Max concurrent instances of a NSURLSessionDataTask - ios

Question

I have a function that calls an API with NSURLSessionDataTask you can see it here:
- (void)getExplorerUrl:(void (^)(NSString *))measurement_url {
NSString *path = [NSString stringWithFormat:#"https://api.ooni.io/api/v1/measurements?report_id=%#&input=%#", self.report_id, self.url_id.url];
NSURL *url = [NSURL URLWithString:path];
NSURLSessionDataTask *downloadTask = [[NSURLSession sharedSession] dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if (!error) {
NSDictionary *dic = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];
NSArray *resultsArray = [dic objectForKey:#"results"];
if ([resultsArray count] == 0)
measurement_url(nil);
measurement_url([[resultsArray objectAtIndex:0] objectForKey:#"measurement_url"]);
}
else {
// Fail
measurement_url(nil);
NSLog(#"error : %#", error.description);
}
}];
[downloadTask resume];
}
This function uses a completion handler to return a value when the async call is finished.
Now I want a for cycle to loop many objects and call this API for every object:
for (Measurement *measurement in [Measurement measurementsWithJson]){
[measurement getExplorerUrl:^(NSString *measurement_url) {
if (measurement_url != nil){
//Do something
NSLog(#"%# measurement_url %#",measurement.Id, measurement_url);
}
else {
NSLog(#"%# measurement_url null", measurement.Id);
}
}];
}
Is there a way to set a max concurrent async calls to 10? And then execute the next call as soon as one call finishes.

Answer 1

I agree with #Rob that he can create his own configuration for URLSession. However, if this sharedSession is used across different jobs and he wanted this job to run with max concurrent async calls to 10, I would suggest to use either NSOperationQueue or dispatch_semaphore to solve this problem. Please refer to the example below to have draftily understand on these approaches
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 10;
for (int i = 1; i <= 30; i++) {
[queue addOperationWithBlock:^{
NSLog(#"[Q] %d", i);
sleep(1);
}];
}
or
dispatch_queue_t q = dispatch_queue_create("q.q", DISPATCH_QUEUE_CONCURRENT);
dispatch_semaphore_t s = dispatch_semaphore_create(10);
for (int i = 1; i <= 30; i++) {
dispatch_async(q, ^{
NSLog(#"[Q] %d", i);
sleep(1);
dispatch_semaphore_signal(s);
});
}
You can observe from the console to see the results. Basically the 2 approaches will perform maximum 10 calls at the same time, and as long as one finished, others will be enter to the execution queue.
I hope this will help you to address your problem. Can have any discussion as needed.!!!