SOURCE

/**
 * 动手实现一个promise
*/

class MyPromise {
    // 构造方法
    constructor(executor) {
        // 初始化值
        this.initValue()
        // 初始化this指向
        this.initBind()
        try {
            // 执行传进来的函数
            executor(this.resolve, this.reject)
        } catch (e) {
            this.reject(e)
        }
    }

    initBind() {
        // 绑定this
        this.resolve = this.resolve.bind(this)
        this.reject = this.reject.bind(this)
    }

    initValue() {
        // 初始化值
        this.PromiseResult = null // 终值
        this.PromiseState = 'pending' // 状态
        this.onFulfilledCallbacks = [] // 保存成功回调
        this.onRejectedCallbacks = [] // 保存失败回调
    }

    resolve(value) {
        if(this.PromiseState !== 'pending') {
            return
        }
        // 如果执行resolve，状态变为fulfilled
        this.PromiseState = 'fulfilled'
        // 终值为传进来的值
        this.PromiseResult = value
        // 
        while(this.onFulfilledCallbacks.length) {
            this.onFulfilledCallbacks.shift()(this.PromiseResult)
        }
    }

    reject(reason) {
        if(this.PromiseState !== 'pending') {
            return
        }
        // 如果执行reject，状态变为rejected
        this.PromiseState = 'rejected'
        // 终值为传进来的reason
        this.PromiseResult = reason
        // 
        while(this.onRejectedCallbacks.length) {
            this.onRejectedCallbacks.shift()(this.PromiseResult)
        }
    }
    
    // 非链式调用
    then_old(onFulfilled, onRejected) {
        // 接收两个回调 onFulfilled, onRejected
        
        // 参数校验，确保一定是函数
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : val => val
        onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason }

        if (this.PromiseState === 'fulfilled') {
            // 如果当前为成功状态，执行第一个回调
            onFulfilled(this.PromiseResult)
        } else if (this.PromiseState === 'rejected') {
            // 如果当前为失败状态，执行第二哥回调
            onRejected(this.PromiseResult)
        } else if (this.PromiseState === 'pending') {
            // 如果状态为待定状态，暂时保存两个回调
            this.onFulfilledCallbacks.push(onFulfilled.bind(this))
            this.onRejectedCallbacks.push(onRejected.bind(this))
        }
    }

    // 链式调用，返回一个promise实例
    then(onFulfilled, onRejected) {
        // 接收两个回调 onFulfilled, onRejected

        // 参数校验，确保一定是函数
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : val => val
        onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason }


        var thenPromise = new MyPromise((resolve, reject) => {

            const resolvePromise = cb => {
                try {
                    const x = cb(this.PromiseResult)
                    if (x === thenPromise) {
                        // 不能返回自身
                        throw new Error('不能返回自身')
                    }
                    if (x instanceof MyPromise) {
                        // 如果返回值是Promise
                        // 如果返回值是promise对象，返回值为成功，新promise就是成功
                        // 如果返回值是promise对象，返回值为失败，新promise就是失败
                        // 谁知道返回的promise是失败成功？只有then知道
                        x.then(resolve, reject)
                    } else {
                        // 非Promise就直接成功
                        resolve(x)
                    }
                } catch (err) {
                    // 处理报错
                    reject(err)
                    throw new Error(err)
                }
            }

            if (this.PromiseState === 'fulfilled') {
                // 如果当前为成功状态，执行第一个回调
                resolvePromise(onFulfilled)
            } else if (this.PromiseState === 'rejected') {
                // 如果当前为失败状态，执行第二个回调
                resolvePromise(onRejected)
            } else if (this.PromiseState === 'pending') {
                // 如果状态为待定状态，暂时保存两个回调
                // 如果状态为待定状态，暂时保存两个回调
                this.onFulfilledCallbacks.push(resolvePromise.bind(this, onFulfilled))
                this.onRejectedCallbacks.push(resolvePromise.bind(this, onRejected))
            }
        })

        // 返回这个包装的Promise
        return thenPromise

    }
}

const promise = new MyPromise((resolve, reject) => {
    resolve('success')
})
 
// 这个时候将promise定义一个p1，然后返回的时候返回p1这个promise
const p1 = promise.then(value => {
   console.log(1)
   console.log('resolve', value)
   return p1
})
 
// 运行的时候会走reject
p1.then(value => {
  console.log(2)
  console.log('resolve', value)
}, reason => {
  console.log(3)
  console.log(reason.message)
})