这是我放在一起的一个宏:

http://github.com/cjhanson/Objective-C-Optimized-Singleton

它基于马特·加拉格尔的作品 但是将实现改为使用谷歌的Dave MacLachlan所描述的方法swizzling。

我欢迎评论/贡献。

KLSingleton是: 可子类化(到n阶) 弧相容 使用alloc和init是安全的 装载 线程安全的 无锁(使用+initialize，而不是@synchronize) Macro-free Swizzle-free 简单的

克林格尔顿

我已经将单例转换为一个类，这样其他类就可以继承单例属性。

Singleton.h:

static id sharedInstance = nil;

#define DEFINE_SHARED_INSTANCE + (id) sharedInstance {  return [self sharedInstance:&sharedInstance]; } \
                               + (id) allocWithZone:(NSZone *)zone { return [self allocWithZone:zone forInstance:&sharedInstance]; }

@interface Singleton : NSObject {

}

+ (id) sharedInstance;
+ (id) sharedInstance:(id*)inst;

+ (id) allocWithZone:(NSZone *)zone forInstance:(id*)inst;

@end

单例。m:

#import "Singleton.h"


@implementation Singleton


+ (id) sharedInstance { 
    return [self sharedInstance:&sharedInstance];
}

+ (id) sharedInstance:(id*)inst {
    @synchronized(self)
    {
        if (*inst == nil)
            *inst = [[self alloc] init];
    }
    return *inst;
}

+ (id) allocWithZone:(NSZone *)zone forInstance:(id*)inst {
    @synchronized(self) {
        if (*inst == nil) {
            *inst = [super allocWithZone:zone];
            return *inst;  // assignment and return on first allocation
        }
    }
    return nil; // on subsequent allocation attempts return nil
}

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

- (id)retain {
    return self;
}

- (unsigned)retainCount {
    return UINT_MAX;  // denotes an object that cannot be released
}

- (void)release {
    //do nothing
}

- (id)autorelease {
    return self;
}


@end

这是一个类的例子，你想变成单例。

#import "Singleton.h"

@interface SomeClass : Singleton {

}

@end

@implementation SomeClass 

DEFINE_SHARED_INSTANCE;

@end

单例类唯一的限制是它是NSObject的子类。但大多数时候我在我的代码中使用单例对象，它们实际上是NSObject的子类，所以这个类真的简化了我的生活，使代码更干净。

由于Kendall发布了一个线程安全的单例，试图避免锁定成本，我想我也会抛出一个:

#import <libkern/OSAtomic.h>

static void * volatile sharedInstance = nil;                                                

+ (className *) sharedInstance {                                                                    
  while (!sharedInstance) {                                                                          
    className *temp = [[self alloc] init];                                                                 
    if(!OSAtomicCompareAndSwapPtrBarrier(0x0, temp, &sharedInstance)) {
      [temp release];                                                                                   
    }                                                                                                    
  }                                                                                                        
  return sharedInstance;                                                                        
}

好吧，让我来解释一下这是怎么回事:

Fast case: In normal execution sharedInstance has already been set, so the while loop is never executed and the function returns after simply testing for the variable's existence; Slow case: If sharedInstance doesn't exist, then an instance is allocated and copied into it using a Compare And Swap ('CAS'); Contended case: If two threads both attempt to call sharedInstance at the same time AND sharedInstance doesn't exist at the same time then they will both initialize new instances of the singleton and attempt to CAS it into position. Whichever one wins the CAS returns immediately, whichever one loses releases the instance it just allocated and returns the (now set) sharedInstance. The single OSAtomicCompareAndSwapPtrBarrier acts as both a write barrier for the setting thread and a read barrier from the testing thread.

这是我放在一起的一个宏:

http://github.com/cjhanson/Objective-C-Optimized-Singleton

它基于马特·加拉格尔的作品 但是将实现改为使用谷歌的Dave MacLachlan所描述的方法swizzling。

我欢迎评论/贡献。

@interface MySingleton : NSObject
{
}

+ (MySingleton *)sharedSingleton;
@end

@implementation MySingleton

+ (MySingleton *)sharedSingleton
{
  static MySingleton *sharedSingleton;

  @synchronized(self)
  {
    if (!sharedSingleton)
      sharedSingleton = [[MySingleton alloc] init];

    return sharedSingleton;
  }
}

@end

(来源)