Thread-safe, forced initialization for singletons

Foo.m

#import "Foo.h"

static dispatch_once_t FooSharedDispatchOnceToken;
static Foo *FooSharedInstance;

+ (void)prepareService
{
    [self fetchSharedServiceAndInitialize:YES];
}

+ (Foo *)sharedInstance {
    
    return [self fetchSharedServiceAndInitialize:NO];
}

+ (Foo *)fetchSharedServiceAndInitialize:(BOOL)initialize
{
    dispatch_once(&FooSharedDispatchOnceToken, ^{
        NSAssert(initialize, @"%s: Instance must be initialized with +prepareService!", __PRETTY_FUNCTION__);
        
        FooSharedInstance = [[self alloc] init];
        
        // other initialization logic
    });
    
    return FooSharedInstance;
}