Promise原理及实现

Promise标准

1. Promise 规范有很多，如Promise/A，Promise/B，Promise/D 以及 Promise/A 的升级版 Promise/A+。ES6 中采用了 Promise/A+ 规范。
   英文版规范: Promises/A+规范
   中文版规范: Promises/A+规范(中文)
2. Promise标准解读
   (1) 一个promise的当前状态只能是pending、fulfilled和rejected三种之一。状态改变只能是pending到fulfilled或者pending到rejected。状态改变不可逆。
   (2) promise的then方法接收两个可选参数，表示该promise状态改变时的回调(promise.then(onFulfilled, onRejected))。then方法返回一个promise。then 方法可以被同一个 promise 调用多次。

实现Promise

Promise标准仅描述了then方法的行为，未对catch、all、race方法进行描述，也未规范如何创建一个Promise对象。ES6中Promise提供了以下API。
(1) 构造函数

function Promise(resolver) {}

(2) 原型方法

Promise.prototype.then = function() {}
Promise.prototype.catch = function() {}

(3) 静态方法

Promise.resolve = function() {}
Promise.reject = function() {}
Promise.all = function() {}
Promise.race = function() {}

构造函数

1. Promise/A+标准并没有指定如何构造一个Promise对象，我们以ES6原生Promise模块里通过构造函数创建Promise对象的方式实现Promise构造函数。ES6中通过构造函数创建Promise对象的简单用法如下：

const promise = new Promise((resolve) => {
    setTimeout(()=> {
        resolve(1);
    }, 2000);
});
promise.then(a=> alert(a));
promise.then(a => alert(a+1));

构造函数用法总结：
(1) 构造函数接收一个executor立即执行函数
(2) executor立即执行函数接收一个resolve函数
(3) promise对象的then方法绑定状态变为fulfilled时的回调
(4) resolve函数被调用时会触发then方法中的回调

2. 构造函数的简单实现

function Promise(executor) {
    var self = this;
    self.status = 'pending'; //promise当前的状态
    self.data = undefined; //promise的值
    self.onResolvedCallback = [];
    //promise状态变为resolve时的回调函数集，可能有多个
   function resolve(value) {
       if(self.status === 'pending') {
           self.status = 'resolved';
           self.data = value;
           for(var i = 0; i < self.onResolvedCallback.length; i++) {
               self.onResolvedCallback[i](value);
           }
       }
   }
   executor(resolve);
};
Promise.prototype.then = function (resolve) {
    this.onResolvedCallback.push(resolve);
};

3. executor自执行函数接收的第二个参数为reject函数且reject函数在promise对象状态变为rejected时或executor抛出异常时触发。

function Promise(executor) {
    var self = this;
    self.status = 'pending'; //promise当前的状态
    self.data = undefined; //promise的值
    self.onResolvedCallback = [];
    //promise状态变为resolve时的回调函数集，可能有多个
    self.onRejectedCallback = [];
    //promise状态变为reject时的回调函数集，可能有多个
   function resolve(value) {
       if(self.status === 'pending') {
           self.status = 'resolved';
           self.data = value;
           for(var i = 0; i < self.onResolvedCallback.length; i++) {
               self.onResolvedCallback[i](value);
           }
       }
   }

   function reject(reason) {
        if(self.status === 'pending') {
            self.status = 'rejected';
            self.data = reason;
            for(var i = 0; i < self.onRejectedCallback.length; i++) {
                self.onRejectedCallback[i](reason);
            }
        }
   }

   try {
       executor(resolve, reject);
   } catch (e){
       reject(e);
   }
};
Promise.prototype.then = function (onResolve, onReject) {
    this.onResolvedCallback.push(onResolve);
    this.onRejectedCallback.push(onReject);
};

总结：①executor函数作为实参在创建Promise对象时传入Promise构造函数。②resolve和reject函数作为实参传入executor函数。③value作为实参传入resolve和reject函数。

4. 如果executor自执行函数中的resolve函数立即触发时，发现Promise失效，例如：

const promise = new Promise((resolve) => {
    resolve(1);
});
promise.then((a) => alert(a));

