这也适用于非垃圾收集环境。

@interface MySingleton : NSObject {
}

+(MySingleton *)sharedManager;

@end


@implementation MySingleton

static MySingleton *sharedMySingleton = nil;

+(MySingleton*)sharedManager {
    @synchronized(self) {
        if (sharedMySingleton == nil) {
            [[self alloc] init]; // assignment not done here
        }
    }
    return sharedMySingleton;
}


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


-(void)dealloc {
    [super dealloc];
}

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


-(id)retain {
    return self;
}


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


-(void)release {
    //do nothing    
}


-(id)autorelease {
    return self;    
}


-(id)init {
    self = [super init];
    sharedMySingleton = self;

    //initialize here

    return self;
}

@end

关于Objective-C中单例模式的深入讨论，请看这里:

在Objective-C中使用单例模式

根据我下面的另一个回答，我认为你应该这样做:

+ (id)sharedFoo
{
    static dispatch_once_t once;
    static MyFoo *sharedFoo;
    dispatch_once(&once, ^ { sharedFoo = [[self alloc] init]; });
    return sharedFoo;
}

如何

static MyClass *gInstance = NULL;

+ (MyClass *)instance
{
    if (gInstance == NULL) {
        @synchronized(self)
        {
            if (gInstance == NULL)
                gInstance = [[self alloc] init];
        }
    }

    return(gInstance);
}

这样就避免了初始化后的同步成本?

我知道有很多关于这个“问题”的评论，但是我没有看到很多人建议使用宏来定义单例对象。这是一个非常常见的模式，一个宏极大地简化了单例。

下面是我根据我见过的几个Objc实现编写的宏。

Singeton。h

/**
 @abstract  Helps define the interface of a singleton.
 @param  TYPE  The type of this singleton.
 @param  NAME  The name of the singleton accessor.  Must match the name used in the implementation.
 @discussion
 Typcially the NAME is something like 'sharedThing' where 'Thing' is the prefix-removed type name of the class.
 */
#define SingletonInterface(TYPE, NAME) \
+ (TYPE *)NAME;


/**
 @abstract  Helps define the implementation of a singleton.
 @param  TYPE  The type of this singleton.
 @param  NAME  The name of the singleton accessor.  Must match the name used in the interface.
 @discussion
 Typcially the NAME is something like 'sharedThing' where 'Thing' is the prefix-removed type name of the class.
 */
#define SingletonImplementation(TYPE, NAME) \
static TYPE *__ ## NAME; \
\
\
+ (void)initialize \
{ \
    static BOOL initialized = NO; \
    if(!initialized) \
    { \
        initialized = YES; \
        __ ## NAME = [[TYPE alloc] init]; \
    } \
} \
\
\
+ (TYPE *)NAME \
{ \
    return __ ## NAME; \
}

使用示例:

我的经理

@interface MyManager

SingletonInterface(MyManager, sharedManager);

// ...

@end

我的经理

@implementation MyManager

- (id)init
{
    self = [super init];
    if (self) {
        // Initialization code here.
    }

    return self;
}

SingletonImplementation(MyManager, sharedManager);

// ...

@end

为什么界面宏几乎是空的?头文件和代码文件之间的代码一致性;可维护性，以防您想添加更多的自动方法或更改它。

我使用initialize方法创建单例，就像这里(在撰写本文时)最流行的答案中使用的那样。

简单回答:太棒了。

长话短说:类似....

static SomeSingleton *instance = NULL;

@implementation SomeSingleton

+ (id) instance {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (instance == NULL){
            instance = [[super allocWithZone:NULL] init];
        }
    });
    return instance;
}

+ (id) allocWithZone:(NSZone *)paramZone {
    return [[self instance] retain];
}

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

- (id) autorelease {
    return self;
}

- (NSUInteger) retainCount {
    return NSUIntegerMax;
}

- (id) retain {
    return self;
}

@end

一定要阅读dispatch/once.h头文件以了解发生了什么。在这种情况下，标题注释比文档或手册页更适用。