浅析es6-promise源码

主要逻辑:
本质上还是回调函数那一套。通过_subscribers的判断完成异步和同步的区分。通过 resolve，reject -> publish -> invokeCallback -> resolve，reject的递归和下一条then的parent是上一条的child来完成then链的流转
同步情况下函数流转： constructor -> resolver -> publish -> then -> invokeCallback 异步情况下函数流转： constructor -> then -> resolver -> publish -> invokeCallback
主要函数解析
1. constructor作用： 把resolve，reject绑定到 resolver上-
constructor(resolver) {
this[PROMISE_ID] = nextId();
this._result = this._state = undefined;
this._subscribers = [];
<!– 判断resolver是不是一个空对象 –>
if (noop !== resolver) {
typeof resolver !== ‘function’ && needsResolver();
<!– 把resolve，reject绑定到 resolver上–>
this instanceof Promise ? initializePromise(this, resolver) : needsNew();
}
}
2 then作用： 把回调函数绑定在_subscribers上，catch和finally本质是then的语法糖 _subscribers的参数是一个数组，[0]是他的child，绑定下一个then链的parent，用于publish递归调用， 第二个是resolve 回调， 第三个是reject回调
export default function then(onFulfillment, onRejection) {
const parent = this;
<!– 用于then链的返回值，下一条then就是当前parent的child –>
const child = new this.constructor(noop);

if (child[PROMISE_ID] === undefined) {
makePromise(child);
}

const { _state } = parent;
<!– 判断_state的状态，是不是PENDING –>
if (_state) {
const callback = arguments[_state – 1];
asap(() => invokeCallback(_state, child, callback, parent._result));
} else {
subscribe(parent, child, onFulfillment, onRejection);
}

return child;
}
3 publish作用： resolve，reject的触发会调用publish，publish继续调用invokeCallback，通过返回值继续调用resolve，reject，形成一个递归，完成then链的流转

function publish(promise) {
let subscribers = promise._subscribers;
let settled = promise._state;

if (subscribers.length === 0) { return; }

let child, callback, detail = promise._result;

for (let i = 0; i < subscribers.length; i += 3) {
child = subscribers[i];
callback = subscribers[i + settled];

if (child) {
invokeCallback(settled, child, callback, detail);
} else {
callback(detail);
}
}

promise._subscribers.length = 0;
}
tip: finally的callback是没有返回参数的，基于
return promise.then(value => constructor.resolve(callback()).then(() => value),
reason => constructor.resolve(callback()).then(() => { throw reason; }));