(转自：http://blog.csdn.net/arthurchenjs/article/details/6699598)
如果你准备写一个类，希望保证只有一个实例存在，同时可以得到这个特定实例提供服务的入口，那么可以使用单态设计模式。
单态模式在Java、C++中很常用，在Cocoa里，也可以实现。
但是，
Objective-C的单例模式绝对和你所想象不一样，他的写法和你所见过的所有语言的写法都不一样。
官方建议
由于自己设计单态模式存在一定风险，主要是考虑到可能在多线程情况下会出现的问题，因此苹果官方建议使用以下方式来实现单态模式：
static MyGizmoClass *sharedGizmoManager = nil;
+ (MyGizmoClass*)sharedManager { @synchronized(self) { if (sharedGizmoManager == nil) { [[self alloc] init]; // assignment not done here } } return sharedGizmoManager; } + (id)allocWithZone:(NSZone *)zone { @synchronized(self) { if (sharedGizmoManager == nil) { sharedGizmoManager = [super allocWithZone:zone]; return sharedGizmoManager; // 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; }
开源模板（附下载地址）
程序员都是偷懒的，现在流行使用一个宏定义来搞定这许多的事，而且考虑的更加周全。
单例包含以下接口
+ (MyClass*) sharedInstance; + (void) purgeSharedInstance;
调用sharedInstance会创建并返回单例
调用purgeSharedInstance会销毁单例
手动调用alloc也可以保证是单例，你可以这样调用
[[MyClass alloc] initWithParam:firstParam secondParam:secondParam];
只是要保证在sharedInstance之前调用，因为只有一次创建机会。
下面是使用宏的写法“
MyClass.h: ======================================== #import "SynthesizeSingleton.h" @interface MyClass: SomeSuperclass { ... } SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(MyClass); @end ======================================== MyClass.m: ======================================== #import "MyClass.h" @implementation MyClass SYNTHESIZE_SINGLETON_FOR_CLASS(MyClass); ... @end ========================================下载地址
以上转载完毕。
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以下是本人（xiaou）补充的：
上面下载地址下载下来的开源模板内容如下：(经本人在xcode4环境下验证修改了一两处compiled error)
文件名：SynthesizeSingleton.h
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#ifndef SYNTHESIZE_SINGLETON_FOR_CLASS #import#pragma mark - #pragma mark Singleton /* Synthesize Singleton For Class * * Creates a singleton interface for the specified class with the following methods: * * + (MyClass*) sharedInstance; * + (void) purgeSharedInstance; * * Calling sharedInstance will instantiate the class and swizzle some methods to ensure * that only a single instance ever exists. * Calling purgeSharedInstance will destroy the shared instance and return the swizzled * methods to their former selves. * * * Usage: * * MyClass.h: * ======================================== * #import "SynthesizeSingleton.h" * * @interface MyClass: SomeSuperclass * { * ... * } * SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(MyClass); * * @end * ======================================== * * * MyClass.m: * ======================================== * #import "MyClass.h" * * @implementation MyClass * * SYNTHESIZE_SINGLETON_FOR_CLASS(MyClass); * * ... * * @end * ======================================== * * * Note: Calling alloc manually will also initialize the singleton, so you * can call a more complex init routine to initialize the singleton like so: * * [[MyClass alloc] initWithParam:firstParam secondParam:secondParam]; * * Just be sure to make such a call BEFORE you call "sharedInstance" in * your program. */ #define SYNTHESIZE_SINGLETON_FOR_CLASS_HEADER(__CLASSNAME__) \ \ + (__CLASSNAME__*) sharedInstance; \ + (void) purgeSharedInstance; #define SYNTHESIZE_SINGLETON_FOR_CLASS(__CLASSNAME__) \ \ static __CLASSNAME__* volatile _##__CLASSNAME__##_sharedInstance = nil; \ \ + (__CLASSNAME__*) sharedInstanceNoSynch \ { \ return (__CLASSNAME__*) _##__CLASSNAME__##_sharedInstance; \ } \ \ + (__CLASSNAME__*) sharedInstanceSynch \ { \ @synchronized(self) \ { \ if(nil == _##__CLASSNAME__##_sharedInstance) \ { \ _##__CLASSNAME__##_sharedInstance = [[self alloc] init]; \ } \ else \ { \ NSAssert2(1==0, @"SynthesizeSingleton: %@ ERROR: +(%@ *)sharedInstance method did not get swizzled.", self, self); \ } \ } \ return (__CLASSNAME__*) _##__CLASSNAME__##_sharedInstance; \ } \ \ + (__CLASSNAME__*) sharedInstance \ { \ return [self sharedInstanceSynch]; \ } \ \ + (id)allocWithZone:(NSZone*) zone \ { \ @synchronized(self) \ { \ if (nil == _##__CLASSNAME__##_sharedInstance) \ { \ _##__CLASSNAME__##_sharedInstance = [super allocWithZone:zone]; \ if(nil != _##__CLASSNAME__##_sharedInstance) \ { \ Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceNoSynch)); \ method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod)); \ method_setImplementation(class_getInstanceMethod(self, @selector(retainCount)), class_getMethodImplementation(self, @selector(retainCountDoNothing))); \ method_setImplementation(class_getInstanceMethod(self, @selector(release)), class_getMethodImplementation(self, @selector(releaseDoNothing))); \ method_setImplementation(class_getInstanceMethod(self, @selector(autorelease)), class_getMethodImplementation(self, @selector(autoreleaseDoNothing))); \ } \ } \ } \ return _##__CLASSNAME__##_sharedInstance; \ } \ \ + (void)purgeSharedInstance \ { \ @synchronized(self) \ { \ if(nil != _##__CLASSNAME__##_sharedInstance) \ { \ Method newSharedInstanceMethod = class_getClassMethod(self, @selector(sharedInstanceSynch)); \ method_setImplementation(class_getClassMethod(self, @selector(sharedInstance)), method_getImplementation(newSharedInstanceMethod)); \ method_setImplementation(class_getInstanceMethod(self, @selector(retainCount)), class_getMethodImplementation(self, @selector(retainCountDoSomething))); \ method_setImplementation(class_getInstanceMethod(self, @selector(release)), class_getMethodImplementation(self, @selector(releaseDoSomething))); \ method_setImplementation(class_getInstanceMethod(self, @selector(autorelease)), class_getMethodImplementation(self, @selector(autoreleaseDoSomething))); \ [_##__CLASSNAME__##_sharedInstance release]; \ _##__CLASSNAME__##_sharedInstance = nil; \ } \ } \ } \ \ - (id)copyWithZone:(NSZone *)zone \ { \ return self; \ } \ \ - (id)retain \ { \ return self; \ } \ \ - (NSUInteger)retainCount \ { \ NSAssert1(1==0, @"SynthesizeSingleton: %@ ERROR: -(NSUInteger)retainCount method did not get swizzled.", self); \ return NSUIntegerMax; \ } \ \ - (NSUInteger)retainCountDoNothing \ { \ return NSUIntegerMax; \ } \ - (NSUInteger)retainCountDoSomething \ { \ return [super retainCount]; \ } \ \ - (oneway void)release \ { \ NSAssert1(1==0, @"SynthesizeSingleton: %@ ERROR: -(void)release method did not get swizzled.", self); \ } \ \ - (void)releaseDoNothing{} \ \ - (void)releaseDoSomething \ { \ @synchronized(self) \ { \ [super release]; \ } \ } \ \ - (id)autorelease \ { \ NSAssert1(1==0, @"SynthesizeSingleton: %@ ERROR: -(id)autorelease method did not get swizzled.", self); \ return self; \ } \ \ - (id)autoreleaseDoNothing \ { \ return self; \ } \ \ - (id)autoreleaseDoSomething \ { \ return [super autorelease]; \ } #endif