从零手撕Promise,掌握Promise的实现原理(15)之手撕Promise的race方法

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传送门

从零手撕Promise,掌握Promise的实现原理(1)之promise基本结构的实现

从零手撕Promise,掌握Promise的实现原理(2)之基础版本的promise实现

从零手撕Promise,掌握Promise的实现原理(3)之回调地狱是什么

从零手撕Promise,掌握Promise的实现原理(4)之then方法链式调用的初步实现

从零手撕Promise,掌握Promise的实现原理(5)之then方法链式调用的进阶实现 

从零手撕Promise,掌握Promise的实现原理(6)之then方法的回调为什么是异步微任务

从零手撕Promise,掌握Promise的实现原理(7)then方法链式调用之核心方法resolvePromise

从零手撕Promise,掌握Promise的实现原理(8)then方法链式调用之核心方法resolvePromise再探究

从零手撕Promise,掌握Promise的实现原理(9)then方法链式调用之核心方法resolvePromise完全体

从零手撕Promise,掌握Promise的实现原理(10)then方法完全体

从零手撕Promise,掌握Promise的实现原理(11)之测试完全体Promise是否符合PromiseA+

什么?Promise.resolve()还可以返回失败的Promise(12)

从零手撕Promise,掌握Promise的实现原理(13)之手撕Promise的finally方法

从零手撕Promise,掌握Promise的实现原理(14)之手撕Promise的all方法

回顾

经过上次的文章介绍,我们的Promise完成了all方法,现在的Promsie已经长这样了。

const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
const resolvePromise = (promise, x, resolve, reject) => {
  //*******************************************************判断x与promsie是否是同一个promise,防止进入死循环*********************//////*********
  if(x === promise) throw new TypeError('Chaining cycle detected for promise #<Promise>')
  // 1.首先判断`x`是基础类型数据,还是引用类型,基础类型的数据直接`resolve`,即可。  
  if(x !== null && /^(object|function)$/.test(typeof x)){
    let then 
    // 2. 如果是引用类型的数据,尝试获取`x`上的`then`属性(`x.then`),如果在获取属性的时候报异常则`reject`
    try{
      then = x.then
    }catch(e){
      reject(e)
    }
    //3. 判断`then`是否是函数,如果是一个函数则我们认定它为`Promise`,如果不是则`resolve`
    if(typeof then === 'function'){
      let called = false //**************************************这里加了变量*************************//////
      try{
        then.call(x, (y) => {
          if(called) return //**************************************这里加了变量*************************//////
          called = true
          resolvePromise(promise, y, resolve, reject)
        },(r) => {
          if(called) return//**************************************这里加了变量*************************//////
          reject(r)
        })
      }catch(e){
        if(called) return//**************************************这里加了变量*************************//////
        reject(e)
      }
    }else{
      resolve(x)
    }
  }else{
    //基础类型数据直接resolve
    resolve(x)
  }
}
class Promise{
  constructor(executor){

    this.state = PENDING
    this.value = undefined
    this.reason = undefined
    //存放onFulfilled
    this.onResolvedCallbacks = []
    //存放onRejected
    this.onRejectedCallbacks = []
    const resolve = (value) => {
       //如果发现value是一个Promise,我们需要调用value的then方法去递归解析
      if(value instanceof Promise){
          return value.then(resolve,reject); // 递归解析
      }
      if (this.state === PENDING) {
        this.value = value
        this.state = FULFILLED
        //promise实例状态改变后调用暂存的onFulfilled
        this.onResolvedCallbacks.forEach(fn => fn())
      }
    }

    const reject = (reason) => {
      if (this.state === PENDING) {
        this.reason = reason
        this.state = REJECTED
        //promise实例状态改变后调用的onRejected
        this.onRejectedCallbacks.forEach(fn => fn())
      }
    }
    try {
      //executor函数执行过程中出错,将会导致Promise失败
      executor(resolve,reject)
    } catch (error) {
      reject(error)
    }
  }
  then(onFulfilled, onRejected){
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err }
    let promise = new Promise((resolve, reject) => {
        
