make 3 "for" loops wait for async answer. obj-c ios - objective-c

Question

I'm working on an app for iPad iOS 8, and I need to make my app wait for an answer from:
[directions calculateETAWithCompletionHandler:^(MKETAResponse *response, NSError *error) {}]
This method is inside 3 loops. I tried dispatch_semaphore_t, but the app can't continue after this line:
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
and gets stuck. I tried with dispatch_group_t and got the same result.
I guess that I'm doing something wrong, but I don't know what. I tried to search SO for similar problems and but found nothing. Can someone explain how I could accomplish this?
-(void)setTimesMissions {
for (Driver *d in self.dataList) {
for (Period *p in d.periods) {
for (Mission *m in p.missions) {
MKDirections *directions = ....
// HERE i want the for loop stop until this completionHandler finish
[directions calculateETAWithCompletionHandler:^(MKETAResponse *response, NSError *error) {
//and when he finish here continue
}];
}
}
}
}

Answer 1

Call your method in dispatch_async block.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[youClassInstance setTimesMissions];
});
And use dispatch_semaphore_wait in your loop
- (void)setTimesMissions {
Mission *home = [[Mission alloc]init];
Mission *next = [[Mission alloc]init];
for (Driver *d in self.dataList) {
home.clientLat = d.centralPointLat;
home.clientLon = d.centralPointLon;
home.clientPaddres = d.centralAddress;
for (Period *p in d.periods) {
home.times = [[NSMutableArray alloc]init];
if ([p.periodIx isEqualToString:self.thisPeriodIX]) {
for (Mission *m in p.missions) {
Mission *source = home;
Mission *destination = m ;
MKPlacemark *placemarkSource = [[MKPlacemark alloc] initWithCoordinate:CLLocationCoordinate2DMake([source.clientLat doubleValue], [source.clientLon doubleValue]) addressDictionary:nil] ;
MKMapItem *mapItemSource = [[MKMapItem alloc] initWithPlacemark:placemarkSource];
MKPlacemark *placemarkDestination = [[MKPlacemark alloc] initWithCoordinate:CLLocationCoordinate2DMake([destination.clientLat doubleValue], [destination.clientLon doubleValue])addressDictionary:nil] ;
MKMapItem *mapItemDestination = [[MKMapItem alloc] initWithPlacemark:placemarkDestination];
MKDirectionsRequest *directionsRequest = [[MKDirectionsRequest alloc] init];
[directionsRequest setSource:mapItemSource];
[directionsRequest setDestination:mapItemDestination];
directionsRequest.transportType = MKDirectionsTransportTypeAutomobile;
[directionsRequest setRequestsAlternateRoutes:NO];
MKDirections *directions = [[MKDirections alloc] initWithRequest:directionsRequest];
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
__block double timeTo;
[directions calculateETAWithCompletionHandler:^(MKETAResponse *response, NSError *error) {
if ( response.expectedTravelTime) {
timeTo = response.expectedTravelTime;
double ans = timeTo;
Time *t = [[Time alloc]init];
t.ix = m.serviceIX;
t.time = ans;
[home.times addObject:t];
}
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}
dispatch_async(dispatch_get_main_queue(), ^{
// code that should be executed on main queue
});
if (next.clientPaddres) {
home = next;
}
}
}
}
}