宏定义抽取单例

// cmd + n 新建Header File 文件名: Singleton.h

// .h
#define singleton_interface(className) + (instancetype)shared##className;

// .m
// 注意：最后一句不要斜线
#define singleton_implementation(className) 
static className *_instace; 

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

+ (instancetype)shared##className 
{ 
    if (_instace == nil) { 
        _instace = [[className alloc] init]; 
    } 
    
    return _instace; 
}