单例的两种写法

第一种

@implementation Mooc

+ (id)sharedInstance
{
    // 静态局部变量
    static Mooc *instance = nil;
    
    // 通过dispatch_once方式 确保instance在多线程环境下只被创建一次
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // 创建实例
        instance = [[super allocWithZone:NULL] init];
    });
    return instance;
}

// 重写方法【必不可少】
+ (id)allocWithZone:(struct _NSZone *)zone{
    return [self sharedInstance];
}

// 重写方法【必不可少】
- (id)copyWithZone:(nullable NSZone *)zone{
    return self;
}
- (id)mutableCopyWithZone:(NSZone *)zone {
    return self;
}
@end

第二种写法

static XMGDownLoaderManager *_shareInstance;
+ (instancetype)shareInstance {
    if (_shareInstance == nil) {
        _shareInstance = [[self alloc] init];
    }
    return _shareInstance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    if (!_shareInstance) {
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            _shareInstance = [super allocWithZone:zone];
        });
    }
    return _shareInstance;
}

- (id)copyWithZone:(NSZone *)zone {
    return _shareInstance;
}

- (id)mutableCopyWithZone:(NSZone *)zone {
    return _shareInstance;
}