We always make singleton object through Class method.What will happened if i make that method instance(use - instead of +) and call that method through a nil object or simply through an object?
Conceptually a the instance method is not attached to any specific instance of a class.
This means that from a design point of view it doesn't make sense to invoke it on an existing object. It's like requiring that a +(int) sum:(int)a with:(int)b should be an instance method when it doesn't need to know anything about an object.
In addition usually a singleton class constructor shouldn't be accessible from clients, if you need to instantiate one to call -instance on it this would defeat the purpose.
Last thing, a singleton should be the only possible instance of an object, if you need to have one to instantiate one then you are in a deadlock: you need an instance to create an instance but you are no allowed to have more than an instance.
Regarding calling it on nil object, if I remember correctly a pointer returned from a message sent to nil will always be nil too.
Related
I'm working with game center and wanted to have a singleton class for accessing the GK functionality which I've setup, but I then introduced a couple of methods which needed a delegate. Obviously delegates can't really work properly with a singleton, but I want/need the data loaded in this class to be loaded once and be there all the time.
Is there a nice way that I'm missing of keeping the data there all the time, but having the class instantiated as and when it's needed?
Yoy say "singleton class", and by that I assume you mean that this class only has class methods. That's fine, you can still use it, since class objects are still objects. That said, you will probably need to maintain state. Each delegate call will include some parameter that allows the object to identify the sender.
What I would probably do myself is create a NSMutableDictionary in an "initialize" method, then have objects register themselves before sending delegate methods, and when they register create another mutableDictionary, and save that in the first one with the sending object as the key (or some other unique identifier).
Every delegate call has to include the sender, and with that you can retrieve the dictionary associated with that object.
I'm struggling to forward a class method through a facade class.
To clarify, I'm overriding all the following methods:
-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
-(void)forwardInvocation:(NSInvocation *)anInvocation
+(BOOL)instancesRespondToSelector:(SEL)aSelector
+(NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector
+(IMP)instanceMethodForSelector:(SEL)aSelector
+(BOOL)resolveClassMethod:(SEL)sel
+(BOOL)resolveInstanceMethod:(SEL)sel
.. and yet, for the class method, the only one to be called is +resolveClassMethod. From there, I immediately get an unrecognized selector exception regardless of whether I return YES or NO.
What's going on?
Does class message forwarding work differently to instance message forwarding?
Similarly, why isn't there a +forwardInvocation class method?
Any help would be greatly appreciated.
So you already know that to make an object do forwardInvocation for instance methods, you have to implement the instance methods -forwardInvocation: and -methodSignatureForSelector: (the others are unnecessary). Well, class methods are just methods on the class object. Classes are objects and work like any other objects, and support all the instance methods of the root class (NSObject in this case).
The Objective-C runtime doesn't care that an object is a class object or non-class object. The message forwarding mechanism is the same. So when you send a message to an object, whatever it is, and it can't find it, it just looks for the forwardInvocation: and methodSignatureForSelector: methods on the object. So you need to have these methods on your class object.
i.e. implement the class methods +forwardInvocation: and +methodSignatureForSelector:
I have seen the Apple's example of Singleton and couple of other examples.
People say that it is too strict!
But the point is ..even if it is too strict, I want to understand it.
I dont understand that when we call allocWithZone on super, What happens ?
Memory will be created according to super's instance size.
What if our Singleton has ivars ?
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW32
I also dont understand that, Why does allocWithZone return the object with a retain call when retain itself is returning the object as it is.
Whenever a method is called in Obj-C it is passed an object reference via the hidden parameter self. For an instance method self refers to the object the method was invoked on, for a class method self refers to the the class object (of type Class) the method was invoked on. Calls to super implicitly pass on self.
Therefore in Apple's example code the call [super allocWithZone:NULL] calls the super implementation of allocWithZone passing the current value of self, which is MyGizmoClass's class object as it is a static method. The implementation of allocWithZone can determine the required memory size from the passed Class object – the details of how are private.
As you've correctly spotted, the call to retain in allocWithZone is pointless but harmless.
I have a custom ViewController class and many instances of it, and I want them all to be able to message the same Model (another custom class, only one instance). Passing pointers to the Model along to new instances of the ViewController seems impractical, especially since the model is lazily instantiated. What is the cleanest, most idiomatic, ARC way to do this?
Usually a singleton in ObjC will have a class method that serves as an accessor for the single instance. The convention is for this to be called either defaultX or sharedX. If your model class is indeed a singleton, you should already have such a method. Since class names are globally available, all you have to do to access the instance anywhere in your program is [MyModelClass sharedModel].
I was wondering when and when not to use class methods and instance methods. I need some practical examples. I am really confused. Another question: can't we do exactly the same things with instance methods that we can with class methods?
Class methods: The method isn't tied to any specific object. In a way it acts like a free function in the class's namespace. No 'self' pointer. For instance [UIScreen mainScreen] is a class method because there's only one screen and there's no need to care about multiple 'screen instances'.
Instance method: Tied to a specific object.
This applies to most OO languages, not just obj-C.
At the implementation level, an instance method call contains a hidden pointer to a data structure (the object), a class method does not.
The practical question to ask is whether your call requires sending the call some specific data which is or could best be encapsulated as instance data inside an object, or not.
You (usually) can do the same thing with class methods as instance methods, but then you have to explicitly pass the object as a visible parameter in the call, which is uglier looking and also potentially disables some method override features of the Objective C language.
Use class methods for utility functions and Instance methods for object oriented stuff.
Eg. For Mathematical calculation (eg sin x ) use class method. But for invoking a behavior specific to an object.. use instance method ..
A class method as the name implies is bounded to the class. You can invoke them just with the name of the particular class. These can be normally exposed methods of a class.
For example
NSArray
+ (id)arrayWithArray:(NSArray *)array;.
You call it with the class name NSArray. What you expect is just a creation of a object of the type of that particular class. This doesn't need an object to invoke. Also these are very basic method required so its better to make it as a class method.
On the other hand instance method as the name implies is very much bound to the instance. Object is an entity that encapsulates state (ivars) and behaviors (methods) of a class. This can be very specific to the object.
For example
- (NSUInteger)count;
Lets take NSArray *a and NSArray *b. If a contains 5 items whereas b contains 4, instance methods called upon these instances will produce different results. And thats why we need instances to be initialized while invoking instance method. They work on the context(or state) of the object they are been called upon. Also they are not exposed as the class methods are.
Hope this helps.
If you want to use instance objet or instance variable you have to go with instance Methods.
Bcz Inside the class you cant access the Instance instance objet or instance variable.
Class methods are static methods.