singleton in objc

objc-singleton.m

+ (instancetype)sharePreference {
    static TFUserPreference *sharePreference = nil;
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharePreference = [[self alloc] init];
    });
    
    return sharePreference;
}

- (id)init {
    self = [super init];
    if (self) {
        self.userDefaults = [NSUserDefaults standardUserDefaults];
    }
    return self;
}