How to create straight forward Singleton classes in Objective C?

Question

I am used to Java and currently on the process of learning Objective-C.

Basically I would create Singleton classes in Java like this:

public class SingletonClass{
  private static instance; //Step 1

  public static SingletonClass getInstance(){ //Step 2
    if(instance == null)
      instance = new SingletonClass();

    return instance;
  }
}

Very straightforward right?

But I find a hard time creating straight forward solution for that in Objective-C

I did like this:

@implementation SingletonClass(){
  //I want to do step 1 here which is to make a private static instance;
  //it is said that private variables are declared here
  static SingletonClass *instance; //but it is said that static keyword is different here
}

//then I would do something like step 2
+ (id)getInstance{
  if(instance == nil)
    instance = self;

  return instance;
}

@end

The problem is that there is an error :Type name does not allow storage class to be specified

How do you guys make straight forward Singleton classes in Objective-C?

Google. http://www.idev101.com/code/Objective-C/singletons.html — carloabelli, Aug 01 '14 at 02:40
See http://stackoverflow.com/questions/5720029/create-singleton-using-gcds-dispatch-once-in-objective-c?rq=1 — rmaddy, Aug 01 '14 at 02:43
@cabellicar123, show anything you want there I am interested btw — Rick Royd Aban, Aug 01 '14 at 02:57
It was from the list of related questions to the right. Those would have also appeared before you even submitted your question. It's always a good idea to review those before posting. :) — rmaddy, Aug 01 '14 at 03:00
That Java singleton you made isn't thread safe. If the singleton creation has side effects, that can result in some hard to reproduce race conditions. — Doug Richardson, Aug 01 '14 at 03:48

zaph · Accepted Answer · 2014-08-01T02:47:36.210

1

Use dispatch_once_t:

From Apple docs dispatch_once(3)
The dispatch_once() function provides a simple and efficient mechanism to run an initializer exactly once, similar to pthread_once(3).

Also see: Secrets of dispatch_once by Mike Ash

+(instancetype)sharedInstance
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[SingletonClass alloc] init];
    });
    return sharedInstance;
}

edited Aug 01 '14 at 02:47

answered Aug 01 '14 at 02:42

zaph

111,848
21
189
228

Sorry but I'm new here in Objective-C, can you please explain about `dispatch_once_t`? Thanks. – Rick Royd Aban Aug 01 '14 at 02:42
Nice answer. Sorry for the late response by the way, I had to read your given links. – Rick Royd Aban Aug 01 '14 at 03:07
also want to give credits to @rmaddy's comment since his link (stackoverflow.com/questions/5720029/…) was helpful – Rick Royd Aban Aug 01 '14 at 03:32

score -3 · Answer 2 · answered Aug 01 '14 at 02:43

-3

They are called external variables in objective c.

In app delegate under #import put

NSString *singleton;

Then in what ever class (view controller) you want to use it in's .h file

extern NSString *singleton;

answered Aug 01 '14 at 02:43

Tom Testicool

563
7
26

1

This has nothing to do with creating a singleton. – rmaddy Aug 01 '14 at 02:45

How to create straight forward Singleton classes in Objective C?

2 Answers2