手写 Promise-技术圈

作者：Waxiangyu

来源：SegmentFault 思否社区

Promise 的用法：

let p = new Promise(function(resolve, reject){
    console.log(111)
    setTimeout(function(){
        resolve(2)
    }, 1000)
})
p.then(function(res){
    console.log('suc',res)
},function(err){
    console.log('err',err)
})

可以看到new了一个Promise,里面有一个回调函数，回调函数里有2个参数，分别又是另外2个函数。其实里面很多也都是回调函数的封装调用。

const [PENDING,RESOLVED,REJECTED]=["PENDING","RESOLVED","REJECTED"];`

class Promise1 {
    
    constructor(executor){
        state = PENDING;
        value = undefined;
        reason = undefined;
        
        resolve = (value) => {
            if(this.state === PENDING){
                this.state = RESOLVED;
                this.value = value;
            }
        }
        reject = (reason) => {
            if(this.state === PENDING){
                this.state = REJECTED;
                this.value = reason;
            }
        }
        
        try{
            executor(this.resolve,this.reject)
        }catch(e){
            reject(e)
        }
    }
 }

测试一下执行情况：

let p1 = new Promise1((resolve,reject)=>{//function:executor
    console.log(111)
    setTimeout(()=>{
        resolve(111)//this.resolve,直接传参调用
    }, 1000)
})

下面就是 then 了

class Promise1 {
    constructor(executor){
        ...
        onResolvedCallbacks=[]; //then,pending里先存起来
        onRejectedCallbacks=[];

        ...

        resolve = (value) => {
            if(this.state === PENDING){
                ...
                //executor里如果是异步会把then里存起来,然后executor走完了resolved的话,会再调then里回调函数
                this.onResolvedCallbacks.forEach(fn => fn(value))
            }
        }
        reject = (reason) => {
            if(this.state === PENDING){
                ...
               this.onRejectedCallbacks.forEach(fn => fn(reason))
            }
        }
    }
    
    then(onResolved,onRejected){
        onResolved = typeof onResolved === 'function' ? onResolved : value => value;
        onRejected = typeof onRejected === 'function' ? onRejected : reason => {
            throw reason
        };
        
        switch(this.state){
            case RESOLVED : //状态变了,就直接把结果给出去
                onResolved(this.value);
            case REJECTED :
                onRejected(this.reason);
            case PENDING : 
            //如果是pending的话,说明executor还没走完,先存起来,走完再调用
                this.onResolvedCallbacks.push(onResolved);
                this.onRejectedCallbacks.push(onRejected)
        }
    }
}

测试一下：

let p2 = new Promise1((resolve, reject) => {
    setTimeout(()=>{
        resolve(222)
    }, 1000)
})
p2.then(res => console.log(res, 'res')) //222

链式调用：


class Promise1 {
    ...
    
    
    then(onResolved,onRejected){
        ...
        //返回一个promise1
        let newPromise=new Promise1((resolve,reject)=>{
            switch(this.state){
                case RESOLVED : //状态变了,就直接把结果给出去
                    setTimeout(()=>{//then,放后面处理
                        try{
                        let reValue=onResolved(this.value);
 rePromise(newPromise,reValue,resolve,reject)
                        }catch(e){reject(e)}
                    },0)
                    
                case REJECTED :
                    setTimeout(()=>{
                        try{
                            let reValue=onRejected(this.reason);
                            rePromise(newPromise,reValue,resolve,reject)
                        }catch(e){reject(e)}
                    },0)
                    
                case PENDING : 
                //如果是pending的话,说明executor还没走完,先存起来,走完再调用
                    this.onResolvedCallbacks.push(()=>{
                        setTimeout(()=>{
                            try{
                                let reValue=onResolved(this.value);
 rePromise(newPromise,reValue,resolve,reject)
                            }catch(e){reject(e)}
                        },0)
                    });
                    this.onRejectedCallbacks.push(()=>{
                        setTimeout(()=>{
                            try{
                                let reValue=onRejected(this.reason);
                            rePromise(newPromise,reValue,resolve,reject)
                            }catch(e){reject(e)}
                        },0)
                    })
            }
        })
        return newPromise;
    }
}

//newPromise:新的promise1对象,reValue:上一个then的返回值,newPromise.resolve,newPromise.reject
function rePromise(newPromise,reValue,resolve,reject){
    //相当于把自己return出去了,let p2 = p.then(res => p2);
    if(newPromise === reValue){
        reject(new TypeError('循环了'))
    }
    if(reValue !== null && (typeof reValue === 'object' || typeof reValue === 'function')){
        try{
    //判断一下reValue是不是promise1,是个对象,还带有then函数
    //如果有then并是个函数,就调then; 
            let then = reValue.then;
            if(typeof then === 'function'){
                then.call(reValue, res=>{
                    resolve(res)
                },err=>{
                    reject(err)
                })
            }else{//then不是函数,直接用resolve返回值
                resolve(reValue)
            }
        }catch(e){
            reject(e)
        }
    }else{//只是个普通值
        resolve(reValue)
    }
}

测试一下：


p3.then(res => {
    console.log(res)
    return new Promise1((resolve, reject) => {
        resolve('p3')
    })
}).then(res => console.log(res, 'pp3'))//p3

但还有个问题

...

function rePromise(newPromise,reValue,resolve,reject){
    ...
    let called;//是否调用过,防止多次调用
    if(reValue !== null && (typeof reValue === 'object' || typeof reValue === 'function')){
        try{
   
            let then = reValue.then;
            if(typeof then === 'function'){
                then.call(reValue, res=>{
                    if(called) return;
                    called = true;
                   //promise1就继续下一个,直到then走完,变成普通的值 
                   rePromise(newPromise,res,resolve,reject)
                },err=>{
                    if(called) return;
                    called = true;
                    reject(err)
                })
            }else{//then不是函数,直接用resolve返回值
                resolve(reValue)
            }
        }catch(e){
            if(called) return;
            called = true;
            reject(e)
        }
    }else{//只是个普通值
        resolve(reValue)
    }
}

测试一下

let p1=new Promise1((resolve,reject)=>{
        resolve(111)
    })
    let p2=p1.then(res=>{console.log(res,'p1')},err=>console.log(err,'p1'));
    p2.then(res=>console.log(res,'p2'))
    .then(res=>console.log(res,'p3))

还有其它属性：


class Promise1 {
    ...
    catch(onReject){
        return this.then(null,onReject)
    }
    defer(){
        let defer={};
        defer.promise=new Promise1((resolve,reject)=>{
            defer.resolve=resolve;
            defer.reject=reject;
        })
        return defer;
    }
    deffered(){
        return this.defer;
    }
    all(promises){
        return new Promise1((resolve,reject)=>{
            let done=gen(promises.length,resolve);
            for(let i=0;i                promises[i].then(res=>{
                    done(i,res)
                },reject)
            }
        })
    }

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