Using a method a second time gives me an error (IOS) - ios

Question

Im writing this app in objective c (Xcode).
when i use a method in the first time everything goes well, but when i use it the second time it gives me an error.
I tried to debug it and the error goes in the method add tutor in the line if([tutor.userName isEqualToString:userName])
this is the error:
-[__NSCFConstantString userName]: unrecognized selector sent to
instance 0xad14c 2016-02-26 20:10:44.043 project[1258:35474] ***
Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: '-[__NSCFConstantString
userName]: unrecognized selector sent to instance 0xad14c'
this is my code:
- (IBAction)addToFavorite:(id)sender {
AppDelegate *delegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
NSString * userName = delegate.delegateStr;
Student * student = [[Model instance]getStudentByUserName:userName];
[student addTutor:self.tutor.userName];
UIStoryboard* sb = [UIStoryboard storyboardWithName:#"Main" bundle:nil];
FavoritesTableViewController * ftvc = [sb instantiateViewControllerWithIdentifier:#"favoritesTableViewController"];
ftvc.favTutors = student.favTutors;
[[NSNotificationCenter defaultCenter] postNotificationName:#"RefreshTable" object:nil userInfo:nil];
}
-(void)addTutor:(NSString*)userName {
BOOL check = YES;
for (Tutor * tutor in self.favTutors) {
if([tutor.userName isEqualToString:userName])
check = NO;
}
if(check)
[self.favTutors addObject:userName];
}
Please help!!
Thanks.

Answer 1

That's failing because you are adding username in your favTutors array instead of objects of Tutor. So when you add first object and then next time your enumerate through self.favTutors you actually have a string object which will not respond to username property.

Answer 2

problem is here at the line i added comment. you are adding an NSString to objects full of Tutor objects. and in second call of this method, in for loop, tutor is actually an NSString and you are trying to call tutor.userName and it doesnt exist in NSString object.
-(void)addTutor:(NSString*)userName {
BOOL check = YES;
for (Tutor * tutor in self.favTutors) {
if([tutor.userName isEqualToString:userName])
check = NO;
}
if(check)
[self.favTutors addObject:userName]; //problem here!!!!
}
My advice is to create tutor object and add it to your array.
-(void)addTutor:(NSString*)userName {
BOOL check = YES;
for (Tutor * tutor in self.favTutors) {
if([tutor.userName isEqualToString:userName])
check = NO;
}
if(check)
{
// create Tutor object and add it to array dont add NSString directly
}
}