Usage of dispatch_once

Question

I understand that dispatch_once is something that is equivalent to a static variable and that the piece of code inside dispatch_once is executed only once throughout the application.

I am going through a huge code base and came across something like this

+ (DBHelper *)sharedInstance {

    static DBHelper *sharedDBHelper = nil;
    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{
        sharedDBHelper = [[super alloc] initUniqueInstance];
    });

    return sharedDBHelper;
}

DBHelper.sharedInstance is used to get an object and is used in a lot of places ot generate objects.

I'm confused as to why dispatch_once is being used here since that would mean you could have only one object of the class?

the `sharedDBHelper` is assigned only once in runtime – that is a standard pattern of creating _singleton_ instances. — holex, Jun 29 '17 at 15:08
So what exactly happens when multiple classes use DBHelper.sharedInstance() and get an instance in return? Do they all refer to the same object? — rowana, Jun 29 '17 at 15:11
that means that at first occasion the new instance is created and every further occasion the already existing instance will be used only – this is why it called singleton, because there is only one instance exists in the memory of the actual object during runtime. — holex, Jun 29 '17 at 15:16

CRD · Accepted Answer · 2017-06-29T20:20:03.817

This is the standard pattern for a shared instance, otherwise known as a faux singleton.

In many cases programmers choose to use a single object that can be easily accessed from any part of an application - by calling a static method which returns back a reference to the shared object, i.e. sharedInstance in your example - as a means to provide communication/shared data between otherwise independent parts of the application.

It is a faux singleton pattern as it does not prevent other instances of the same type - DBHelper in your example - from being created. A true singleton model is one in which only a single instance of the type can ever be created. (Apple used to have sample code showing how to create true singletons, but it was never updated for the post-ARC world, for more details on that including an ARC version see this answer.)

HTH

score 0 · Answer 2 · answered Jun 29 '17 at 15:00

0

It's a singleton (a design pattern). You only need one instance of the class instantiated, so that's all you create.

answered Jun 29 '17 at 15:00

Cesar

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So what exactly happens when multiple classes use DBHelper.sharedInstance() and get an instance in return? Do they all refer to the same object? – rowana Jun 29 '17 at 15:11
2

@rowana - Yes, they all refer to the same instance. And, BTW, the reason they use `dispatch_once` pattern is precisely in case multiple threads call this at the same time. This thread-safe pattern ensures if multiple threads call this at the same time, that the first will instantiate the object and all the rest will retrieve that same, single instance. – Rob Jun 29 '17 at 15:41
That helps! Thanks Rob! – rowana Jun 29 '17 at 15:59

Usage of dispatch_once

2 Answers2