Cross Domain AJAX (getJSON) with long polling? - php

Question

I was wondering if it's possible to long poll using $.getJSON and what the proper front and back end logic would be.
I've come up with this so far but haven't tested it yet since I'm pretty sure there is wrong and/or missing logic.
Here is the JS:
function lpOnComplete(data) {
console.log(data);
if (!data.success) {
lpStart();
}
else {
alert("Works!");
}
};
function lpStart() {
$.getJSON("http://path.to.my.URL.php?jsoncall=?", function(data) {
// What happens when no data is returned
// This is more than likely since there
// is no fall back in the PHP.
lpOnComplete(data);
});
};
PHP:
$time = time();
while((time() - $time) < 30) {
// only returns data when it's new.
$data = checkCode();
// What would be the proper way to break out
// and send back $data['success'] = false
// so the JS loop can continue?
if(!empty($data)) {
echo $_GET["jsoncall"] . "(" . json_encode($data) . ")";
break;
}
usleep(25000);
}

Answer 1

From what you've got there, the Javascript is going to make multiple requests to the server and each one is going to spin up that infinite loop, and never go anywhere. I'd suggest something like: js:
$.getJSON("http://my.site/startAsyncWork.php", null, function(data){
waitUntilServerDone(data.token, function(response){
alert("done");
});
});
function waitUntilServerDone(token, doneCallback){
$.getJSON("http://my.site/checkIfWorkIsDone.php", {"token": token}, function(response){
if(response.isDone){
doneCallback(response);
}
else{
setTimeout(function(){
waitUntilServerDone(token, doneCallback);
}, 1000);
}
});
}
I don't know php, so I'm not going to write sample code for that side, but basically, startAsycWork.php makes up a random token that associates to the request. Then it spawns a thread that does all the work needed, and returns the token back to the response.
When the worker thread is done, it writes the results of the work out to a file like token.dat (or puts it in a cache or whatever).
checkIfWorkIsDone.php checks for the existence of token.dat, and returns false if it doesn't exist, or returns the contents if it does.