单例的写法 ，可直接用

Singleton.h
 
@interface Singleton : NSObject
 
+(instancetype) shareInstance ;
 
@end

Singleton.m
@interface Singleton()<NSCopying,NSMutableCopying>
@end
 
@implementation Singleton
 
static Singleton* _instance = nil;
 
+(instancetype) shareInstance
{
    static dispatch_once_t onceToken ;
    dispatch_once(&onceToken, ^{
        _instance = [[super allocWithZone:NULL] init] ;
        //不是使用alloc方法，而是调用[[super allocWithZone:NULL] init] 
        //已经重载allocWithZone基本的对象分配方法，所以要借用父类（NSObject）的功能来帮助出处理底层内存分配的杂物
    }) ;
     
    return _instance ;
}
 
+(id) allocWithZone:(struct _NSZone *)zone
{
    return [Singleton shareInstance] ;
}
 
-(id) copyWithZone:(NSZone *)zone
{
    return [Singleton shareInstance] ;//return _instance;
}
 
-(id) mutablecopyWithZone:(NSZone *)zone
{
    return [Singleton shareInstance] ;
}
@end