)Objective-C的單例模式（singleton）

(轉自：http://blog.csdn.net/arthurchenjs/article/details/6699598)

如果你準備寫一個類，希望保證只有一個執行個體存在，同時可以得到這個特定執行個體提供服務的入口，那麼可以使用單態設計模式。

單態模式在Java、C++中很常用，在Cocoa裡，也可以實現。

但是，

Objective-C的單例模式絕對和你所想象不一樣，他的寫法和你所見過的所有語言的寫法都不一樣。

 

官方建議

 

由於自己設計單態模式存在一定風險，主要是考慮到可能在多線程情況下會出現的問題，因此蘋果官方建議使用以下方式來實現單態模式：

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; 
}

 

開源模板（附）

程式員都是偷懶的，現在流行使用一個宏定義來搞定這許多的事，而且考慮的更加周全。

單例包含以下介面 

+ (MyClass*) sharedInstance; 
+ (void) purgeSharedInstance;

 

調用sharedInstance會建立並返回單例

調用purgeSharedInstance會銷毀單例

手動調用alloc也可以保證是單例，你可以這樣調用

[[MyClass alloc] initWithParam:firstParam secondParam:secondParam];

只是要保證在sharedInstance之前調用，因為只有一次建立機會。

下面是使用宏的寫法“ 

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 
========================================

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

 

以上轉載完畢。

----------------------------------分割線------------------------------------

以下是本人（xiaou）補充的：

      上面下載下來的開源模板內容如下：(經本人在xcode4環境下驗證修改了一兩處compiled error)

檔案名稱：SynthesizeSingleton.h

----------------------------------分割線------------------------------------

#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