Question

In my extension’s content script, I request data from background.js like so:

fireOnNewTopic ();  // Initial run on cold start or full reload.

window.addEventListener ("hashchange", fireOnNewTopic,  false);

function fireOnNewTopic () {
/*-- For the pages we want, location.hash will contain values
    like: "#!newtopic/{group title}"
*/
  if (location.hash) {
    var locHashParts    = location.hash.split ('/');
    if (locHashParts.length > 1  &&  locHashParts[0] == '#!newtopic') {
        var subjectStr  = '';
        var bodyStr  = '';
        switch (locHashParts[1]) {
            case 'opencomments-site-discussions':
            chrome.extension.sendMessage({name:"domain"},
                function(response)
                {
                subjectStr = response.domain;   
                });
            chrome.extension.sendMessage({name:"url"},
                function(response)
                {
                bodyStr = "URL of last page visited: " +                        response.url;   
                });
                break;
            default:
                break;
        }

        if (subjectStr && bodyStr) {
            runPayloadCode (subjectStr, bodyStr);
        }
    }
  }
}

Unfortunately, since sendMessage() runs asynchronously with the callback, at the time the code reaches runPayloadCode(), subjectStr and bodyStr are still null, since the code in background.js hasn’t completed. What’s the best way to synchronize the code so that subjectStr and bodyStr are filled in by the time runPayloadCode() is called?

Answer 1

To elaborate on what Sudarshan said about combining the two requests into one (sorry, need code so couldn’t just comment) here’s what you could do…
send

chrome.extension.sendMessage({url: true, domain:true}, function(response) {
  console.debug('The url is "'+response.url+'" and the domain is"'+response.domain+'"');
  if (repsone.url && response.domain) {
            runPayloadCode (subjectStr, "URL of last page visited: " + response.domain);
        }
});

or, if you wanted it more like your case way for some reason, maybe this is the sorta thing you’d like…

    if(location.hash) {
        var locHashParts = location.hash.split('/');
        if(locHashParts.length > 1 && locHashParts[0] == '#!newtopic') {
            var subjectStr = '';
            var bodyStr = '';
            var request = {};
            switch(locHashParts[1]) {
            case 'opencomments-site-discussions':

                request = {
                    url: true,
                    domain: true
                }
                break;
            default:
                break;
            }

            chrome.extension.sendMessage(request, function(response) {
                console.debug('The url is "' + response.url + '" and the domain is"' + response.domain + '"');
                if(repsone.url && response.domain) {
                    runPayloadCode(subjectStr, "URL of last page visited: " + response.domain);
                }
            });

        }
    }
}  

listen

chrome.extension.onMessage.addListener(
function(request, sender, sendResponse) {
    var response = {};
    if(request.url) {
        response.url = "someURL";
    }
    if(request.domain) {
        response.domain = "someDomain";
    }
    sendResponse(response);
});