Objective-C：静态字段和实现单例模式

Question

好日子，朋友。

有关对象 - 从新手:)

我想实现的OBJ-C单例设计模式再一次愚蠢的问题：

@interface SampleSingleton : NSObject { 
@private 
    static SampleSingleton* instance; 
} 
+(SampleSingleton*) getInstance; 

编译器返回错误：“预期说明符限定符在'静态'之前列出。

Answer 1

请在下面找到我使用的Objective-C的代码片段，对正确的线程安全的单身实现

头文件：

/* 
* 
* Singleton interface that match Cocoa recommendation 
* @ http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW32 
* extended with thread-safe pattern 
*/ 
@interface MyCustomManager : NSObject { 
} 

#pragma mark Singleton Thred-Safe Pattern 

+ (MyCustomManager *) sharedInstance; 
+ (id)allocWithZone:(NSZone *)zone; 
- (id)copyWithZone:(NSZone *)zone; 
- (id)retain; 
- (NSUInteger)retainCount; 
- (void)release; 
- (id)autorelease; 

#pragma mark - 

实现文件：

/* 
* My custom manager Class singleton implementation 
*/ 
@implementation MyCustomManager 

#pragma mark Initializers 

/* 
* specific initialize goes here 
*/ 
- (void) specificInitialize 
{ 
    // ... 
} 

/* 
* Ensure any owned object is properly released 
*/ 
- (void) dealloc 
{ 
[super dealloc]; 
} 

#pragma mark - 

#pragma mark Singleton Thred-Safe Pattern 

//- use Volatile to make sure we are not foiled by CPU caches 
static void * volatile sharedInstance = nil;             

/* 
* retrieve sharedInstance based on OSAtomicCompareAndSwapPtrBarrier that 
* acts as both a write barrier for the setting thread and a read barrier from the testing thread 
* more info @ http://stackoverflow.com/questions/145154/what-does-your-objective-c-singleton-look-like/2449664#2449664 
* and http://stackoverflow.com/questions/6915/thread-safe-lazy-contruction-of-a-singleton-in-c/6943#6943 
*/ 
+ (MyCustomManager *) sharedInstance { 
    //- check sharedInstance existenz 
    while (!sharedInstance) { 
     //- create a temporary instance of the singleton  
     id temp = [super allocWithZone:NSDefaultMallocZone()]; 
     //- The OSAtomicCompareAndSwapPtrBarrier function provided on Mac OS X 
     //- checks whether sharedInstance is NULL and only actually sets it to temp to it if it is. 
     //- This uses hardware support to really, literally only perform the swap once and tell whether it happened. 
     if(OSAtomicCompareAndSwapPtrBarrier(0x0, (void *)temp, &sharedInstance)) { 
      //- compute singleton initialize 
     MyCustomManager *singleton = (MyCustomManager *) sharedInstance; 
      [singleton specificInitialize]; 
     } 
     else { 
      //- if the swap didn't take place, delete the temporary instance 
      [temp release]; 
      temp = nil; 
     }                         
    } 
    //- return computed sharedInstance 
    return sharedInstance;                   
} 

/* 
* method to ensure that another instance is not allocated if someone tries to allocate 
* and initialize an instance of your class directly instead of using the class factory method. 
* Instead, it just returns the shared object. 
*/ 
+ (id)allocWithZone:(NSZone *)zone 
{ 
    return [[self sharedInstance] retain]; 
} 

/* 
* Implements the base protocol methods to do the appropriate things to ensure singleton  status. 
* Applies to memory-managed code, not to garbage-collected code 
*/ 
- (id)copyWithZone:(NSZone *)zone 
{ 
    return self; 
} 

/* 
* Implements the base protocol methods to do the appropriate things to ensure singleton status. 
* Applies to memory-managed code, not to garbage-collected code 
*/ 
- (id)retain 
{ 
    return self; 
} 