        switch(this.state){
          case FULFILLED:
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          case REJECTED:
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          default:
            this.onResolvedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
            this.onRejectedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
        }
    })
    return promise
  }
  catch(onRejected){
    return this.then(null, onRejected)
  }

  finally(cb){
    return this.then(
        (y) => {
        return Promise.resolve(cb()).then(() => y)
        },
        (r) => {
        return Promise.resolve(cb()).then(() => {throw r})
        }
    )
  }  

  static rsolve(value){
    return new Promise((resolve,reject)=>{
              resolve(value);
    })
  }
  static reject(reason){
      return new Promise((resolve,reject)=>{
          reject(reason);
      })
  }

  static all (promsies){
        return new Promise((resolve, reject) => {
            const arr = []

            let count = 0
            const processResult = (i, val) => {
                arr[i] = val
                if(++times === promises.length) resolve(arr)
            }

            for(let i = 0; i < promises.length; i++){
                let val = promises[i]
                if(typeof val.then === 'function'){
                    val.then(y => processResult(i, y), reject)
                }else{
                    processResult(i, val)
                }
            }
        })
    }

}

Promise.race实现原理

  • Promise.race方法也是Promise的静态方法,它的参数也是一个Promise数组。只不过与Promise.all方法不同的是,Promise.all的作用是所有成功才是成功,有一个失败就失败。Promise.race的作用是采用最快完成的Promise的结果作为最终的结果。

Promise.race采用最快完成的Promsie的结果作为结果,但是并不会中断其他Promise的执行,只是不采用其他的结果了,没有中断效果。 Promsie.all的特点是全成功才成功,有一个失败了,其他的Promsie也还是执行的。

  • 原理实现
static race(promises) {
    return new Promsie((resolve, reject)=>{
        for(let i = 0; i < promise.length; i++){
            let p = promises[i]
            Promise.resolve(p).then(resolve, reject)
        }
    })
}

整理

至此我们的Promsie完成了resolverejectcatchfinallyallrace这几个方法,现在的Promise已经算是比较完善了。经过一番整理现在的Promise长这样。

const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
const resolvePromise = (promise, x, resolve, reject) => {
  //*******************************************************判断x与promsie是否是同一个promise,防止进入死循环*********************//////*********
  if(x === promise) throw new TypeError('Chaining cycle detected for promise #<Promise>')
  // 1.首先判断`x`是基础类型数据,还是引用类型,基础类型的数据直接`resolve`,即可。  
  if(x !== null && /^(object|function)$/.test(typeof x)){
    let then 
    // 2. 如果是引用类型的数据,尝试获取`x`上的`then`属性(`x.then`),如果在获取属性的时候报异常则`reject`
    try{
      then = x.then
    }catch(e){
      reject(e)
    }
    //3. 判断`then`是否是函数,如果是一个函数则我们认定它为`Promise`,如果不是则`resolve`
    if(typeof then === 'function'){
      let called = false //**************************************这里加了变量*************************//////
      try{
        then.call(x, (y) => {
          if(called) return //**************************************这里加了变量*************************//////
          called = true
          resolvePromise(promise, y, resolve, reject)
        },(r) => {
          if(called) return//**************************************这里加了变量*************************//////
          reject(r)
        })
      }catch(e){
        if(called) return//**************************************这里加了变量*************************//////
        reject(e)
      }
    }else{
      resolve(x)
    }
  }else{
    //基础类型数据直接resolve
    resolve(x)
  }
}
class Promise{
  constructor(executor){

