单例类是一种特殊的类。在一个进程种仅仅会存在一个该类的对象，在iOS应用中仅仅会出现一个对象。这样的设计模式在系统框架中很多地方都使用了。如NSFileManager、UIApplication等。

• 在ARC的环境下，接口文件为：

//

//  DVISingleton.h

//

//  Copyright (c) 2014 长沙戴维营教育. All rights reserved.

//

#import <Foundation/Foundation.h>

@interface DVISingleton : NSObject

+ (instancetype)sharedSingleton;

@end

实现文件：

//

//  DVISingleton.m

//

//  Copyright (c) 2014 长沙戴维营教育. All rights reserved.

//

#import "DVISingleton.h"

@implementation DVISingleton

+ (instancetype)sharedSingleton

{

    static DVISingleton *sharedObject = nil;

    //线程安全。仅仅同意运行依次

    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{

        //使用父类的allocWithZone:方法创建对象

        sharedObject = [[super allocWithZone:NULL] init];

    });

    return sharedObject;

}

- (id)init

{

    if (self = [super init]) {

    }

    return self;

}

+ (id)allocWithZone:(struct _NSZone *)zone

{

    return [self sharedSingleton];

}

- (id)copy

{

    return self;

}

- (void)dealloc

{

}

@end

• 在非ARC环境下的实现文件：

<code class="objectivec hljs" data-origin="" <pre><code="" "dvisingleton.h""="" style="margin: 0px; padding: 0px; border: 0px; font-size: inherit; font-variant: inherit; font-weight: bold; line-height: inherit; vertical-align: baseline; color: rgb(110, 107, 94); font-family: Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace !important;">#import "DVISingleton.h"

@implementation DVISingleton

+ (instancetype)sharedSingleton

{

    static DVISingleton *sharedObject = nil;

    //线程安全。仅仅同意运行依次

    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{

        //使用父类的allocWithZone:方法创建对象

        sharedObject = [[super allocWithZone:NULL] init];

    });

    return sharedObject;

}

+ (id)allocWithZone:(NSZone *)zone {

  return [[self sharedSingleton] retain];

}

- (id)copyWithZone:(NSZone *)zone {

  return self;

}

- (id)retain {

  return self;

}

- (unsigned)retainCount {

  return UINT_MAX; //denotes an object that cannot be released

}

- (oneway void)release {

  // never release

}

- (id)autorelease {

  return self;

}

- (id)init {

  if (self = [super init]) {

  }

  return self;

}

- (void)dealloc {

  [super dealloc];

}

@end