解决办法：需要将promise的resolve和reject异步执行。

function resolve(value) {
    setTimeout(function () {
        if(self.status === 'pending') {
            self.status = 'resolved';
            self.data = value;
            for(var i = 0; i < self.onResolvedCallback.length; i++) {
                self.onResolvedCallback[i](value);
            }
        }
    })
}

function reject(reason) {
    setTimeout(function () {
        if(self.status === 'pending') {
            self.status = 'rejected';
            self.data = reason;
            for(var i = 0; i < self.onRejectedCallback.length; i++) {
                self.onRejectedCallback[i](reason);
            }
        }
    })
}

then方法

1. 在构造函数中的示例中，then方法并没有返回一个promise对象，而 Promise/A+ 规范中规定then方法用来注册promise对象状态改变时的回调，且返回一个新的promise对象。
   (1) then方法返回一个新的promise对象。
   (2) executor自执行函数中的resolve参数调用时执行then方法的第一个回调函数onResolved。
   (3) executor自执行函数中的reject参数调用时执行then方法的第二个回调函数onRejected。

Promise.prototype.then = function (onResolved, onRejected) {
    var self = this;
    var promise2;
    onResolved = typeof onResolved === 'function' 
                ? onResolved 
                : function (value) {return value};
    onRejected = typeof onRejected === 'function' 
                ? onRejected 
                : function (reason) {throw reason};
    //promise对象当前状态为resolved
    if(self.status === 'resolved') {
        return promise2 = new Promise(function (resolve, reject) {
            try {
                //调用onResolve回调函数
                var x = onResolved(self.data);
                //如果onResolve回调函数返回值为一个promise对象
                if(x instanceof  Promise) {
                    //将它的结果作为promise2的结果
                    x.then(resolve, reject);
                } else {
                    resolve(x);//执行promise2的onResolve回调
           }        
            } catch (e) {
                reject(e); //执行promise2的onReject回调
            }
        })
    }
    //promise对象当前状态为rejected
    if(self.status === 'rejected') {
        return promise2 = new Promise(function (resolve, reject) {
            try {
                var x = onRejected(self.data);
                if (x instanceof Promise) {
                    x.then(resolve, reject)
                } else {
                    resolve(x)
                }
            } catch (e) {
                reject(e)
            }
        })
    }
    //promise对象当前状态为pending
    //此时并不能确定调用onResolved还是onRejected，需要等当前Promise状态确定。
    //所以需要将callBack放入promise1的回调数组中
    if(self.status === 'pending') {
        return promise2 = new Promise(function (resolve, reject) {
            self.onResolvedCallback.push(function (value) {
                try {
                    var x = onResolved(self.data);
                    if (x instanceof Promise) {
                        x.then(resolve, reject);
                    } else {
                        resolve(x);
          }
                } catch (e) {
                    reject(e);
                }
            })
            self.onRejectedCallback.push(function(reason) {
                try {
                    var x = onRejected(self.data);
                    if (x instanceof Promise) {
                        x.then(resolve, reject)
                    } else {
                        resolve(x);
                    }
                } catch (e) {
                    reject(e)
                }
            })
        })
    }
};

2. 参照Promise/A+标准对promise进行改写

function resolvePromise(promise2, x, resolve, reject) {
    var then
    var thenCalledOrThrow = false

    if (promise2 === x) {
        return reject(new TypeError('Chaining cycle detected for promise!'))
    }
    if (x instanceof Promise) {
        if (x.status === 'pending') { //because x could resolved by a Promise Object
            x.then(function(v) {
                resolvePromise(promise2, v, resolve, reject)
            }, reject)
        } else { //but if it is resolved, it will never resolved by a Promise Object but a static value;
            x.then(resolve, reject)
        }
        return
    }

    if ((x !== null) && ((typeof x === 'object') || (typeof x === 'function'))) {
        try {
            then = x.then //because x.then could be a getter
            if (typeof then === 'function') {
                then.call(x, function rs(y) {
                    if (thenCalledOrThrow) return
                    thenCalledOrThrow = true
                    return resolvePromise(promise2, y, resolve, reject)
                }, function rj(r) {
                    if (thenCalledOrThrow) return
                    thenCalledOrThrow = true
                    return reject(r)
                })
            } else {
                resolve(x)
            }
        } catch (e) {
            if (thenCalledOrThrow) return
            thenCalledOrThrow = true
            return reject(e)
        }
    } else {
        resolve(x)
    }
}

