redux

代码片段

// todo.js
function todos(state = [], action) {
  switch (action.type) {
    case ADD_TODO:
      ...
      default:
        return state
  }
}

// reducer.js
import { combineReducers } from 'redux'
import todos from './todos'
import visibilityFilter from './visibilityFilter'

const rootReducer = combineReducers({
  todos,
  visibilityFilter
})

// index.js
import { createStore } from 'redux'
import reducer from './reducers'
...
const store = createStore(reducer)

function createStore(reducer, preloadedState, enhancer) {
  let currentReducer = reducer
  let currentState = preloadedState
  let currentListeners = []
  let nextListeners = currentListeners
  let isDispatching = false
  
  function getState() {
    return currentState
  }
  
  function dispatch(action) {
  ...
    try {
      isDispatching = true
      // currentReducer即为combineReducers
      currentState = currentReducer(currentState, action)
    } finally {
      isDispatching = false
    }
    ...
  }
  
  dispatch({ type: ActionTypes.INIT })
  
  return {
    dispatch,
    getState,
    ...
  }
}

// combineReducer.js
funciton combineReducer (reducers) {
  const reducerKeys = Object.keys(reducers)
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]
    ...
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  const finalReducerKeys = Object.keys(finalReducers)
  return function combination(state = {}, action) {
  
    let hasChanged = false
    const nextState = {}
    for (let i = 0; i < finalReducerKeys.length; i++) {
      const key = finalReducerKeys[i]
      const reducer = finalReducers[key]
      const previousStateForKey = state[key]
      // previousStateForKey为undefined， action为{ type: ActionTypes.INIT }。 因此每个reducer会执行default语句。
      const nextStateForKey = reducer(previousStateForKey, action)
      nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    return hasChanged ? nextState : state
  }

}

1. combineReducers函数对每个reducer做校验，返回combination函数
2. 在createStore中执行 dispatch({ type: ActionTypes.INIT })即执行combination函数。
3. combination函数遍历执行每一个reducer函数，const nextStateForKey = reducer(previousStateForKey, action)
4. 在每个reducer函数中，action为{ type: ActionTypes.INIT }， state为undefined
5. state为undefined，因此会用默认值
6. 自己定义的action中不会有与之对应的case语句，因此执行switch的default语句块中.