combineReducer

对于将多个reducer合并为一个的做法，redux为我们提供了一个方便的API叫combineReducer ，同样，为了搞懂它到底是怎么运作的，我们可以尝试实现它。

在之前的例子里，我们注意到最后返回的reducer的key与子reducer同名，所以函数内部过程应该是：

1. 向combineReducer传入一个对象，假设我们有子reducer A，B， 那么我们这样传入： combineReducer({A, B}) ，它的对等形式就是： combineReducer({A: A, B: B})，这是ES6的语法糖 - Destructing Assignment.
2. 获取key数组
3. 以这个key数组为数据，获取一个合并后的reducer 函数

/**
* combineReducer => merge all reducer function into one
* @param [Object] reducers object, { reducerVaribale, ... }
* @return [function] mixed reducer function
*/

const combineReducer = (reducers) => {
  return (state = {}, action) => { // return a reduce function
    return Object.keys(reducers).reduce(
      (nextState, key) => {  // When gave an initial value(here is {}), nextState will be {}
        nextState[key] = reducers[key](state[key], action)
        // This is actually very similar to 'todos: todos(state.todos, action)' in todos reducer
        return nextState
      }, 
      {} //initial value => offered a container for mixed state
    )
  }
}

OK，我们把其中最麻烦的一部分拿出来分析：

Object.keys(reducers).reduce(
 (nextState, key) => {
    nextState[key] = reducers[key](state[key], action)
    return nextState
    },
  {} //initial value => offered a container for mixed state
)

我们假设{ReducerA, ReducerB}是参数reducers：

1. Object.keys(reducers) => ['ReducerA', 'ReducerB']
2. ['ReducerA', 'ReducerB'].reduce()
3. 我们把里面的函数提取出来： function (accumulator, currentValue) { }
4. 如果我们不给定reduce函数一个initialValue， 那么初始状态下accumulator会是第一个key，显然，我们的操作对象不是key，而是一个对象，这个对象将包含所有的state.
5. 给定initialValue： {} ，接上面第三步，函数体：accumulator[currentValue] = reducers[currentValue](state[currentValue], action)
6. 手动跑reduce结果：
   1. Initial State 对应关系: accumulator = {} , currentValue = 'ReducerA'
      1. {}['ReducerA'] = ReducerA(state['ReducerA]，action) 此时state['ReducerA] 为undefined ，前面解释过，ReducerA 自带默认state ，会正常返回值.
   2. Second State 对应关系: accumulator = { 'ReducerA' : ReducerA返回的state } , currentValue = 'ReducerB

现在清晰了吗？ 最后的返回结果便会是{ 'ReducerA' : ReducerA返回的state， 'ReducerB' : ReducerB返回的state }

测试一下我们的函数是否可用：

It works !