服务工作者错误：事件已经回答了

问题描述：

Uncaught (in promise) DOMException: Failed to execute 'respondWith' on 'FetchEvent': The event has already been responded to.

我知道，如果异步东西的获取函数内的推移该服务人员自动响应，但我不能完全制定出哪些位将是此代码中的罪犯：

importScripts('cache-polyfill.js'); 

self.addEventListener('fetch', function(event) { 

    var location = self.location; 

    console.log("loc", location) 

    self.clients.matchAll({includeUncontrolled: true}).then(clients => { 
    for (const client of clients) { 
     const clientUrl = new URL(client.url); 
     console.log("SO", clientUrl); 
     if(clientUrl.searchParams.get("url") != undefined && clientUrl.searchParams.get("url") != '') { 
     location = client.url; 
     } 
    } 

    console.log("loc2", location) 

    var url = new URL(location).searchParams.get('url').toString(); 

    console.log(event.request.hostname); 
    var toRequest = event.request.url; 
    console.log("Req:", toRequest); 

    var parser2 = new URL(location); 
    var parser3 = new URL(url); 

    var parser = new URL(toRequest); 

    console.log("if",parser.host,parser2.host,parser.host === parser2.host); 
    if(parser.host === parser2.host) { 
    toRequest = toRequest.replace('https://booligoosh.github.io',parser3.protocol + '//' + parser3.host); 
    console.log("ifdone",toRequest); 
    } 

    console.log("toRequest:",toRequest); 

    event.respondWith(httpGet('https://cors-anywhere.herokuapp.com/' + toRequest)); 
    }); 
}); 

function httpGet(theUrl) { 
    /*var xmlHttp = new XMLHttpRequest(); 
    xmlHttp.open("GET", theUrl, false); // false for synchronous request 
    xmlHttp.send(null); 
    return xmlHttp.responseText;*/ 
    return(fetch(theUrl)); 
}

任何帮助，将不胜感激。

答

问题是，您致电event.respondWith()的内容位于您的顶级承诺的.then()子句内，这意味着它将在顶层承诺解决后异步执行。为了获得您期望的行为，event.respondWith()需要作为fetch事件处理程序执行的一部分同步执行。

你的诺言里面的逻辑是有点难以遵循，所以我不完全相信你想实现什么，但一般你可以按照这个模式：

self.addEventListerner('fetch', event => { 
    // Perform any synchronous checks to see whether you want to respond. 
    // E.g., check the value of event.request.url. 
    if (event.request.url.includes('something')) { 
    const promiseChain = doSomethingAsync() 
     .then(() => doSomethingAsyncThatReturnsAURL()) 
     .then(someUrl => fetch(someUrl)); 
     // Instead of fetch(), you could have called caches.match(), 
     // or anything else that returns a promise for a Response. 

    // Synchronously call event.respondWith(), passing in the 
    // async promise chain. 
    event.respondWith(promiseChain); 
    } 
});

这就是大概的概念。（如果您最终以async/await代替承诺，则代码看起来更清晰。）

服务工作者错误：事件已经回答了

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