ios - Does using blocks auto create new threads? -

just started doing work blocks... confusing. using block this:

-(void)tableview:(uitableview *)tableview didselectrowatindexpath:(nsindexpath *)indexpath { nsdictionary *mydictionary = [[mysingleton arraypeoplearoundme] objectatindex:indexpath.row];  nsmutablestring *mystring = [[nsmutablestring alloc] initwithstring:@"http://www.domain.com/4daction/pp_profiledetail/"]; [mystring appendstring:[mydictionary objectforkey:@"userid"]]; nsurlrequest *urlrequest = [nsurlrequest requestwithurl:[nsurl urlwithstring:[mystring stringbyaddingpercentescapesusingencoding: nsutf8stringencoding]]                                             cachepolicy:nsurlrequestuseprotocolcachepolicy                                         timeoutinterval:60.0]; nsoperationqueue *queue = [[nsoperationqueue alloc] init]; [nsurlconnection  sendasynchronousrequest:urlrequest  queue:queue  completionhandler: ^( nsurlresponse *response,                       nsdata *data,                       nserror *error)  {      [[mysingleton dictionaryuserdetail] removeallobjects];      [[mysingleton arrayuserdetail] removeallobjects];       if ([data length] > 0 && error == nil) // no error , received data      {          nserror* error;          nsdictionary *mydic = [nsjsonserialization jsonobjectwithdata:data options:kniloptions error:&error];          [mysingleton setdictionaryuserdetail:[mydic mutablecopy]];           nsarray *myarray = [mydic objectforkey:@"searchresults"];          [mysingleton setarrayuserdetail:[myarray mutablecopy]];           [self userdetailcomplete];      } else if          ([data length] == 0 && error == nil) // no error , did not receive data      {          [self servererror];      } else if          (error != nil) // error      {          [self servererror];      }  }]; } 

once connection completed, called:

-(void)userdetailcomplete { viewprofile *vpvc = [[viewprofile alloc] init]; [vpvc setpassedinstructions:@"viewdetail"]; [[self navigationcontroller] pushviewcontroller:vpvc animated:yes]; } 

which caused error pop up: "tried obtain web lock thread other main thread or web thread. may result of calling uikit secondary thread." way got rid of error changing userdetailcomplete this:

-(void)userdetailcomplete { dispatch_async(dispatch_get_main_queue(), ^{ viewprofile *vpvc = [[viewprofile alloc] init]; [vpvc setpassedinstructions:@"viewdetail"]; [[self navigationcontroller] pushviewcontroller:vpvc animated:yes]; }); } 

my question: new thread started automatically every time block used? there other pitfalls should aware of when using blocks?

blocks not create threads. closures; contain runnable code can run @ future point.

this running on background thread because that's asked do:

nsoperationqueue *queue = [[nsoperationqueue alloc] init]; [nsurlconnection   sendasynchronousrequest:urlrequest   queue:queue   ... 

you created new queue , asked nsurlconnection call on queue. if want called on main thread, pass [nsoperationqueue mainqueue]. that's waht want.