{ type: 'LIKE_ARTICLE', articleId: 42 }
{ type: 'FETCH_USER_SUCCESS', response: { id: 3, name: 'Mary' } }
{ type: 'ADD_TODO', text: '金融前端.' }

export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose,
  __DO_NOT_USE__ActionTypes
}

export default function createStore(reducer, preloadedState, enhancer) {
...
}

/**
 * Reads the state tree managed by the store.
 *
 * @returns {any} The current state tree of your application.
 */
function getState() {
  if (isDispatching) {
    throw new Error(
      'You may not call store.getState() while the reducer is executing. ' +
        'The reducer has already received the state as an argument. ' +
        'Pass it down from the top reducer instead of reading it from the store.'
    )
  }
  return currentState
}

/**
 * Turns an object whose values are different reducer functions, into a single
 * reducer function. It will call every child reducer, and gather their results
 * into a single state object, whose keys correspond to the keys of the passed
 * reducer functions.
 */
export default function combineReducers(reducers) {
  const reducerKeys = Object.keys(reducers)
     ...
  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]
      const nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      nextState[key] = nextStateForKey
      //判断state是否发生改变
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    //根据是否发生改变，来决定返回新的state还是老的state
    return hasChanged ? nextState : state
  }
}

/**
 * Turns an object whose values are action creators, into an object with the
 * same keys, but with every function wrapped into a `dispatch` call so they
 * may be invoked directly. This is just a convenience method, as you can call
 * `store.dispatch(MyActionCreators.doSomething())` yourself just fine.
 */
function bindActionCreator(actionCreator, dispatch) {
  return function() {
    return dispatch(actionCreator.apply(this, arguments))
  }
}
 
export default function bindActionCreators(actionCreators, dispatch) {
  if (typeof actionCreators === 'function') {
    return bindActionCreator(actionCreators, dispatch)
  }
    ...
    ...
  const keys = Object.keys(actionCreators)
  const boundActionCreators = {}
  for (let i = 0; i < keys.length; i++) {
    const key = keys[i]
    const actionCreator = actionCreators[key]
    if (typeof actionCreator === 'function') {
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    }
  }
  return boundActionCreators
}

/**
 * Composes single-argument functions from right to left. The rightmost
 * function can take multiple arguments as it provides the signature for
 * the resulting composite function.
 *
 * @param {...Function} funcs The functions to compose.
 * @returns {Function} A function obtained by composing the argument functions
 * from right to left. For example, compose(f, g, h) is identical to doing
 * (...args) => f(g(h(...args))).
 */
 
export default function compose(...funcs) {
  if (funcs.length === 0) {
    return arg => arg
  }
 
  if (funcs.length === 1) {
    return funcs[0]
  }
 
  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

/**
 * Creates a store enhancer that applies middleware to the dispatch method
 * of the Redux store. This is handy for a variety of tasks, such as expressing
 * asynchronous actions in a concise manner, or logging every action payload.
 */
export default function applyMiddleware(...middlewares) {
  return createStore => (...args) => {
    const store = createStore(...args)
    ...
    ...
    return {
      ...store,
      dispatch
    }
  }
}

const store = createStore(reducer);
const next = store.dispatch;
 
// 重写store.dispatch
store.dispatch = (action) => {
    try {
        console.log('action:', action);
        console.log('current state:', store.getState());
        next(action);
        console.log('next state', store.getState());
    } catch (error){
        console.error('msg:', error);
    }
}

let state = {
    count : 1
}
//修改之前
console.log (state.count);
//修改count的值为2
state.count = 2；
//修改之后
console.log (state.count);

let state = {
    count: 1
};
//订阅
function subscribe (listener) {
    listeners.push(listener);
}
function changeState(count) {
    state.count = count;
    for (let i = 0; i < listeners.length; i++) {
        const listener = listeners[i];
        listener();//监听
    }
}

const createStore = function (initState) {
    let state = initState;
    //订阅
    function subscribe (listener) {
        listeners.push(listener);
    }
    function changeState (count) {
        state.count = count;
        for (let i = 0; i < listeners.length; i++) {
            const listener = listeners[i];
            listener();//通知
        }
    }
    function getState () {
        return state;
    }
    return {
        subscribe,
        changeState,
        getState
    }
}

let initState = {
    counter: {
        count : 0
    },
    info: {
        name: '',
        description: ''
    }
}
let store = createStore(initState);
//输出count
store.subscribe(()=>{
    let state = store.getState();
    console.log(state.counter.count);
});
//输出info
store.subscribe(()=>{
    let state = store.getState();
    console.log(`${state.info.name}:${state.info.description}`);
});

//自增
store.changeState({
    count: store.getState().count + 1
});
//自减
store.changeState({
    count: store.getState().count - 1
});
//随便改成什么
store.changeState({
    count: 金融
});

function plan (state, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return {
        ...state,
        count: state.count + 1
      }
    case 'DECREMENT':
      return {
        ...state,
        count: state.count - 1
      }
    default:
      return state
  }
}
let store = createStore(plan, initState);
//自增
store.changeState({
    type： 'INCREMENT'
});
//自减
store.changeState({
    type： 'DECREMENT'
});