IOS单例实现

@implementation MyTeststatic id instance = nil;+ (instancetype)allocWithZone:(struct _NSZone *)zone{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        instance = [super allocWithZone:zone];    });    return instance;}+ (instancetype)share{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        instance = [[MyTest alloc] init];    });    return instance;}- (instancetype)copyWithZone:(struct _NSZone*)zone{    return instance;}- (instancetype)mutableCopyWithZone:(struct _NSZone*)zone{    return instance;}@end