Javascript:如何检查异步操作是否仍在执行中/正在进​​行中? [英] Javascript: How to check if async operation is still pending / In progress?

问题描述

我想知道是否有可能检查Javascript中的异步操作是否仍在等待中. 因为我正在执行有关调用特定URL的数据库请求...，而db调用仍在进行中，所以我想停止所有其他传入的db-calls(这意味着，停止所有对该URL的进一步调用，以防db-请求仍待处理). 有可能吗?

I would like to know if it is somehow possible to check if an asynchronous operation in Javascript is still pending.. Because I am doing a database request on calling a specific URL... While the db call is still in progress, I want to stop any other incoming db-calls (which means, stop any further calls to that URL in case the db-request is still pending). Is that somehow possible?

因为数据库调用最多需要几分钟，并且我不想在第一个数据库调用仍在进行时启动另一个数据库调用.问题是，我以某种方式无法弄清楚如何检查该调用是否已开始并且仍在进行中，因为只有在过程完成后，响应才在.then()子句之后出现.

Because the database call takes up to minutes, and I don't want to launch another database-call while the first is still in progress.. The problem is, I somehow cannot figure out how to check if the call has started and is still in progress, because the response comes only after the .then() clause when the process has already finished.

这是我的数据库调用函数:

this is my db-call function:

const getWriteIndex = async () =>  {   
    return Promise.all(someFunction1, someFunction2...).then(...) {  

        writeMessageObject = checkDocuments(...);

        return Promise.resolve(writeMessageObject);       
       })).catch((err) => {           
           return Promise.reject(err);
       });
}

这是我的Express的URL/路由呼叫功能:

This is my URL/Route Call function with express:

router.get("/v1/...", someMiddleware(), async function(req,res,next) {    

    if (read_cached() && initialised_read) {
        res.setHeader('Content-Type', 'application/json');
        res.json(readmsg_obj);
    } else {    
        try {   
            //HOW CAN I CHECK HERE IF THE DB-CALL IS ALREADY IN PROGRESS?
                readmsg_obj.message = '';  
                getReadIndex().then((message) => {                       
                    initialised_read = true;
                    readmsg_obj = {...message};                
                    res.setHeader('Content-Type', 'application/json');
                    res.json(readmsg_obj);
                }).catch((reject) => {                  
                    logger.error(`/../... : ${reject}`);
                    initialised_read = false;
                    res.status(500).send(reject);
                });
            } catch(err)  {
                logger.error(`/v1/... : ${err}`);
                res.status(500).send(err);
            };

    } 
});

推荐答案

hmm我在这里找到了解决方法:

hmm I found a workaround here: https://ourcodeworld.com/articles/read/317/how-to-check-if-a-javascript-promise-has-been-fulfilled-rejected-or-resolved

所以我写了那个函数来检查承诺状态，但是我仍然想知道是否不可能以某种方式查询静态的promise属性以获取其实际状态;)(但是很奇怪，我没有在网络上找不到任何内容.

so I wrote that function to check for promise stati, but I am still wondering if it's not somehow possible to query for static promise properties to get their actual state ;) (but weirdly, I didn't find any on the web).

const checkPendingRequest= (promise) => {
    if (promise.isResolved) return promise;    
        // Set initial state
        var isPending = true;
        var isRejected = false;
        var isFulfilled = false;

        // Observe the promise, saving the fulfillment in a closure scope.
        var result = promise.then(
            function(v) {
                isFulfilled = true;
                isPending = false;
                return v; 
            }, 
            function(e) {
                isRejected = true;
                isPending = false;
                throw e; 
            }
        );

        result.isFulfilled = function() { return isFulfilled; };
        result.isPending = function() { return isPending; };
        result.isRejected = function() { return isRejected; };
        return result;    
}

所以我为请求修复了功能:

So I fixed my function for the request:

router.get("/v1/...", someMiddleware(), async function(req,res,next) {    

    if (read_cached() && initialised_read) {
        res.setHeader('Content-Type', 'application/json');
        res.json(readmsg_obj);
    } else {    
        try {   
           readmsg_obj.message = '';  

            if ((dbQueryPromiseRead != null) && dbQueryPromiseRead.isPending()) {
                logger.info(`Database request for Index-read is still pending!`);
                return;
            }

            dbQueryPromiseRead =  checkPendingRequest(getReadIndex());

            dbQueryPromiseRead.then((message) => {  
                initialised_read = true;
                readmsg_obj = {...message};

                res.setHeader('Content-Type', 'application/json');
                res.json(readmsg_obj);
                }).catch((reject) => {                  
                    logger.error(`/../... : ${reject}`);
                    initialised_read = false;
                    res.status(500).send(reject);
                });
            } catch(err)  {
                logger.error(`/v1/... : ${err}`);
                res.status(500).send(err);
            };

    } 
});

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