手写一个符合promise/A+规范的promise库

摘要：使用及原理分析通过关键字创建实例接受一个参数方法返回两个方法可用通过在方法中通过调用使成功或调用使失败来控制状态中可以执行同步代码也可以执行异步代码原型对象上有方法供实例调用方法接受两个参数默认为一个函数默认为一个函数当状态为时执行用户传入

通过new关键字创建promise实例, 接受一个executor参数, executor方法返回两个方法 resolve, reject, 可用通过在executor方法中通过调用resolve(使成功)或调用reject(使失败),来控制promise状态

let p = new Promise((resolve, reject) => {
    resolve(100)
})

executor中可以执行同步代码也可以执行异步代码

let p = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve(100)
    })
})

promise原型对象上有then方法供promise实例调用, then方法接受两个参数onFulfilled, onRejected

onFulfilled默认为一个函数 data => data

onRejected默认为一个函数 err => {throw err}

当promise状态为fulfilled时, 执行用户传入的onFulfilled方法

当promise状态为rejected时, 执行用户传入的onRected方法

当用户创建promise对象时采用异步,那么执行then方法时没有调用resolve方法或reject方法,所以promise状态依然为pending状态,所以需要在then方法中采取发布订阅模式,先保存then方法传来的onFulfilled和onReje

又因为同一个promise实例可以调用多次then方法,从而传多个onFulfilled和onRected,所以发布订阅采用数组保存

onFulfilled和onRejected方法中可能throw Error, 所以在执行onFulfilled和onRejected时需要try catch捕获

then方法返回一个新的Promise实现链式编程

统一判断then方法传入的onFulfilled方法和onRejected方法中return的类型(resolvePromise)

p.then(data => {
    console.log(data)
}, err => {
    console.log(err)
})

let p1 = p.then(data => {
    return p1   // 报类型错误
})

p.then(data => {
    return new Promise((resolve, reject) => {
        resolve(100)
    })
})

catch方法接受一个错误回调,可以用then方法实现(语法糖)

ES2018 中新增finally方法 也可以通过then方法实现(语法糖) finally要实现值得穿透, finally前如果有then方法,其返回值要穿过finally方法传给之后的then

p.then(data => {
    return 100
}).finally(() => {
    console.log("finally")
}).then(data => {
    console.log(data)  // 100
})

all, race 方法都接受一个promise数组

all方法要所有promise都返回才resolve一个全部是成功态的数组,只要有一个rejected就直接reject

race方法只要有一个promise resolve就直接resolve

class Promise {
    constructor(executor) {
        this.status = Promise.PENDING
        this.value = undefined
        this.reason = undefined
        // 发布订阅的存储器onResolvedCallbacks, onRejectedCallbacks
        this.onResolvedCallbacks = []
        this.onRejectedCallbacks = []
        this.initBind()
        this.init(executor)
    }
    initBind() {
        this.resolve = this.resolve.bind(this)
        this.reject = this.reject.bind(this)
    }
    init(executor) {
        // 防止executor中抛错
        try {
            executor(this.resolve, this.reject)
        } catch (e) {
            this.reject(e)
        }
    }
    resolve(data) {
        // 如果resolve中传入一个promise, 那么返回改promise结果
        if (data instanceof Promise) data.then(this.resolve, this.reject)
        if (this.status === Promise.PENDING) {
            this.status = Promise.FULFILLED
            this.value = data
            this.onResolvedCallbacks.forEach(fn => fn())
        }
    }
    reject(reason) {
        if (this.status === Promise.PENDING) {
            this.status = Promise.REJECTED
            this.reason = reason
            this.onRejectedCallbacks.forEach(fn => fn())
        }
    }
    then(onFulfilled, onRejected) {
        const fulfilledHandle = (resolve, reject) => {
            // 此处用setTimeout异步才能拿到promise2
            setTimeout(() => {
                try {
                    let x = onFulfilled(this.value)
                    Promise.resolvePromise(promise2, x, resolve, reject)
                } catch (e) {
                    reject(e)
                }
            })
        }
        const rejectHandle = (resolve, reject) => {
            setTimeout(() => {
                try {
                    let x = onRejected(this.reason)
                    Promise.resolvePromise(promise2, x, resolve, reject)
                } catch (e) {
                    reject(e)
                }
            })
        }
        // onFulfilled和onRejected定义默认值
        onFulfilled = typeof onFulfilled === "function" ? onFulfilled : value => value
        onRejected = typeof onRejected === "function" ? onRejected : reason => {
            throw reason
        }
        let promise2 = new Promise((resolve, reject) => {
            if (this.status === Promise.FULFILLED) {
                fulfilledHandle(resolve, reject)
            }
            if (this.status === Promise.REJECTED) {
                rejectHandle(resolve, reject)
            }
            if (this.status === Promise.PENDING) {
                this.onResolvedCallbacks.push(() => {
                    fulfilledHandle(resolve, reject)
                })
                this.onRejectedCallbacks.push(() => {
                    rejectHandle(resolve, reject)
                })
            }
        })
        // 返回一个新的promise
        return promise2
    }
    catch (onRejected) {
        return this.then(null, onRejected)
    }
    static resolve() {
        return new Promise((resolve, reject) => {
            resolve()
        })
    }
    static reject() {
        return new Promise((resolve, reject) => {
            reject()
        })
    }
    finally(callback) {
        return this.then(
            data => Promise.resolve(callback()).then(() => data),
            err => Promise.resolve(callback()).then(() => {
                throw err
            })
        )
    }
    static all(promises) {
        return new Promise((resolve, reject) => {
            let result = []
            let count = 0
            const setResult = (key, value) => {
                result[key] = value
                if (++count === promises.length) {
                    resolve(result)
                }
            }
            for (let i = 0; i < promises.length; i++) {
                let current = promises[i]
                if (Promise.isPromise(current)) {
                    current.then(data => {
                        setResult(i, data)
                    }, reject)
                } else {
                    setResult(i, current)
                }
            }
        })
    }
    static race(promises) {
        return new Promise((resolve, reject) => {
            for (let i = 0; i < promises.length; i++) {
                let current = promises[i]
                if (Promise.isPromise(current)) {
                    current.then(resolve, reject)
                } else {
                    resolve(current)
                }
            }
        })
    }
}
Promise.PENDING = "pending"
Promise.FULFILLED = "fulfilled"
Promise.REJECTED = "rejected"
Promise.resolvePromise = (promise2, x, resolve, reject) => {
    // called防止他人的promise即执行resolve又执行 reject
    let called
    if (promise2 === x) throw new TypeError("xxx")
    if (typeof x === "function" || typeof x === "object" && x !== null) {
        try {
            let then = x.then
            if (typeof then === "function") {
                then.call(x, y => {
                    if (called) return
                    called = true
                    // 递归解析,总有一个结果then方法返回一个普通值
                    Promise.resolvePromise(promise2, y, resolve, reject)
                }, e => {
                    if (called) return
                    called = true
                    reject(e)
                })
            } else {
                resolve(x)
            }
        } catch (e) {
            if (called) return
            called = true
            reject(e)
        }
    } else {
        resolve(x)
    }
}
Promise.isPromise = (obj) => {
    return typeof obj === "function" || typeof obj === "object" && obj !== null && obj.then && typeof obj.then === "function"
}
// 延迟对象
Promise.deferred = () => {
    const defer = {}
    defer.promise = new Promise((resolve, reject) => {
        defer.resolve = resolve
        defer.reject = reject
    })
    return defer
}

module.exports = Promise