Promise.prototype.then = function(onResolved, onRejected) {
    var self = this
    var promise2
    onResolved = typeof onResolved === 'function' ? onResolved : function(v) {
        return v
    }
    onRejected = typeof onRejected === 'function' ? onRejected : function(r) {
        throw r
    }

    if (self.status === 'resolved') {
        return promise2 = new Promise(function(resolve, reject) {
            setTimeout(function() { // 异步执行onResolved
                try {
                    var x = onResolved(self.data)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (reason) {
                    reject(reason)
                }
            })
        })
    }

    if (self.status === 'rejected') {
        return promise2 = new Promise(function(resolve, reject) {
            setTimeout(function() { // 异步执行onRejected
                try {
                    var x = onRejected(self.data)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (reason) {
                    reject(reason)
                }
            })
        })
    }

    if (self.status === 'pending') {
        // 这里之所以没有异步执行，是因为这些函数必然会被resolve或reject调用，而resolve或reject函数里的内容已是异步执行，构造函数里的定义
        return promise2 = new Promise(function(resolve, reject) {
            self.onResolvedCallback.push(function(value) {
                try {
                    var x = onResolved(value)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (r) {
                    reject(r)
                }
            })

            self.onRejectedCallback.push(function(reason) {
                try {
                    var x = onRejected(reason)
                    resolvePromise(promise2, x, resolve, reject)
                } catch (r) {
                    reject(r)
                }
            })
        })
    }
}

完整代码

var Promise = (function() {
    function Promise(resolver) {
        if (typeof resolver !== 'function') { //resolver必须是函数
            throw new TypeError('Promise resolver ' + resolver + ' is not a function')
        }
        if (!(this instanceof Promise)) return new Promise(resolver)

        var self = this //保存this
        self.callbacks = [] //保存onResolve和onReject函数集合
        self.status = 'pending' //当前状态

        function resolve(value) {
            setTimeout(function() { //异步调用
                if (self.status !== 'pending') {
                    return
                }
                self.status = 'resolved' //修改状态
                self.data = value

                for (var i = 0; i < self.callbacks.length; i++) {
                    self.callbacks[i].onResolved(value)
                }
            })
        }

        function reject(reason) {
            setTimeout(function(){ //异步调用
                if (self.status !== 'pending') {
                    return
                }
                self.status = 'rejected' //修改状态
                self.data = reason

                for (var i = 0; i < self.callbacks.length; i++) {
                    self.callbacks[i].onRejected(reason)
                }
            })
        }

        try{
            resolver(resolve, reject) //执行resolver函数
        } catch(e) {
            reject(e)
        }
    }

    function resolvePromise(promise, x, resolve, reject) {
        var then
        var thenCalledOrThrow = false

        if (promise === x) {
            return reject(new TypeError('Chaining cycle detected for promise!'))
        }

        if ((x !== null) && ((typeof x === 'object') || (typeof x === 'function'))) {
            try {
                then = x.then
                if (typeof then === 'function') {
                    then.call(x, function rs(y) {
                        if (thenCalledOrThrow) return
                        thenCalledOrThrow = true
                        return resolvePromise(promise, y, resolve, reject)
                    }, function rj(r) {
                        if (thenCalledOrThrow) return
                        thenCalledOrThrow = true
                        return reject(r)
                    })
                } else {
                    return resolve(x)
                }
            } catch(e) {
                if (thenCalledOrThrow) return
                thenCalledOrThrow = true
                return reject(e)
            }
        } else {
            return resolve(x)
        }
    }

    Promise.prototype.then = function(onResolved, onRejected) {
        //健壮性处理，处理点击穿透
        onResolved = typeof onResolved === 'function' ? onResolved : function(v){return v}
        onRejected = typeof onRejected === 'function' ? onRejected : function(r){throw r}
        var self = this
        var promise2

        //promise状态为resolved
        if (self.status === 'resolved') {
            return promise2 = new Promise(function(resolve, reject) {
                setTimeout(function() {
                    try {
                        //调用then方法的onResolved回调
                        var x = onResolved(self.data)
                        //根据x的值修改promise2的状态
                        resolvePromise(promise2, x, resolve, reject)
                    } catch(e) {
                        //promise2状态变为rejected
                        return reject(e)
                    }
                })
            })
        }

