如何等待一个异步方法的回调返回值？ [英] How to wait for an asynchronous method's callback return value?

问题描述

我知道，等待一个异步方法是愚蠢的，<一个href=\"http://stackoverflow.com/questions/5010288/how-to-make-a-function-wait-until-a-callback-has-been-called-using-node-js\">one应该使用回调，而不是。但是，如果一个第三方API的力量，你将成为怎样的同步？

I know that waiting for an asynchronous method is stupid, one should use callbacks instead. But what if an third party API forces you to be synchronous?

我正在开发一个Chrome扩展，将prevent从访问网站中已有的另一个标签中打开用户。基本上，我需要先删除基于在打开的标签页的URL的请求。我想用 chrome.webRequest.onBeforeRequest 是这样的：

I'm developing a Chrome Extension that will prevent the user from visiting websites that's already open in another tab. I basically need to cancel requests based on the urls in open tabs. I want to use chrome.webRequest.onBeforeRequest like this:

function onBeforeRequest(details) {
  var websiteAlreadyOpenInOtherTab;

  // Here i want to set `websiteAlreadyOpenInOtherTab` by using the `chrome.tabs`
  // API. It's asynchronous though and that's my problem. I cant return a value
  // from an asynchronous method call.

  if (websiteAlreadyOpenInOtherTab) {
    return { cancel: true };
  }
}

chrome.webRequest.onBeforeRequest.addListener(
  onBeforeRequest,
  { urls: ['<all_urls>'], types: ['main_frame'] },
  ['blocking']);

希望你看到上面的code我的困境。我需要返回基于异步方法调用的结果的对象。是否有可能实现这一目标？

Hopefully you see my dilemma in the code above. I need to return an object based on the result of asynchronous method calls. Is it possible to achieve this?

推荐答案

的 也许的就可以解决通过跟踪标签URL的问题：

Maybe you can solve the problem by keeping track of the tab URLs?:

1. 当应用程序启动时，通过获取所有打开的标签页网址 chrome.tabs.query


2. 订阅 chrome.tabs.onUpdated 和 chrome.tabs.onRemoved 添加/删除/更新URL当他们改变。

1. When the app starts, get all open tab URLs by using chrome.tabs.query
2. Subscribe to chrome.tabs.onUpdated and chrome.tabs.onRemoved and add/remove/update the URLs when they change.

某种code例子基础上，文档：

Some kind of code example based on the documentation:

var tabUrlHandler = (function() {
   // All opened urls
   var urls = {},

   queryTabsCallback = function(allTabs) {
      allTabs && allTabs.forEach(function(tab) {
          urls[tab.id] = tab.url;
      });
   },

   updateTabCallback = function(tabId, changeinfo, tab) {
       urls[tabId] = tab.url;
   },

   removeTabCallback = function(tabId, removeinfo) {
       delete urls[tabId]; 
   };

   // init
   chrome.tabs.query({ active: true }, queryTabsCallback);
   chrome.tabs.onUpdated.addListener(updateTabCallback);
   chrome.tabs.onRemoved.addListener(removeTabCallback);

   return {
     contains: function(url) {
        for (var urlId in urls) {
           if (urls[urlId] == url) {
              return true;
           }
        }

        return false;
     }
   };

}());

现在你应该可以问 tabUrlHandler 直接在 onBeforeRequestMethod ：

Now you should be able to ask the tabUrlHandler directly in your onBeforeRequestMethod:

function onBeforeRequest(details) {
  var websiteAlreadyOpenInOtherTab = tabUrlHandler.contains(details.url);

  if (websiteAlreadyOpenInOtherTab) {
    return { cancel: true };
  }
}

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