Objective-C单例模式

单例类是一种特殊的类，在操作系统中的一个进程内只会存在一个该类的对象，在iOS应用中同样也只会出现一个对象。这种设计模式在Apple系统框架中许多地方也都使用了，如NSFileManager、UIApplication

• 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

• 在ARC的环境下，实现文件:

//
//  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，只允许执行依次
    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：

#import "DVISingleton.h"

@implementation DVISingleton

+ (instancetype)sharedSingleton
{
    static DVISingleton *sharedObject = nil;

    //线程安全，只允许执行依次
    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