//promise里面只有三个状态，且三个状态的转换形式有两种

//由pending转换为fulfilled，由pending转换为rejected

//Promise的构造函数参数是一个函数，函数的参数分别为resolve和reject，两者也均为一个函数

//then中是实际要执行的函数，将传递进来的值传给resolve和reject对应的参数

const PENDING = 'PENDING'

const FULFILLED = 'FULFILLED'

const REJECTED = 'REJECTED'

class YPromise {

    constructor(cb) {

        this.state = PENDING;

        this.value = null;

        //承诺完成的回调列表

        this.fulfilledCbs = [];

        //承诺被拒绝的回调列表

        this.rejectedCbs = [];

        let resolve = data => {

            setTimeout(() => {

                if (this.state !== PENDING) return;

                //更改状态

                this.state = FULFILLED;

                this.value = data;

                this.fulfilledCbs.forEach(c => {

                    this.value = c(this.value);

                })

            }, 0);

        }

        let reject = reason => {

            setTimeout(() => {

                if (this.state !== PENDING) return;

                this.state = REJECTED;

                this.reason = reason;

                this.rejectedCbs.forEach(c => {

                    this.reason = c(this.reason);

                })

            }, 0);

        }

        cb(resolve, reject);

    };

    then(onFulfilled, onRejected) {

        if (typeof onFulfilled === 'function') {

            this.fulfilledCbs.push(onFulfilled);

        }

        if (typeof onRejected === 'function') {

            this.rejectedCbs.push(onRejected);

        }

        return this;//返回整个构造函数可以继续调用then方法

    }

}

let promise = new YPromise((resolve, reject) => {

    if (4 > 1) {

        resolve('hi');

    } else {

        reject(4大于1')

    }

})

fulfilledCbs = [data => data + 'world']

promise.then(data => {

    return data + ' world';

}).then(data => {

    return data + '!';

}).then(data => {

    console.log(data);

})