iOS 创建单例

#import "SingleInstance.h"

@interface SingleInstance ()<NSCopying,NSMutableCopying>

@end

//定义一个当前单例对象的一个实例，并赋值为nil
static SingleInstance *instance = nil;

@implementation SingleInstance
+ (instancetype)sharedInstance
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc] init];
    });

    return instance;
}
/**************真正的单例需重载所有和创建有关的方法**************/
//alloc会触发，防止通过alloc创建一个不同的实例
+ (instancetype)allocWithZone:(struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [super allocWithZone:zone];
    });

    return instance;
}

- (id)copyWithZone:(nullable NSZone *)zone
{
    return self;
}
- (id)mutableCopyWithZone:(nullable NSZone *)zone
{
    return self;
}

/**************手动内存管理需做的操作**************/

- (instancetype)retain
{
    return self;
}

- (oneway void)release
{

}

- (instancetype)autorelease
{
    return self;
}
- (NSUInteger)retainCount
{
    return MAXFLOAT;
}

- (void)dealloc
{

}
@end