
          }
          return item;
        });
      case ACTIONS.DELETE_TODO:
        return todos.filter((item: todoItem) => item.id !== action.item.id);
      default:
        return todos;
    }
  }

  function newTodo(name: string) {
    const _name = name;
    setTodoName("");
    return { id: new Date().getTime(), name: _name, isFinish: false };
  }

  function handleSumit(e: FormEvent) {
    e.preventDefault();
    dispatch({ type: ACTIONS.ADD_TODO });
  }

  return (
    <>
      <form onSubmit={handleSumit}>
        <input
          type="text"
          value={todoName}
          onChange={(e) => setTodoName(e.target.value)}
        />
        <button type="submit">提交</button>
      </form>
      <div>
        {todos.map((item: todoItem) => (
          <div key={item.id}>
            <span style={{ color: item.isFinish ? "#aaa" : "#333" }}>
              {item.name} - {item.isFinish ? "1" : "2"}
            </span>
            <button
              onClick={() => dispatch({ type: ACTIONS.SWITCH_TODO, item })}
            >
              切换
            </button>
            <button
              onClick={() => dispatch({ type: ACTIONS.DELETE_TODO, item })}
            >
              删除
            </button>
          </div>
        ))}
      </div>
    </>
  );
};