Objective-C:静态字段和实现单例模式

Question

美好的一天,朋友们.

从新手回来关于Obj-C的愚蠢问题:)

我正在尝试在Obj-C中实现单例设计模式:

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

编译器返回错误:"在'静态'之前的预期说明符 - 限定符列表".

Answer 1

您不能在类接口声明中使用static.单例应该在文件中声明为静态独立变量.m.我通常这样做(如果我觉得我不能避免单身):

@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 2

请在下面找到我正在使用的Objective-C代码片段,以获得正确的线程安全单例实现

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 -

实施文件:

/*
 * 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 -

只是为了帮助您从objective-c开始而不是迷失在您的项目结构中,您可以考虑让项目结构与您的文件系统相匹配,这样您的项目就会变得更大,您不会迷路.

另外请考虑使用适当的类命名约定,并坚持下去.

我向你提供我的样本:

• 任何匹配单例模式的类都使用Manager后缀(例如MyCustomManager)命名.

• 任何静态类都使用Helper后缀命名(例如MyCustomHelper).

• 
任何专用于控制特定进程的类都使用Controller后缀(例如MyParticularTaskConstroller)命名.


• 从其他控件继承的任何UI控件都需要提供控件后缀(例如,继承自UITableViewCell的MyCustomDetailCell)

希望这可以帮助.