Objective-C: Syntax to call method from another class

Question

iOS newb trying to pull an array from another class and a getting error. Fundamentally, I don't really understand the difference between methods and properties, I realize, so this might help me get a better understanding of that.

I have created a method that returns an array. I want to use that array in the other class (like a function returning a value in other languages) so I thought I would do it as follows:

class1
- (id) getTags{
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Tags"];
    fetchRequest.resultType = NSDictionaryResultType;

    NSError *error      = nil;
    NSArray *results    = [self.managedObjectContext executeFetchRequest:fetchRequest
                                                                   error:&error];

    NSMutableArray * tags = [[results valueForKey:@"tag"] mutableCopy];
    return tags;
}

In other words, I want to use what that method returns as a sort of variable which in Objective C is called properties. The method is also in the .h file so that it is public.

In class 2 I import class 1 at top and then try to call method as follows:

NSArray *newarray = [class1 getTags];

However I get error "No known class method for selector getTags"

Can anyone tell me what I am doing wrong?

Thanks.

Answer 1

I suspect this line;

NSArray *newarray = [class1 getTags];

Actually, you should use class instance, not class itself. To use this kind of function call, you should declare getTags as class level function(+ function), not - function

So your code should be look like this;

class1 *class1Instance = [[class1 alloc] init];
NSArray *newarray = [class1Instance getTags];

Answer 2

The problem is the code NSArray *newarray = [class1 getTags];

You are calling class method getTags in class1 ; however you only have an instance method called getTags in class1 .

To solve it, change the - to + in the method signature, which will change your method from an instance method to a class method. (In java terms: + is where you put a static in the method).

However, there are quite a lot of style issues in your code. First, I don't think putting code in .h is a good practice. What you might want is

.h

@interface Class1
+ (NSArray *)getTags;
@end

.m

@implementation Class1
+ (NSArray *)getTags { ... }
@end

so you declare your public interface in .h file and implementation in .m. Also, you probably want to capitalize your class name.

You shouldn't use id as a return type and assign that id from [class1 getTags] to NSArray * . There is a implicit cast there, which sometimes could lead to some good debug time.

NSArray CAN be used as return type of your method because NSMutableArray is inherited from NSArray - Apple Documentation of NSMutableArray