performselectoronmainthread and get return value

gistfile1.m

// http://stackoverflow.com/a/3346157/490794


// call
NSMutableDictionary *myDict;
[object performSelectorOnMainThread:@selector(getElements:)
                         withObject:&myDict waitUntilDone:YES];

// define
- (void)getElements:(NSMutableDictionary **)objects;