Problems with dispatch iOS - ios

Question

I'm new to iOS and I have trouble understanding and applying well dispatch ... I have an application I need to query a website (api) within a for loop, the end of that cycle I need to make further inquiries in another cycle, and finally, at the end of both cycles need to switch views.
I now have this code (after much trial and error but still does not work):
dispatch_queue_t queue = dispatch_queue_create("threadServicios", DISPATCH_QUEUE_SERIAL);
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_async(queue, ^(void) {
NSLog(#"llego a buscar servicios por local");
for (NSDictionary *local in _arrLocalesTmp) {
[self getListaServiciosPorLocal:[local objectForKey:#"idLocal"]];
//this function calls another function that consumes a web service and get a json
}
procced = YES;
NSLog(#"llego a buscar profesionales por local");
for (NSDictionary *local in _arrLocalesTmp) {
[self getListaProfesionalesPorLocal:[local objectForKey:#"idLocal"]];
//this function calls another function that consumes a web service and get a json
}
procced2 = YES;
dispatch_group_leave(group);
});
dispatch_group_notify(group, dispatch_get_main_queue(),^{
NSLog(#"dispatch procced 1");
if (procced && procced2) {
[self setFormularioConsultaCompleto];
}
});
The function [self getListaServiciosPorLocal: [Local objectForKey: # "idLocal"]]; is as follows:
dispatch_async(dispatch_get_main_queue(), ^(void) {
NSURL *url = [NSURL URLWithString:urlConnection];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfiguration.timeoutIntervalForRequest = 30;
sessionConfiguration.timeoutIntervalForResource = 60;
NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];
NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
__block NSError *jsonError;
NSHTTPURLResponse *urlResponse = (NSHTTPURLResponse *) response;
if(!error) {
if(urlResponse.statusCode == 200) {
NSDictionary *response = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:&jsonError];
if(response) {
NSString *resp = [NSString stringWithFormat:#"%#", [dataResponse objectForKey:#"resp"]];
if([resp isEqualToString:#"1"]) {
_json = [dataResponse objectForKey:#"data"];
[_arrServiciosTmp addObjectsFromArray:(NSArray *)_json];
} else {
NSString *message = [dataResponse objectForKey:#"description"];
}
} else {
self.lblMensaje.text = #"Ha ocurrido un error al obtener la información, por favor, vuelva a intentarlo en unos momentos.";
}
} else {
completion(nil);
}
} else {
NSLog(#"Error en Task");
}
});
And the function [self getListaProfesionalesPorLocal: [Local objectForKey: # "idLocal"]]; is similar but obtains other information
The problem is that the app calls this function [self setFormularioConsultaCompleto]; (changing the view) but the above functions still do not quite get all the data from webservice.
Sorry for this, but I gave up, I hope can help me!
Thanks!

Answer 1

The below uses dispatch groups to hold off starting another block till the groups work has been completed.
First change your data methods to not be wrapped in dispatch_async and accept a completion block, calling that at the end of the NSURLSessionDataTasks completionHandler:
-(void)getListaServiciosPorLocal:(id)whatEver withCompletionBlock:(dispatch_block_t)block
{
NSURL *url = [NSURL URLWithString:urlConnection];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfiguration.timeoutIntervalForRequest = 30;
sessionConfiguration.timeoutIntervalForResource = 60;
NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];
NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
__block NSError *jsonError;
NSHTTPURLResponse *urlResponse = (NSHTTPURLResponse *) response;
if(!error) {
if(urlResponse.statusCode == 200) {
NSDictionary *response = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:&jsonError];
if(response) {
NSString *resp = [NSString stringWithFormat:#"%#", [dataResponse objectForKey:#"resp"]];
if([resp isEqualToString:#"1"]) {
_json = [dataResponse objectForKey:#"data"];
[_arrServiciosTmp addObjectsFromArray:(NSArray *)_json];
} else {
NSString *message = [dataResponse objectForKey:#"description"];
}
} else {
self.lblMensaje.text = #"Ha ocurrido un error al obtener la información, por favor, vuelva a intentarlo en unos momentos.";
}
} else {
completion(nil);
}
} else {
NSLog(#"Error en Task");
}
block(); // Notify completion block
});
}
Now when you call these methods:
dispatch_group_t group = dispatch_group_create();
dispatch_async(queue, ^(void) {
NSLog(#"llego a buscar servicios por local");
for (NSDictionary *local in _arrLocalesTmp) {
dispatch_group_enter(group);
[self getListaServiciosPorLocal:[local objectForKey:#"idLocal"] withCompletionBlock:^{
dispatch_group_leave(group);
}];
}
NSLog(#"llego a buscar profesionales por local");
for (NSDictionary *local in _arrLocalesTmp) {
dispatch_group_enter(group);
[self getListaProfesionalesPorLocal:[local objectForKey:#"idLocal"] withCompletionBlock:^{
dispatch_group_leave(group);
}];
}
});
dispatch_group_notify(group, dispatch_get_main_queue(),^{
[self setFormularioConsultaCompleto];
});
(Adapted from this answer)