Synchronised Ajax requests with JQuery in a loop - ajax

Question

I have the following situation: I need to make synchronized Ajax requests within a loop and display the returned result after each iteration in a div-element (appended on top with the previous results at the bottom). The response time of each request can be different but the order in which it should be displayed should be the same as issued. Here is an example with 3 requests. Lets say request "A" needs 3 seconds, "B" needs 1 second and "C" needs 5 seconds. The order I want to display the result is A, B, C as the requests were issued but the code I use shows the results in B, A, C.
Here is the code (JQuery Ajax request):
$(document).ready(function(){
var json = document.getElementById("hCategories").value;
var categories = eval( '(' + json + ')' );
for(curCat in categories) {
curCatKey = categories[curCat]['grKey'];
$.ajax({
type: "POST",
url: "get_results.php",
data: "category=" + escape(curCatKey) +
"&search=" + escape($("#hQuery").val()),
timeout: 8000,
async: false,
success: function(data) {
$("#content").append(data);
}
});
});
I thought it would work with "async:false" but then it waits until every Ajax call is finished and presents the results after the loop. I hope some of you can point out some different solutions, I am pretty much stuck.
Thanks in advance,
Cheers Chris
EDIT: Thanks for all the possible solutions, I will try these now one by one and come back with that one that fits my problem.

Answer 1

I have two solution proposals for this problem:
Populate generated divs
You could generate divs with ids in the loop and populate them when the request finishes:
$(document).ready(function() {
var json = document.getElementById("hCategories").value;
var categories = eval('(' + json + ')');
for (curCat in categories) {
(function(curCat) {
var curCatKey = categories[curCat]['grKey'];
$('#content').append('<div id="category-"' + escape(curCat) + '/>');
$.ajax({
type: "POST",
url: "get_results.php",
data: "category=" + escape(curCatKey) + "&search=" + escape($("#hQuery").val()),
success: function(data) {
$("#category-" + escape(curCat)).html(data);
}
});
})(curCat);
}
});
Or use a deferred
You can store jqXHR objects in an array and use a deferred to call the success functions in order, when all calls have finished.
$(document).ready(function() {
var json = document.getElementById("hCategories").value;
var categories = eval('(' + json + ')');
var requests;
for (curCat in categories) {
var curCatKey = categories[curCat]['grKey'];
requests.push($.ajax({
type: "POST",
url: "get_results.php",
data: "category=" + escape(curCatKey) + "&search=" + escape($("#hQuery").val())
}));
}
$.when.apply(requests).done(function() {
for (i in requests) {
requests[i].success(function(data) {
$("#content").append(data);
});
}
});
});
The first method has the advantage that it populates the containers continuously. I have not tested either of these function, but the logic should work the way I described it.

Answer 2

This would do the trick
var results = [];
var idx = 0;
for(curCat in categories) {
curCatKey = categories[curCat]['grKey'];
(function( i ) {
$.ajax({
type: "POST",
url: "get_results.php",
data: "category=" + escape(curCatKey) +
"&search=" + escape($("#hQuery").val()),
timeout: 8000,
async: false,
success: function(data) {
results[i] = data;
if (i == idx - 1) { // last one
for (var j=0; j < results.length; j++) {
$("#content").append(results[j]);
}
}
}
});
})(idx++);

Answer 3

I think something like this is what you're looking for. Might need some tweaking, I'm a little rusty on Deferred. Read up on it though, mighty powerful
deferred = $.Deferred()
for(curCat in categories) {
deferred.pipe(
function(resp){
postData = {} // set up your data...
return $.post("get_results.php", {data: postData, timeout: 8000})
.done(function(content){ $("#content").append(content) })
})
)
}
// Trigger the whole chain of requests
deferred.resolve()