redux createStore 源码学习

createStore在redux类库中是篇幅最长的一个函数。它主要的作用是构建redux的store。

当createStore函数遇到enhance参数

function createStore(reducer, preloadedState, enhancer)

if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }

    return enhancer(createStore)(reducer, preloadedState)
  }

在createStore函数中，只要enhancer存在，创建store的过程将会在enhancer函数中完成。这保证了可以在enhancer函数中装饰和包装dispatch核心方法。有兴趣可看：applyMiddleware源码分析

dispatch函数：

 function dispatch(action) {
    if (!isPlainObject(action)) {
      throw new Error(
        'Actions must be plain objects. ' +
        'Use custom middleware for async actions.'
      )
    }

    if (typeof action.type === 'undefined') {
      throw new Error(
        'Actions may not have an undefined "type" property. ' +
        'Have you misspelled a constant?'
      )
    }

    if (isDispatching) {
      throw new Error('Reducers may not dispatch actions.')
    }
    //这就是为什么reducer函数处理返回数据出现错误也不报错的原因。
    //直接在dispatch函数内进行错误处理，reducer处理action的错误并不会报给开发者。
    //（真坑爹,这导致开发者很难进行错误追踪。）
    try {
      isDispatching = true
      currentState = currentReducer(currentState, action)
    } finally {
      isDispatching = false
    }

    const listeners = currentListeners = nextListeners
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      listener()
    }

    return action
  }

初始化store的state

  // When a store is created, an "INIT" action is dispatched so that every
  // reducer returns their initial state. This effectively populates
  // the initial state tree.
  dispatch({ type: ActionTypes.INIT })

源代码中的英文注释，已经说明了这一行代码的作用。