Create singleton using GCD's dispatch_once in Objective-C

Question

If you can target iOS 4.0 or above

Using GCD, is it the best way to create singleton in Objective-C (thread safe)?

+ (instancetype)sharedInstance
{
          


        

Answer 1

If you want to make sure that [[MyClass alloc] init] returns the same object as sharedInstance (not necessary in my opinion, but some folks want it), that can be done very easily and safely using a second dispatch_once:

- (instancetype)init
{
    static dispatch_once_t once;
    static Class *sharedInstance;

    dispatch_once(&once, ^
    {
        // Your normal init code goes here. 
        sharedInstance = self;
    });

    return sharedInstance;
}

This allows any combination of [[MyClass alloc] init] and [MyClass sharedInstance] to return the same object; [MyClass sharedInstance] would just be a bit more efficient. How it works: [MyClass sharedInstance] will call [[MyClass alloc] init] once. Other code could call it as well, any number of times. The first caller to init will do the "normal" initialisation and store the singleton object away in the init method. Any later calls to init will completely ignore what alloc returned and return the same sharedInstance; the result of alloc will be deallocated.

The +sharedInstance method will work as it always did. If it isn't the first caller to call [[MyClass alloc] init], then the result of init is not the result of the alloc call, but that is OK.