IOS 单例模式

#import "Singleton.h"
 
@implementation Singleton
 
static Singleton* _instance = nil;
 
+(instancetype) shareInstance
{
    static dispatch_once_t onceToken ;
    dispatch_once(&onceToken, ^{
        _instance = [[super allocWithZone:NULL] init] ;
    }) ;
     
    return _instance ;
}
 
+(id) allocWithZone:(struct _NSZone *)zone
{
    return [Singleton shareInstance] ;
}
 
-(id) copyWithZone:(struct _NSZone *)zone
{
    return [Singleton shareInstance] ;
}
 
@end

使用：

 Singleton* obj1 = [Singleton shareInstance] ;
 NSLog(@"obj1 = %@.", obj1) ;
         
 Singleton* obj2 = [Singleton shareInstance] ;
 NSLog(@"obj2 = %@.", obj2) ;
         
 Singleton* obj3 = [[Singleton alloc] init] ;
 NSLog(@"obj3 = %@.", obj3) ;
         
 Singleton* obj4 = [[Singleton alloc] init] ;
 NSLog(@"obj4 = %@.", [obj4 copy]) ;