12.Finally

const PENDING = 'pending'
const FUFILLED = 'fulfilled'
const REJECTED = 'rejected'

class MyPromise {
    constructor (executor) {
        try {
            executor(this.resolve, this.reject)
        } catch (error) {
            this.reject(error)
        }
    }
    status = PENDING
    value = undefined
    reason = undefined
    successCallback = []
    failCallback = []

    resolve = value => {
        if (this.status !== PENDING) return
        this.status = FUFILLED
        this.value = value
        while(this.successCallback.length) this.successCallback.shift()()
    }
    reject = reason => {
        if (this.status !== PENDING) return
        this.status = REJECTED
        this.reason = reason
        while(this.failCallback.length) this.failCallback.shift()()
    }
    then(successCallback, failCallback) {
        successCallback = successCallback || (value => value)
        failCallback = failCallback || (reason => { throw reason })
        const promise2 = new MyPromise((resolve, reject) => {
            if (this.status === FUFILLED) {
                setTimeout(() => {
                    try {
                        const successValue = successCallback(this.value)
                        resolvePromise(promise2, successValue, resolve, reject)
                    } catch (error) {
                        reject(error)
                    }
                }, 0)
            } else if (this.status === REJECTED) {
                setTimeout(() => {
                    try {
                        const failReason = failCallback(this.reason)
                        resolvePromise(promise2, failReason, resolve, reject)
                    } catch (error) {
                        reject(error)
                    }
                }, 0)
            } else {
                this.successCallback.push(() => {
                    setTimeout(() => {
                        try {
                            const successValue = successCallback(this.value)
                            resolvePromise(promise2, successValue, resolve, reject)
                        } catch (error) {
                            reject(error)
                        }
                    }, 0)
                })
                this.failCallback.push(() => {
                    setTimeout(() => {
                        try {
                            const failReason = failCallback(this.reason)
                            resolvePromise(promise2, failReason, resolve, reject)
                        } catch (error) {
                            reject(error)
                        }
                    }, 0)
                })
            }
        })
        return promise2
    }
    finally (callback) {
        // this.then 能知道当前Promise的状态
        return this.then(
            value => {
                // 将finally中的值全部处理成promise，以便处理异步的情况
                return MyPromise.resolve(callback()).then(() => value)
            },
            reason => {
                // 将finally中的值全部处理成promise，以便处理异步的情况
                return MyPromise.resolve(callback()).then(() => { throw reason })
            }
        )
    }
    static all (array) {
        const results = []
        let i = 0
        return new MyPromise((resolve, reject) => {
            function addData (key, value) {
                results[key] = value
                i++
                if (i === array.length) {
                    resolve(results)
                }
            }
            array.forEach((value, index) => {
                if (value instanceof MyPromise) {
                    value.then(value => addData(index, value), reason => reject(reason))
                } else {
                    addData(index, value)
                }
            })
        })
    }
    static resolve(value) {
        if (value instanceof MyPromise) return value
        return new MyPromise(resolve => resolve(value))
    }
}
function resolvePromise(promise2, resValue, resolve, reject) {
    if (resValue === promise2) {
        return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
    }
    if (resValue instanceof MyPromise) {
        resValue.then(value => resolve(value), reason => reject(reason))
    } else {
        resolve(resValue)
    }
}

// 验证
function a1() {
    return new MyPromise((resolve, reject) => {
        resolve('hello MyPromise')
    })
}
function a2() {
    return new MyPromise((resolve, reject) => {
        setTimeout(() => {
            resolve('hello MyPromise a2')
        }, 2000)
    })
}

a1().finally(() => {
    console.log('finally')
    return a2()
}).then(value => console.log(value))