(转)ObjectiveC的单例模式（singleton）

(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

http://arthurchen.blog.51cto.com/attachment/201108/2483760_1313658868.rar

以上转载完毕。

----------------------------------分割线------------------------------------

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

----------------------------------分割线------------------------------------

#ifndef SYNTHESIZE_SINGLETON_FOR_CLASS

#import <objc/runtime.h>


#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