Love this Pattern for generating a single instance of an object

QTSomeClient.m

@implementation QTSomeClient

+ (QTSomeClient *)sharedClient {
    static QTSomeClient *_sharedClient = nil;
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedClient = [QTSomeClient new];
    });
    return _sharedClient;
}

- (id)init {
    self = [super init];
    if (!self) {
        return nil;
    }
    return self;
}

@end