promise源码分析

初级入门以及如何使用请看 阮一峰promise对象讲解

先上一坨代码,后面我们要基于这坨代码来实现自定义PRomise

原始方法

setTimeout(function(){
    VAR a=100;
    console.LOG(a);
    setTimeout(function () {
        var b=200;
        console.log(b)
        setTimeout(function () {
            var c=300;
            console.log(c)
        }, 1000);
    }, 1000);
},1000);   

promise实现

 new Promise(function (resolve, reject) {
    setTimeout(function () {
        var a=100;
        resolve(a);
    }, 1000);
}).then(function (res) {
    console.log(res);
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            var b=200;
            resolve(b);
        }, 1000);
    })
}).then(function (res) {
    console.log(res);
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            var c=300
            resolve(c);
        }, 1000);
    })
}).then(function (res) {
        console.log(res);
    }
    )

如何让你的promise能有此魔力

• 让a,b,c的值能在then里面的回调接收到
• 在连续调用异步，如何确保异步函数的执行顺序

如何让异步的value在thenable函数中拿到

• 将resolve/reject函数和onfulfiled/onrejected放入同一个对象(promise对象)里面,resolve/reject的时候将value设置this.value=xxx。onfulfiled/onrejected执行的时候呢,onfulfiled(this.value)即可

如何处理链式的promise且保证顺序

• 每个promise后面链一个对象该对象包含onfulfiled,onfulfiled,子promise三个属性.

当父promise 状态改变完毕,执行完相应的onfulfiled/onfulfiled的时候呢，拿到子promise,在等待这个子promise状态改变，在执行相应的onfulfiled/onfulfiled。依次循环直到当前promise没有子promise

最终代码(内涵注释)

            /*
            我们要满足状态只能三种状态：PENDING,FULFILLED,REJECTED三种状态，且状态只能由PENDING=>FULFILLED,或者PENDING=>REJECTED
            */
            var PENDING = 0;
            var FULFILLED = 1;
            var REJECTED = 2;
            /*
            value状态为执行成功事件的入参，deferreds保存着状态改变之后的需要处理的函数以及promise子节点，构造函数里面应该包含这三个属性的初始化
             */
            function Promise(callback) {
                this.status = PENDING;
                this.value = null;
                this.defferd = [];
                setTimeout(callback.bind(this, this.resolve.bind(this), this.reject.bind(this)), 0);
            }
            
            Promise.prototype = {
                constructor: Promise,
                //触发改变promise状态到FULFILLED
                resolve: function (result) {
                    this.status = FULFILLED;
                    this.value = result;
                    this.done();
                },
                //触发改变promise状态到REJECTED
                reject: function (error) {
                    this.status = REJECTED;
                    this.value = error;
                },
                //处理defferd
                handle: function (fn) {
                    if (!fn) {
                        return;
                    }
                    var value = this.value;
                    var t = this.status;
                    var p;
                    if (t == PENDING) {
                         this.defferd.push(fn);
                    } else {
                        if (t == FULFILLED && typeof fn.onfulfiled == 'function') {
                            p = fn.onfulfiled(value);
                        }
                        if (t == REJECTED && typeof fn.onrejected == 'function') {
                            p = fn.onrejected(value);
                        }
                    var promise = fn.promise;
                    if (promise) {
                        if (p && p.constructor == Promise) {
                            p.defferd = promise.defferd;
                        } else {
                            p = this;
                            p.defferd = promise.defferd;
                            this.done();
                        }
                    }
                    }
                },
                //触发promise defferd里面需要执行的函数
                done: function () {
                    var status = this.status;
                    if (status == PENDING) {
                        return;
                    }
                    var defferd = this.defferd;
                    for (var i = 0; i < defferd.length; i++) {
                        this.handle(defferd[i]);
                    }
                },
                /*储存then函数里面的事件
                返回promise对象
                defferd函数当前promise对象里面
                */
                then: function (success, fail) {
                   var o = {
                        onfulfiled: success,
                        onrejected: fail
                    };
                    var status = this.status;
                    o.promise = new this.constructor(function () {
            
                    });
                    if (status == PENDING) {
                        this.defferd.push(o);
                    } else if (status == FULFILLED || status == REJECTED) {
                        this.handle(o);
                    }
                    return o.promise;
                }
            };

在附上一张手绘的流程图

参考资料

• v8官方实现

源码地址

• github