IOS单例类的实现-优化版

以下是我实现单例类的两个方案，并且每个方案都经过了性能优化，如下：

方案1：互斥锁

通过互斥锁实现的单列代码；不保证只初始化一次！

//此单例方案不能保证只初始一次

//单例指针
static id _instance = nil;

+ (instancetype)sharedInstance
{
    if (_instance == nil) {     //因为加锁操作要比普通操作耗费更多的资源，所以尽量把锁内的执行条件在加锁之前先判断一次，以减少不必根的加锁操作
        
        @synchronized(self) {
            if (_instance == nil) {
                _instance = [[self alloc] init];
            }
        }
    }
    
    return _instance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone
{
    if (_instance == nil) {     //因为加锁操作要比普通操作耗费更多的资源，所以尽量把锁内的执行条件在加锁之前先判断一次，以减少不必根的加锁操作
        
        @synchronized(self) {
            if (_instance == nil) {
                _instance = [super allocWithZone:zone];
            }
        }
    }
    
    return _instance;
}

//copy在底层 会调用copyWithZone:
-(id)copyWithZone:(NSZone*)zone {
    return _instance;
}

+(id)copyWithZone:(struct _NSZone *)zone {
    return _instance;
}

//mutableCopy在底层 会调用mutableCopyWithZone:
-(id)mutableCopyWithZone:(NSZone*)zone {
    return _instance;
}

+ (id)mutableCopyWithZone:(struct _NSZone *)zone{
    return _instance;
}

方案2：GCD

通过GCD方式实现的单列代码

//单列指针
static id _instance = nil;


//获取共享单列
+(instancetype)sharedInstance {
    return [[self alloc] init];
}

//alloc方法会调用allocWithZone
+(instancetype)allocWithZone:(struct _NSZone *)zone {
    
    static dispatch_once_t  onceToken;
    dispatch_once(&onceToken, ^{
        //只执行一次
        _instance = [super allocWithZone: zone];
    });
    
    return _instance;
}

//初始化方法
-(instancetype)init {
    
    static dispatch_once_t  onceToken;
    
    dispatch_once(&onceToken, ^{
        //只执行一次
        _instance = [super init];
    });
    
    return _instance;
    
}

//copy在底层 会调用copyWithZone:
-(id)copyWithZone:(NSZone*)zone {
    return _instance;
}

+(id)copyWithZone:(struct _NSZone *)zone {
    return _instance;
}

//mutableCopy在底层 会调用mutableCopyWithZone:
-(id)mutableCopyWithZone:(NSZone*)zone {
    return _instance;
}

+ (id)mutableCopyWithZone:(struct _NSZone *)zone{
    return _instance;
}