摘要：使用及原理分析通過關鍵字創建實例接受一個參數方法返回兩個方法可用通過在方法中通過調用使成功或調用使失敗來控制狀態中可以執行同步代碼也可以執行異步代碼原型對象上有方法供實例調用方法接受兩個參數默認為一個函數默認為一個函數當狀態為時執行用戶傳入

通過new關鍵字創建promise實例, 接受一個executor參數, executor方法返回兩個方法 resolve, reject, 可用通過在executor方法中通過調用resolve(使成功)或調用reject(使失敗),來控制promise狀態

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

executor中可以執行同步代碼也可以執行異步代碼

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

promise原型對象上有then方法供promise實例調用, then方法接受兩個參數onFulfilled, onRejected

onFulfilled默認為一個函數 data => data

onRejected默認為一個函數 err => {throw err}

當promise狀態為fulfilled時, 執行用戶傳入的onFulfilled方法

當promise狀態為rejected時, 執行用戶傳入的onRected方法

當用戶創建promise對象時采用異步,那么執行then方法時沒有調用resolve方法或reject方法,所以promise狀態依然為pending狀態,所以需要在then方法中采取發布訂閱模式,先保存then方法傳來的onFulfilled和onReje

又因為同一個promise實例可以調用多次then方法,從而傳多個onFulfilled和onRected,所以發布訂閱采用數組保存

onFulfilled和onRejected方法中可能throw Error, 所以在執行onFulfilled和onRejected時需要try catch捕獲

then方法返回一個新的Promise實現鏈式編程

統一判斷then方法傳入的onFulfilled方法和onRejected方法中return的類型(resolvePromise)

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

let p1 = p.then(data => {
    return p1   // 報類型錯誤
})

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

catch方法接受一個錯誤回調,可以用then方法實現(語法糖)

ES2018 中新增finally方法 也可以通過then方法實現(語法糖) finally要實現值得穿透, finally前如果有then方法,其返回值要穿過finally方法傳給之后的then

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

all, race 方法都接受一個promise數組

all方法要所有promise都返回才resolve一個全部是成功態的數組,只要有一個rejected就直接reject

race方法只要有一個promise resolve就直接resolve

class Promise {
    constructor(executor) {
        this.status = Promise.PENDING
        this.value = undefined
        this.reason = undefined
        // 發布訂閱的存儲器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中拋錯
        try {
            executor(this.resolve, this.reject)
        } catch (e) {
            this.reject(e)
        }
    }
    resolve(data) {
        // 如果resolve中傳入一個promise, 那么返回改promise結果
        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) => {
            // 此處用setTimeout異步才能拿到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定義默認值
        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)
                })
            }
        })
        // 返回一個新的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即執行resolve又執行 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
                    // 遞歸解析,總有一個結果then方法返回一個普通值
                    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"
}
// 延遲對象
Promise.deferred = () => {
    const defer = {}
    defer.promise = new Promise((resolve, reject) => {
        defer.resolve = resolve
        defer.reject = reject
    })
    return defer
}

module.exports = Promise