GCD_DISPATCH_ONCE实现单例模式

@implementation TestSingle
+ (instancetype)instance {
    static dispatch_once_t onceToken;
    static TestSingle *ins = nil;
    dispatch_once(&onceToken, ^{
        NSLog(@"init the TestSingle");
        ins = [[TestSingle alloc] init];
    });
    return ins;
}
@end