单例模式在Cocoa和Cocoa Touch中非常常见。比如这两个，[UIApplication sharedApplication]和[NSApplication sharedApplication]，大家应该都见过。但是我们应该如何在代码中实现一个单例模式呢？

如果你对苹果的文档很熟悉的话，你一定知道，在中有一段。大致如下：

/* Singleton.h */#import <Foundation/Foundation.h>@interface Singleton : NSObject+ (Singleton *)instance;@end/* Singleton.m */#import "Singleton.h"static Singleton *instance = nil;@implementation Singleton+ (Singleton *)instance {    if (!instance) {        instance = [[super allocWithZone:NULL] init];    }    return instance;}+ (id)allocWithZone:(NSZone *)zone {    return [self instance];}- (id)copyWithZone:(NSZone *)zone {    return self;}- (id)init {    if (instance) {        return instance;    }    self = [super init];    return self;}- (id)retain {    return self;}- (oneway void)release {    // Do nothing}- (id)autorelease {    return self;}- (NSUInteger)retainCount {    return NSUIntegerMax;}@end

这是一种很标准的Singleton实现，中规中矩。不过这种实现并不是线程安全的。所以各路大神都各显神威，给出了多种单例模式的实现。

Matt Gallagher在中放出了一个Macro，用来实现单例模式。虽然是一个宏定义的代码，但是具体实现还是很清楚的。代码如下：

////  SynthesizeSingleton.h//  CocoaWithLove////  Created by Matt Gallagher on 20/10/08.//  Copyright 2009 Matt Gallagher. All rights reserved.////  Permission is given to use this source code file without charge in any//  project, commercial or otherwise, entirely at your risk, with the condition//  that any redistribution (in part or whole) of source code must retain//  this copyright and permission notice. Attribution in compiled projects is//  appreciated but not required.//#define SYNTHESIZE_SINGLETON_FOR_CLASS(classname) \ \static classname *shared##classname = nil; \ \+ (classname *)shared##classname \{ \    @synchronized(self) \    { \        if (shared##classname == nil) \        { \            shared##classname = [[self alloc] init]; \        } \    } \     \    return shared##classname; \} \ \+ (id)allocWithZone:(NSZone *)zone \{ \    @synchronized(self) \    { \        if (shared##classname == nil) \        { \            shared##classname = [super allocWithZone:zone]; \            return shared##classname; \        } \    } \     \    return nil; \} \ \- (id)copyWithZone:(NSZone *)zone \{ \    return self; \} \ \- (id)retain \{ \    return self; \} \ \- (NSUInteger)retainCount \{ \    return NSUIntegerMax; \} \ \- (void)release \{ \} \ \- (id)autorelease \{ \    return self; \}

@interface SomeManager : NSObject+ (id)sharedManager;@end/* 非线程安全的实现 */@implementation SomeManager+ (id)sharedManager {    static id sharedManager = nil;    if (sharedManager == nil) {        sharedManager = [[self alloc] init];    }    return sharedManager;}@end/* 线程安全的实现 */@implementation SomeManagerstatic id sharedManager = nil;+ (void)initialize {    if (self == [SomeManager class]) {        sharedManager = [[self alloc] init];    }}+ (id)sharedManager {    return sharedManager;}@end

关于为什么上述代码就能实现单例模式，以及关于线程安全问题的考量，请参考他的。

最后介绍一个比较。为什么说现代呢？因为这种实现利用了GCD（Grand Central Dispatch）和ARC（Automatic Reference Counting）。核心代码如下：

+ (id)sharedInstance{  static dispatch_once_t pred = 0;  __strong static id _sharedObject = nil;  dispatch_once(&pred, ^{    _sharedObject = [[self alloc] init]; // or some other init method  });  return _sharedObject;}

作者还写了一个宏（）来方便使用，大家可以阅读作者的博文了解详情。

大多数情况下，Apple官方文档里的单例模式的示例代码实现已经够用了。虽然它最繁琐，但是也是本文介绍的几种单例模式中最容易理解的一个。至于其他的实现就留给读者们根据需要选择和应用了。

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