dispatch_group_notify does not wait for one dispatch_group_async - ios

Question

Two concurrent background tasks need to patch two separate arrays which need to be merged in a dispatch_group_notify block.The problem is that.the first block is exceeded but the dispatch_group_notify is exceeded without waiting for the execution of the second background task.
The only different between them is that the first one make a local search and the second one makes a remote call to a web service.Any clue why the second one is jumped over ?
Edit: I also tried the approach mentioned in https://stackoverflow.com/a/19580584/859742 using dispatch_barrier_async but still same.
dispatch_group_t taskGroup = dispatch_group_create();
dispatch_queue_t mainQueue = dispatch_get_main_queue();
__block NSArray *localAddresses;
__block NSArray *remoteAddresses;
//Get local array in the background
dispatch_group_async(taskGroup, mainQueue, ^{
//localAddresses is set
});
//get remote array from server
dispatch_group_async(taskGroup, mainQueue, ^{
[[MDAddressManager instance] searchForPlacesContainingText:query
location:alocation
completion:^(NSArray* addresses, MDError *error){
//remoteAddresses is set
});
//Merge two arrays
dispatch_group_notify(taskGroup, mainQueue, ^{
//remoteAddresses and local addresses are merged
});
And the remote search method looks like this
- (void)searchForPlacesContainingText:(NSString *)searchText
location:(CLLocation *)alocation
completion:(MDAddressManagerBlock)completionBlock
{
NSDictionary *parameters = [[NSMutableDictionary alloc] init];
[parameters setValue:searchText forKey:#"input"];
[[MDHTTPClient sharedHTTPClient] getPath:#"v1/remotePlaces.json"
parameters:parameters
success:^(AFHTTPRequestOperation *operation, id dict) {
if ([MDHTTPClient isResponseValid:dict])
{
completionBlock(returnArray, nil);
}
else
{
completionBlock(nil, nil);
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
EDLog(#"%#", error);
completionBlock(nil, [MDError errorAFNetworking:error]);
}];
}

Answer 1

This is because your getPath method runs asynchronously. You need it to not leave the group until that completion block runs. So rather than than doing dispatch_group_async, you should manually dispatch_group_enter and dispatch_group_leave.
You can change your code from:
dispatch_group_async(taskGroup, mainQueue, ^{
[[MDAddressManager instance] searchForPlacesContainingText:query
location:alocation
completion:^(NSArray* addresses, MDError *error){
//remoteAddresses is set
}];
});
To:
dispatch_group_enter(taskGroup);
[[MDAddressManager instance] searchForPlacesContainingText:query
location:alocation
completion:^(NSArray* addresses, MDError *error){
//remoteAddresses is set
dispatch_group_leave(taskGroup);
});
That will ensure that you don't leave the group until the completion block is called.
Alternatively, you could change searchForPlacesContainingText to use dispatch_group_t parameter:
- (void)searchForPlacesContainingText:(NSString *)searchText
location:(CLLocation *)alocation
group:(dispatch_group_t)group
completion:(MDAddressManagerBlock)completionBlock
{
dispatch_group_enter(group);
NSDictionary *parameters = [[NSMutableDictionary alloc] init];
[parameters setValue:searchText forKey:#"input"];
[[MDHTTPClient sharedHTTPClient] getPath:#"v1/remotePlaces.json"
parameters:parameters
success:^(AFHTTPRequestOperation *operation, id dict) {
if ([MDHTTPClient isResponseValid:dict])
{
completionBlock(returnArray, nil);
}
else
{
completionBlock(nil, nil);
}
dispatch_group_leave(group);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
EDLog(#"%#", error);
completionBlock(nil, [MDError errorAFNetworking:error]);
dispatch_group_leave(group);
}];
}
and change your invocation so that it doesn't do dispatch_group_async, but rather just passes the taskGroup parameter:
[[MDAddressManager instance] searchForPlacesContainingText:query
location:alocation
group:taskGroup
completion:^(NSArray* addresses, MDError *error) {
//remoteAddresses is set
});