建单例

+ (XXX *)sharedInstance
{
    // 1
    static XXX *_sharedInstance = nil;
    
    // 2
    static dispatch_once_t oncePredicate;
    
    // 3
    dispatch_once(&oncePredicate, ^{
        _sharedInstance = [[XXX alloc] init];
    });
    return _sharedInstance;
}