Does using blocks auto create new threads? - ios

Question

JUST started doing work with blocks... very confusing. I am using a block like 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 and received data back
{
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 and did not receive data back
{
[self serverError];
} else if
(error != nil) // error
{
[self serverError];
}
}];
}
Once the connection is completed, this is called:
-(void)userDetailComplete {
ViewProfile *vpVC = [[ViewProfile alloc] init];
[vpVC setPassedInstructions:#"ViewDetail"];
[[self navigationController] pushViewController:vpVC animated:YES];
}
which caused this error to pop up:
"Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread."
The only way I got rid of the error was by changing userDetailComplete to 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: is a new thread started automatically every time a block is used? Are there any other pitfalls I should aware of when using blocks?

Answer 1

Blocks do not create threads. They are closures; they just contain runnable code that can be run at some future point.
This is running on a background thread because that's what you asked it to do:
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[NSURLConnection
sendAsynchronousRequest:urlRequest
queue:queue
...
You created a new queue and then asked NSURLConnection to call you back on that queue. If you want to be called back on the main thread, pass [NSOperationQueue mainQueue]. That's usually waht you want.