构建并列Reducer

重组前

//todos Reducer
const todos = (state = [], action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return [
        ...state,
        {
          id: action.id,
          text: action.text,
          completed: false
        }
      ];
    case 'TOGGLE_TODO':
      return state.map(todo => {
        if (todo.id !== action.id) {
          return todo;
        }

        return {
          ...todo,
          completed: !todo.completed
        };
      });
    default:
      return state;
  }
};

重组后

//todo Reducer
const todo = (state, action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return {
          id: action.id,
          text: action.text,
          completed: false
        };
    case 'TOGGLE_TODO':
      if (state.id !== action.id) {
        return state;
      }

      return {
        ...state,
        completed: !state.completed
      };
    default:
      return state;
  }
};

//todos Reducer
const todos = (state = [], action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return [
        ...state,
        todo(undefined, action)
      ];
    case 'TOGGLE_TODO':
      return state.map(t => todo(t, action));
    default:
      return state;
  }
};

// 最终代码： http://jsbin.com/sabamob/edit?js,console