redux教程之源码解析2 combineReducers（分析在注释中）

eg:使用代码
//reducers/todos.js
export default function todos(state = [], action) {
  switch (action.type) {
  case 'ADD_TODO':
    return state.concat([action.text])
  default:
    return state
  }
}

//reducers/counter.js
export default function counter(state = 0, action) {
  switch (action.type) {
  case 'INCREMENT':
    return state + 1
  case 'DECREMENT':
    return state - 1
  default:
    return state
  }
}

//reducers/index.js
import { combineReducers } from 'redux'
import todos from './todos'
import counter from './counter'

export default combineReducers({
  todos,
  counter
})

//App.js
import { createStore } from 'redux'
import reducer from './reducers/index'

let store = createStore(reducer)
console.log(store.getState())
// {
//   counter: 0,
//   todos: []
// }

store.dispatch({
  type: 'ADD_TODO',
  text: 'Use Redux'
})
console.log(store.getState())
// {
//   counter: 0,
//   todos: [ 'Use Redux' ]
// }

　　

//源码分析
export default function combineReducers(reducers) {
/**获取reducers的key */
  const reducerKeys = Object.keys(reducers)
  const finalReducers = {}
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]

       ...
       
    /**将传入的reducers赋值给 finalReducers*/
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  /**获取所有的finalReducerKeys */
  const finalReducerKeys = Object.keys(finalReducers)
  
   ...

  let shapeAssertionError
  try {
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }
  
  /*返回合并后的新的reducer函数*/
  return function combination(state = {}, action) {
    if (shapeAssertionError) {
      throw shapeAssertionError
    }

       ...

    let hasChanged = false
    const nextState = {}
    /**遍历执行所有的reducers*/
    for (let i = 0; i < finalReducerKeys.length; i++) {
      const key = finalReducerKeys[i]
      const reducer = finalReducers[key]
      /**获取到之前的state */
      const previousStateForKey = state[key]
      /**生成新的state */
      const nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      /**将新的当前reducer的state赋值给总的state */
      nextState[key] = nextStateForKey
      /**判断当前reducer对应是否state发生变化 */
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    /**如果有一个state发生变化则返回新的state，否则返回旧的state */
    return hasChanged ? nextState : state
  }
}