iOS 单例的实现

1、GCD
static InstanceClass *instance;
+ (InstanceClass *)defaultInstance{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[InstanceClass alloc] init];
    });
    
    return instance;
}
2、@synchronized
static InstanceClass *instance;
+ (InstanceClass *)defaultInstance{
    @synchronized (self){
        if (instance == nil) {
            instance = [[InstanceClass alloc] init];
        }
    }
    
    return instance;
}

3、
#define SYNTHESIZE_SINGLETON_FOR_CLASS(classname) \
\
static classname *shared##classname = nil; \
\
+ (classname *)shared##classname \
{ \
@synchronized(self) \
{ \
if (shared##classname == nil) \
{ \
shared##classname = [[self alloc] init]; \
} \
} \
\
return shared##classname; \
} \
\
+ (id)allocWithZone:(NSZone *)zone \
{ \
@synchronized(self) \
{ \
if (shared##classname == nil) \
{ \
shared##classname = [super allocWithZone:zone]; \
return shared##classname; \
} \
} \
\
return nil; \
} \
\
- (id)copyWithZone:(NSZone *)zone \
{ \
return self; \
}\
- (id)retain\
{\
return self;\
}\
- (unsigned)retainCount\
{\
return UINT_MAX; \
}\
- (oneway void)release\
{\
}\
- (id)autorelease\
{\
return self;\
}
/*
warning: Semantic Issue: Conflicting distributed object modifiers on return type in implementation of 'release'
You need to declare it oneway.


- (oneway void) release {}
oneway is a keyword used with distributed objects to indicate that the call can be made asynchronously. Since the NSObject header uses it when it declares the release method, you must also use it. It won't affect your program unless you use distributed objects, but it will satisfy the compiler.


in your .m file rather than amending your existing


- (void) release {
line with the extra word "oneway".


This would be why you get "Duplicate declaration of release". Yes, this is confusing because it's a duplicate definition that is invisibly creating the duplicate declaration. But I've just tried doing it your wrong way, and I get that "duplicate declaration" message.


I get the impression, perhaps wrongly, that you didn't realise you actually had a release method, particularly when you think adding the line will "release this warning".


Don't take all errors too literally, and always try to think what someone might really mean as it's often different from what they say, but do try and understand what is in your code, even in the classes you've taken off the shelf.


And to address other questions raised, the reason you're overriding release is because it is a singleton which is not usually released. You probably only have a definition in your code, which will suffice.


What Jonathan Grynspan has to say about specifying on both the declaration and the definition is broadly valid (and indeed the root of the issue) but it's important to recognise that in this specific case, the declaration is by Apple's foundation code which has changed.


So, if it's not clear already, amend the line that XCode finds problem with to include the word oneway.


*/