Does using blocks auto create new threads?

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?

score 7 · Accepted Answer · answered Apr 05 '13 at 22:03

7

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.

answered Apr 05 '13 at 22:03

Rob Napier

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And am I correct in assuming that one cannot do UI stuff like push new VC or the likes on a background thread? – sangony Apr 05 '13 at 22:10
1

That is correct. UIKit is not thread-safe. This is not to say you can't do UI work in a block; just that the block must run on the main thread. – Rob Napier Apr 05 '13 at 22:12
Got it. Last follow up question... Is it preferable to use blocks, like in my above code, or stick with the NSURL and its delegates?I am asking because I am still trying to wrap my head around blocks and am wondering if they are really worth the effort at this point. – sangony Apr 05 '13 at 22:19
1

I typically use delegates for NSURLConnection. But blocks are becoming more preferred over delegates for some kinds of problems, and fewer new interfaces include delegates. In the above case, either is fine and you should use the one you're comfortable with. If your completion block gets long, you should break it out into another method and call that from the completion block. But if you do that, you might as well use the delegate. Note that you have a block-retains-self loop here, but for NSURLConnection that will work itself out (another thing you have to pay attention to with blocks). – Rob Napier Apr 05 '13 at 22:25
1

For more on block retain self loops, see http://www.cocoanetics.com/2012/03/block-retain-loop/ – Rob Napier Apr 05 '13 at 22:30

Does using blocks auto create new threads?

1 Answers1