iOS单例设计 MRC&ARC

单例设计思想是iOS开发中常见的设计思想之一，下面总结出MRC&ARC环境下单例代码实现：

#import

@interface AccountManager : NSObject < NSCopying >

+ ( instancetype )sharedAccountManager;

@end

#import "AccountManager.h"

@implementation AccountManager

static AccountManager *instance;

#if __has_feature(objc_arc)

//  MAEK:在ARC中需要重写以下方法

+ ( instancetype )sharedAccountManager
{
    static dispatch_once_t onceToken;
    dispatch_once (&onceToken, ^{
        instance = [[ AccountManager alloc ] init ];
    });
   
    return instance ;
}

+ ( instancetype )allocWithZone:( struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once (&onceToken, ^{
        instance = [ super allocWithZone :zone];
    });
   
    return instance ;
}

- ( id )copyWithZone:( NSZone *)zone
{
    return instance ;
}

#else

// MAEK:在MRC中除了要重写上面的方法以外还要重写以下方法

+ ( instancetype )sharedAccountManager
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[AccountManager alloc] init];
    });
   
    return instance;
}

+ ( instancetype )allocWithZone:( struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [ super allocWithZone:zone];
    });
   
    return instance;
}

- ( id )copyWithZone:(NSZone *)zone
{
    return instance;
}

- ( oneway void )release
{
    // 什么都不做
}

- ( instancetype )retain
{
    return instance;
}

- ( instancetype )autorelease
{
    return instance;
}

/// 永远保证引用计数为 1
- (NSUInteger)retainCount
{
    return 1 ;
}

#endif

@end