JS魔法堂: Native Promise Only源码剖析

function Promise(executor) {
                if (typeof executor != "function") {
                        throw TypeError("Not a function");
                }

                if (this.__NPO__ !== 0) {
                        throw TypeError("Not a promise");
                }

                // instance shadowing the inherited "brand"
                // to signal an already "initialized" promise
                this.__NPO__ = 1;

                // 内部结构体
                var def = new MakeDef(this);

                this["then"] = function then(success,failure) {
                        var o = {
                                success: typeof success == "function" ? success : true,
                                failure: typeof failure == "function" ? failure : false
                        };
                        // Note: `then(..)` itself can be borrowed to be used against
                        // a different promise constructor for making the chained promise,
                        // by substituting a different `this` binding.
                        o.promise = new this.constructor(function extractChain(resolve,reject) {
                                if (typeof resolve != "function" || typeof reject != "function") {
                                        throw TypeError("Not a function");
                                }

                                o.resolve = resolve;
                                o.reject = reject;
                        });
                        // 构建Promise树
                        def.chain.push(o);
 　　　　　　　　　　　　  // 当前Promise节点状态不为pending时,发起异步执行请求事件处理函数
                        if (def.state !== 0) {
                                schedule(notify,def);
                        }

                        return o.promise;
                };
                this["catch"] = function $catch$(failure) {
                        return this.then(void 0,failure);
                };

                try {
                        // 调用工厂方法
                        executor.call(
                                void 0,
                                function publicResolve(msg){
                                        resolve.call(def,msg);
                                },
                                function publicReject(msg) {
                                        reject.call(def,msg);
                                }
                        );
                }
                catch (err) {
                        reject.call(def,err);
                }
        }

 function resolve(msg) {
                var _then, def_wrapper, self = this;

                // already triggered?
                if (self.triggered) { return; }

                self.triggered = true;

                // unwrap
                if (self.def) {
                        self = self.def;
                }

                try {
                        if (_then = isThenable(msg)) {
　　　　　　　　　   // 构造Thenable的内部状态结构体
                                def_wrapper = new MakeDefWrapper(self);
                                _then.call(msg,
                                        function $resolve$(){ resolve.apply(def_wrapper,arguments); },
                                        function $reject$(){ reject.apply(def_wrapper,arguments); }
                                );
                        }
                        else {
                                self.msg = msg;
                                self.state = 1;
                                if (self.chain.length > 0) {
                                        schedule(notify,self);
                                }
                        }
                }
                catch (err) {
                        reject.call(def_wrapper || (new MakeDefWrapper(self)),err);
                }
        }
function reject(msg) {
                var self = this;

                // already triggered?
                if (self.triggered) { return; }

                self.triggered = true;

                // unwrap
                if (self.def) {
                        self = self.def;
                }

                self.msg = msg;
                self.state = 2;
                if (self.chain.length > 0) {
                        schedule(notify,self);
                }
        } 

        function notify() {
                for (var i=0; i<this.chain.length; i++) {
                        notifyIsolated(
                                this,
                                (this.state === 1) ? this.chain[i].success : this.chain[i].failure,
                                this.chain[i]
                        );
                }
                this.chain.length = 0;
        }

        // NOTE: This is a separate function to isolate
        // the `try..catch` so that other code can be
        // optimized better
        function notifyIsolated(self,cb,chain) {
                var ret, _then;
                try {
                        if (cb === false) {
                                chain.reject(self.msg);
                        }
                        else {
                                if (cb === true) {
                                        ret = self.msg;
                                }
                                else {
                                        ret = cb.call(void 0,self.msg);
                                }

                                if (ret === chain.promise) {
                                        chain.reject(TypeError("Promise-chain cycle"));
                                }
                                else if (_then = isThenable(ret)) {
                                        _then.call(ret,chain.resolve,chain.reject);
                                }
                                else {
                                        chain.resolve(ret);
                                }
                        }
                }
                catch (err) {
                        chain.reject(err);
                }
        }

scheduling_queue = (function Queue() {
                var first // 指向队首元素
                  , last  // 指向队尾元素
                  , item;

                function Item(fn,self) {
                        this.fn = fn;
                        this.self = self;
                        this.next = void 0;
                }

                return {
                        // 元素入队
                        add: function add(fn,self) {
                                item = new Item(fn,self);
                                if (last) {
                                        last.next = item;
                                }
                                else {
                                        first = item;
                                }
                                last = item;
                                item = void 0;
                        },
                        // 清空队列 
                        drain: function drain() {
                                var f = first;
                                first = last = cycle = void 0;

                                // 从队首元素开始遍历所有队列元素
                                while (f) {
                                        f.fn.call(f.self);
                                        f = f.next;
                                }
                        }
                };
        })();

        // 安排执行状态变化事件的处理函数
        function schedule(fn,self) {
                scheduling_queue.add(fn,self);
                // 防止重复发起异步执行请求
                if (!cycle) {
                        cycle = timer(scheduling_queue.drain);
                }
        }