怎么写好单例

import “SingleObj.h”

static SingleObj *kSingleObj = nil;

@implementation SingleObj

• (instancetype)sharedInstance
  {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  kSingleObj = [[super allocWithZone:NULL] init];
  });
  return kSingleObj;
  }

• (instancetype)allocWithZone:(struct _NSZone *)zone
  {
  return [self sharedInstance];
  }

• (id)copy
  {
  // return kSingleObj;
  return [self.class sharedInstance];
  }

• (id)mutableCopy
  {
  // return kSingleObj;
  return [self.class sharedInstance];
  }