/* 
* Implements the base protocol methods to do the appropriate things to ensure singleton status. 
* Applies to memory-managed code, not to garbage-collected code 
*/ 
- (NSUInteger)retainCount 
{ 
    return NSUIntegerMax; //denotes an object that cannot be released 
} 

/* 
* Implements the base protocol methods to do the appropriate things to ensure singleton status. 
* Applies to memory-managed code, not to garbage-collected code 
*/ 
- (void)release 
{ 
    //do nothing 
} 

/* 
* Implements the base protocol methods to do the appropriate things to ensure singleton status. 
* Applies to memory-managed code, not to garbage-collected code 
*/ 
- (id)autorelease 
{ 
    return self; 
} 

#pragma mark - 

只是为了帮助你在Objective-C中开始并且不会迷失在你的项目结构中，你可以考虑让项目结构与你的文件系统相匹配，以便你的项目变得更大你不会迷路。

也请考虑使用适当的类命名约定，并坚持下去。

我一个为您提供矿山为样本：

• 符合单例模式是使用管理后缀（例如MyCustomManager）命名任何类。

• 任何静态类都使用助手后缀（例如MyCustomHelper）命名。

• 任何专门用于控制特定进程的类都使用Controller后缀（例如MyParticularTaskConstroller）来命名。

• 从另一个控制需要继承任何UI控件提供控制后缀（例如MyCustomDetailCell自定义UITableViewCell继承）

希望这有助于。

Answer 2

静态SampleSingleton* instance;行不能进入@interface部分。大多数人把它放在上面。

Objective-c并不适用于单例模式以及其他一些语言。然而在this question中有很多不同的实现。有些人认为辛格尔顿根本不是一个很好的模式，我试图让自己放弃使用它 - 但这是我的选择。

Answer 3

您不能在类接口声明中使用静态。单身人士应在.m文件中声明为静态独立变量。我通常这样做（如果我觉得我不能避免一个单）：

@interface SampleSingleton : NSObject 
{ 
@private 
} 

+(SampleSingleton*) theSingleton; 

@end 

// .m file 

@implementation SampleSingleton 

+(SampleSingleton*) theSingleton 
{ 
    static SampleSingleton* theSingleton = nil; 

    if (theSingleton == nil) 
    { 
     theSingleton = [[SampleSingleton alloc] init]; 
    } 
    return theSingleton; 
} 

Answer 4

请看看我的问题here和尼克DeMoore精彩的答案（有很多的意见和代码修复）。拥有一个可以在IB中连接的单例（无论你在XCode 4中调用它）是非常有用的。

很酷的事情是，你可以使用同一个Singleton，并将它的一些插座连接到一个NIB，并将它的一些插座连接到另一个插座......因为它实际上是一个单例，整个事件中只能有一个实例运行时系统。工作得非常好。

注意：每次你使用辛格尔顿的人会说这是一个坏主意。

Answer 5

这是怎么了，我通常实现一个单身方法

+(SampleSingleton *)sampleSingleton 
{ 
    static SampleSingleton * theSampleSingleton = nil; 
    if(theSampleSingleton == nil) 
     theSampleSingleton = [[SampleSingleton alloc] init]; 
    return theSampleSingleton; 
} 

，使这个线程安全的，你会做

+(SampleSingleton *)sampleSingleton 
{ 
    static SampleSingleton * theSampleSingleton = nil; 
    if(theSampleSingleton == nil) 
    { 
     @syncronise([SampleSingleton class]) 
     { 
      if(theSampleSingleton == nil) 
       theSampleSingleton = [[SampleSingleton alloc] init]; 
     } 
    } 
    return theSampleSingleton; 
} 

也是而是采用了单，你已经在形式的单的UIApplicationDelegate，你可以随时添加一个方法给你委托从你的委托中获取SampleSingleton。

另一个要考虑的问题是单身人士需要强制执行单身人士，UIApplication有一个共享应用程序来执行创建单身人士的功能，但是没有什么能真正阻止您创建新的实例。