        //promise状态为rejected
        if (self.status === 'rejected') {
            return promise2 = new Promise(function(resolve, reject) {
                setTimeout(function() {
                    try {
                        //调用then方法的onReject回调
                        var x = onRejected(self.data)
                        //根据x的值修改promise2的状态
                        resolvePromise(promise2, x, resolve, reject)
                    } catch(e) {
                        //promise2状态变为rejected
                        return reject(e)
                    }
                })
            })
        }

        //promise状态为pending
        //需要等待promise的状态改变
        if (self.status === 'pending') {
            return promise2 = new Promise(function(resolve, reject) {
                self.callbacks.push({
                    onResolved: function(value) {
                        try {
                            //调用then方法的onResolved回调
                            var x = onResolved(value)
                            //根据x的值修改promise2的状态
                            resolvePromise(promise2, x, resolve, reject)
                        } catch(e) {
                            //promise2状态变为rejected
                            return reject(e)
                        }
                    },
                    onRejected: function(reason) {
                        try {
                            //调用then方法的onResolved回调
                            var x = onRejected(reason)
                            //根据x的值修改promise2的状态
                            resolvePromise(promise2, x, resolve, reject)
                        } catch(e) {
                            //promise2状态变为rejected
                            return reject(e)
                        }
                    }
                })
            })
        }
    }

    //获取当前Promise传递的值
    Promise.prototype.valueOf = function() {
        return this.data
    }

    //由then方法实现catch方法
    Promise.prototype.catch = function(onRejected) {
        return this.then(null, onRejected)
    }

    //finally方法
    Promise.prototype.finally = function(fn) {
        return this.then(function(v){
            setTimeout(fn)
            return v
        }, function(r){
            setTimeout(fn)
            throw r
        })
    }

    Promise.prototype.spread = function(fn, onRejected) {
        return this.then(function(values) {
            return fn.apply(null, values)
        }, onRejected)
    }

    Promise.prototype.inject = function(fn, onRejected) {
        return this.then(function(v) {
            return fn.apply(null, fn.toString().match(/\((.*?)\)/)[1].split(',').map(function(key){
                return v[key];
            }))
        }, onRejected)
    }

    Promise.prototype.delay = function(duration) {
        return this.then(function(value) {
            return new Promise(function(resolve, reject) {
                setTimeout(function() {
                    resolve(value)
                }, duration)
            })
        }, function(reason) {
            return new Promise(function(resolve, reject) {
                setTimeout(function() {
                    reject(reason)
                }, duration)
            })
        })
    }

    Promise.all = function(promises) {
        return new Promise(function(resolve, reject) {
            var resolvedCounter = 0
            var promiseNum = promises.length
            var resolvedValues = new Array(promiseNum)
            for (var i = 0; i < promiseNum; i++) {
                (function(i) {
                    Promise.resolve(promises[i]).then(function(value) {
                        resolvedCounter++
                        resolvedValues[i] = value
                        if (resolvedCounter == promiseNum) {
                            return resolve(resolvedValues)
                        }
                    }, function(reason) {
                        return reject(reason)
                    })
                })(i)
            }
        })
    }

    Promise.race = function(promises) {
        return new Promise(function(resolve, reject) {
            for (var i = 0; i < promises.length; i++) {
                Promise.resolve(promises[i]).then(function(value) {
                    return resolve(value)
                }, function(reason) {
                    return reject(reason)
                })
            }
        })
    }

    Promise.resolve = function(value) {
        var promise = new Promise(function(resolve, reject) {
            resolvePromise(promise, value, resolve, reject)
        })
        return promise
    }

    Promise.reject = function(reason) {
        return new Promise(function(resolve, reject) {
            reject(reason)
        })
    }

    Promise.fcall = function(fn){
        // 虽然fn可以接收到上一层then里传来的参数，但是其实是undefined，所以跟没有是一样的，因为resolve没参数啊
        return Promise.resolve().then(fn)
    }

    Promise.done = Promise.stop = function(){
        return new Promise(function(){})
    }

    Promise.deferred = Promise.defer = function() {
        var dfd = {}
        dfd.promise = new Promise(function(resolve, reject) {
            dfd.resolve = resolve
            dfd.reject = reject
        })
        return dfd
    }

    try { // CommonJS compliance
        module.exports = Promise
    } catch(e) {}

    return Promise
})()

参考

Promise实现分析
Promise完整代码