首先放一段苹果官方推荐的单例模式写法，兼容iOS 4以下的系统
Creating a Singleton Instance
Some classes of the Foundation and AppKit frameworks create singleton objects. In a strict implementation, a singleton is the sole allowable instance of a class in the current process. But you can also have a more flexible singleton implementation in which a factory method always returns the same instance, but you can allocate and initialize additional instances.The NSFileManager class fits this latter pattern, whereas the UIApplication fits the former. When you ask for an instance of UIApplication, it passes you a reference to the sole instance, allocating and initializing it if it doesn’t yet exist.
A singleton object acts as a kind of control center, directing or coordinating the services of the class. Your class should generate a singleton instance rather than multiple instances when there is conceptually only one instance (as with, for example, NSWorkspace). You use singleton instances rather than factory methods or functions when it is conceivable that there might be multiple instances someday.
To create a singleton as the sole allowable instance of a class in the current process, you need to have an implementation similar to Listing 2-15. This code does the following:
It declares a static instance of your singleton object and initializes it to nil.
In your class factory method for the class (named something like “sharedInstance” or “sharedManager”), it generates an instance of the class but only if the static instance is nil.
It overrides the allocWithZone: 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.
It implements the base protocol methods copyWithZone:, release, retain, retainCount, and autorelease to do the appropriate things to ensure singleton status. (The last four of these methods apply to memory-managed code, not to garbage-collected code.)
Listing 2-15 Strict implementation of a singleton
static MyGizmoClass *sharedGizmoManager = nil;
+ (MyGizmoClass*)sharedManager
{
if (sharedGizmoManager == nil) {
sharedGizmoManager = [[super allocWithZone:NULL] init];
}
return sharedGizmoManager;
}
+ (id)allocWithZone:(NSZone *)zone
{
return [[self sharedManager] retain];
}
- (id)copyWithZone:(NSZone *)zone
{
return self;
}
- (id)retain
{
return self;
}
- (NSUInteger)retainCount
{
return NSUIntegerMax; //denotes an object that cannot be released
}
- (void)release
{
//do nothing
}
- (id)autorelease
{
return self;
}
If you want a singleton instance (created and controlled by the class factory method) but also have the ability to create other instances as needed through allocation and initialization, do not override allocWithZone: and the other methods following it as shown in Listing 2-15.
上文指出，如果需要通过alloc开辟一个新的实例，则不要覆盖allocWithZone函数及其以下的函数。
现实中常用的写法(ARC＋GCD)
Singleton.h
@interface Singleton : NSObject
+(Singleton*)defaultManager;
@end
Singleton.m
@implementation Singleton
//单例类的静态实例对象，因对象需要唯一性，故只能是static类型
static Singleton *defaultManager = nil;
/**单例模式对外的唯一接口，用到的dispatch_once函数在一个应用程序内只会执行一次，且dispatch_once能确保线程安全
*/
+(Singleton*)defaultManager
{
static dispatch_once_t token;
dispatch_once(&token, ^{
if(defaultManager == nil)
{
defaultManager = [[self alloc] init];
}
});
return defaultManager;
}
/**覆盖该方法主要确保当用户通过[[Singleton alloc] init]创建对象时对象的唯一性，alloc方法会调用该方法，只不过zone参数默认为nil，因该类覆盖了allocWithZone方法，所以只能通过其父类分配内存，即[super allocWithZone:zone]
*/
+(id)allocWithZone:(struct _NSZone *)zone
{
static dispatch_once_t token;
dispatch_once(&token, ^{
if(defaultManager == nil)
{
defaultManager = [super allocWithZone:zone];
}
});
return defaultManager;
}
//覆盖该方法主要确保当用户通过copy方法产生对象时对象的唯一性
- (id)copy
{
return self;
}
//覆盖该方法主要确保当用户通过mutableCopy方法产生对象时对象的唯一性
- (id)mutableCopy
{
return self;
}
同样，如果需要通过alloc开辟一个新的实例，则不要覆盖allocWithZone函数及其以下的函数。