    this.state = PENDING
    this.value = undefined
    this.reason = undefined
    //存放onFulfilled
    this.onResolvedCallbacks = []
    //存放onRejected
    this.onRejectedCallbacks = []
    const resolve = (value) => {
       //如果发现value是一个Promise,我们需要调用value的then方法去递归解析
      if(value instanceof Promise){
          return value.then(resolve,reject); // 递归解析
      }
      if (this.state === PENDING) {
        this.value = value
        this.state = FULFILLED
        //promise实例状态改变后调用暂存的onFulfilled
        this.onResolvedCallbacks.forEach(fn => fn())
      }
    }

    const reject = (reason) => {
      if (this.state === PENDING) {
        this.reason = reason
        this.state = REJECTED
        //promise实例状态改变后调用的onRejected
        this.onRejectedCallbacks.forEach(fn => fn())
      }
    }
    try {
      //executor函数执行过程中出错,将会导致Promise失败
      executor(resolve,reject)
    } catch (error) {
      reject(error)
    }
  }
  then(onFulfilled, onRejected){
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v
    onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err }
    let promise = new Promise((resolve, reject) => {
        
        switch(this.state){
          case FULFILLED:
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          case REJECTED:
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })   
              break
          default:
            this.onResolvedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onFulfilled(this.value)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
            this.onRejectedCallbacks.push(() => {
              setTimeout(() => {
                try{
                  let x = onRejected(this.reason)
                  resolvePromise(promise, x, resolve, reject)
                } catch(e){
                  reject(e)
                }
              })
            })
        }
    })
    return promise
  }
  catch(onRejected){
    return this.then(null, onRejected)
  }

  finally(cb){
    return this.then(
        (y) => {
        return Promise.resolve(cb()).then(() => y)
        },
        (r) => {
        return Promise.resolve(cb()).then(() => {throw r})
        }
    )
  }  

  static rsolve(value){
    return new Promise((resolve,reject)=>{
              resolve(value);
    })
  }
  static reject(reason){
      return new Promise((resolve,reject)=>{
          reject(reason);
      })
  }

  static all (promsies){
        return new Promise((resolve, reject) => {
            const arr = []

            let count = 0
            const processResult = (i, val) => {
                arr[i] = val
                if(++times === promises.length) resolve(arr)
            }

            for(let i = 0; i < promises.length; i++){
                let val = promises[i]
                if(typeof val.then === 'function'){
                    val.then(y => processResult(i, y), reject)
                }else{
                    processResult(i, val)
                }
            }
        })
  }
  static race(promises) {
        return new Promsie((resolve, reject)=>{
            for(let i = 0; i < promise.length; i++){
                let p = promises[i]
                Promise.resolve(p).then(resolve, reject)
            }
        })
  }

}

传送门

从零手撕Promise,掌握Promise的实现原理(1)之promise基本结构的实现

从零手撕Promise,掌握Promise的实现原理(2)之基础版本的promise实现

从零手撕Promise,掌握Promise的实现原理(3)之回调地狱是什么

从零手撕Promise,掌握Promise的实现原理(4)之then方法链式调用的初步实现

从零手撕Promise,掌握Promise的实现原理(5)之then方法链式调用的进阶实现 

从零手撕Promise,掌握Promise的实现原理(6)之then方法的回调为什么是异步微任务

从零手撕Promise,掌握Promise的实现原理(7)then方法链式调用之核心方法resolvePromise

从零手撕Promise,掌握Promise的实现原理(8)then方法链式调用之核心方法resolvePromise再探究

从零手撕Promise,掌握Promise的实现原理(9)then方法链式调用之核心方法resolvePromise完全体

从零手撕Promise,掌握Promise的实现原理(10)then方法完全体

从零手撕Promise,掌握Promise的实现原理(11)之测试完全体Promise是否符合PromiseA+

什么?Promise.resolve()还可以返回失败的Promise(12)

从零手撕Promise,掌握Promise的实现原理(13)之手撕Promise的finally方法

从零手撕Promise,掌握Promise的实现原理(14)之手撕Promise的all方法