Objective-c Calling subclasses by baseclass - objective-c

I have an attack method I am trying to implement in class 'Enemy', in which it will call upon the subclasses of the class 'Paragon'.
The subclasses are created via #interface in the .h file, and implemented in .m.
This is my current attack method in stated in the Enemy class:
-(void)chosenParagonForAttack:(Paragon*)paragon{
_paragonLink = nil; //_paragonLink, is an object of class Paragon.
_paragonLink = paragon;
[self attackParagon];
[paragon underAttackByEnemy :self];
}
The problem I am getting is that since the actual paragon is created by a subclass, the attack is not occuring. The subclasses define a different "paragon", with differing images and data etc.
Is there a way to call the subclasses just stating the baseclass (Paragon)?
I can work around this by creating an if statement containing every subclass of the Paragon class, however I am interested to find out if there is a better way to go about this.
I was thinking that 'isSubclassOfClass' or 'isKindOfClass' would sort it out but I haven't been successful using them.
Thanks.

This sounds like a general object-oriented design problem, rather than something that's specific to Objective-C.
You shouldn't use a series of if statements that are aware of every Paragon class. Doing so would mean that your Enemy class is tightly-coupled to every Paragon subclass: if you ever wanted to add a new Paragon subclass, you'd also have to update Enemy.
Instead, each of your Paragon subclasses should override the underAttackByEnemy: method and probably any methods on Paragon that are called from attackParagon. That way, your sub-classes will know that the attack is occurring and can perform any logic that's specific to them while allowing Enemy to only know about Paragon, rather than all of Paragon's sub-classes. Any attack logic that's common among all the Paragon sub-classes should be implemented in Paragon and then called from the sub-classes: for example, [super underAttackByEnemy:enemy]

Related

Objective-C - Effective Subclassing of Cocoa Class Clusters

I have an object that used to be an NSMutableSet but needed some more stuff attached to it. The obvious (and obviously not supported) thing to do is to subclass NSMutableSet and tack on the two additional properties. Since NSMutableSet, like basically all Cocoa data structures, is a class cluster I cannot subclass it in the usual way, since the super class just throws exceptions. This led me down several paths.
The first path was to create sort of a composite object that declared itself as a subclass of NSMutableSet but really just forwarded the invocations to an internal NSMutableSet. I didn't want to have to implement every method on NSMutableSet, so I thought forwardInvocation: would be a good way to accomplish my mission. Unfortunately, the abstract class of NSMutableSet implements all of the methods on the interface and their implementations throw exceptions, so I was never getting to the point where I could forward an invocation.
The second path was to subclass NSProxy and forward the invocation from there. This solution falls short in that I need to copy the interface of NSMutableSet over unless there's a way to declare "this class implements this interface" that I don't know about (this could very well be the solution).
The third path was to create a category on NSMutableSet and import it just for the class that needs to use it but that falls short since you cannot add non-dynamic properties via a category. That led me to using associated objects in a category. I'm willing to admit that that is the correct solution for this use case, but I wish it weren't since it's kind of clunky. It's doubly clunky since the properties I'm adding are primitive so I'll have to wrap and unwrap them when setting and getting the association (unless there's a way to associate primitives which I'm unfamiliar with).
Essentially, what I would like is something that behaves functionally as a subclass of NSMutableSet (and all class clusters) but cannot figure out the best approach. Thanks!
Trying to subclass Cocoa class clusters will just create an awful lot of hurt. It may seem a good idea, but you will forever run into problems.
Just create an NSObject with an NSMutableSet as the first member object.
Subclassing Cocoa class cluster is kind of discouraged. Not without reasons. Please do not enter this crashy world.
Either of your solutions will work. I've successfully used the first path with NSArray and NSDictionary, so I believe it should work fine for NSMutableSet as well. Just remember that you need to override not only forwardInvocation:, but a few of other methods as well. Please consult Surrogate Objects sections of Apple docs:
Although forwarding mimics inheritance, the NSObject class never confuses the two. Methods like respondsToSelector: and isKindOfClass: look only at the inheritance hierarchy, never at the forwarding chain.
https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtForwarding.html
In my case, I've overridden:
conformsToProtocol:
isKindOfClass:
isMemberOfClass:
respondsToSelector:
instancesRespondToSelector:
forwardInvocation:
methodSignatureForSelector:
instanceMethodSignatureForSelector:
from which isKindOfClass:, conformsToProtocol: and respondsToSelector: are definitely crucial.
I've also used the third path with good results, but I admit the associated objects API is clunky.
First, gnasher729 is correct. Don't subclass class clusters. Just don't do it. Can you do it? If I tell you that you can't, will it help you convince yourself that you shouldn't? I can lie if it helps you make good choices.
But in all seriousness, it is almost always meaningless as well. Is your subclass really a specific kind of set? Or is it really kind of like a set. Consider NSAttributedString. It isn't a kind of string, it has-a string. This is almost always better.
And also, class clusters happen to be a royal pain to subclass.
That said, adding associated values onto a data structure, as you've already discovered, is generally just fine, because what you really want is "hey, I have some data that needs to go along with this other data." Wrapping has gotten so easy that it shouldn't really slow you down. See https://stackoverflow.com/a/14918158/97337:
objc_setAssociatedObject(self, animatingKey, #(value), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
And with "one weird trick", you can make this really easy:
#interface NSObject (BoolVal)
#property (nonatomic, readwrite, assign) BOOL boolVal;
#end
#implementation NSObject (BoolVal)
- (BOOL)boolVal {
return [objc_getAssociatedObject(self, _cmd) boolValue];
}
- (void)setBoolVal:(BOOL)value {
objc_setAssociatedObject(self, #selector(boolVal), #(value), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
#end
But I'd still come back to the question of whether this is really a kind of set (rather than just like a set), and whether it really needs to respond to every message that can be sent to a set. As with NSAttributedString, your real needs are often much smaller than that in practice, and wrapping the handful of methods you need is often worth the simplicity and control.
For completeness, let's look at your first path:
create sort of a composite object that declared itself as a subclass of NSMutableSet but really just forwarded the invocations to an internal NSMutableSet
Can you subclass an NSMutableSet? Yes, but should you? The documentation for NSMutableSet says:
Subclassing Notes
There should be little need of subclassing. If you need to customize behavior, it is often better to consider composition instead of subclassing.
So weigh that up and if you want to subclass refer again to the documentation:
Methods to Override
In a subclass, you must override both of its primitive methods:
addObject:
removeObject:
You must also override the primitive methods of the NSSet class.
And looking at the NSSet class documentation we find its primitive methods are:
Methods to Override
In a subclass, you must override all of its primitive methods:
count
member:
objectEnumerator
That's it, 5 methods.
You can define your own class as a subclass of NSMutableSet, add an instance variable which is an instance of NSMutableSet, implement 5 methods and redirect them to the set instance, add whatever init methods you wish, and then add your additional properties.
If performance is of concern then the tradeoff is between redirecting those five methods and accessing associated objects for your additional properties. You'll need to profile to work that out, but if and only if performance becomes an issue.

confusion over Objective-c views, delegates, and outlets

I am trying to teach myself objective-c, but coming from a Python/Java background, it is proving very difficult. I tried to post a bunch of points I am confused on, but it was marked as too vague, so I'll break it into sections.
First, every example of a delegate and outlet I've found uses cocoa view code to get the idea across. Unfortunately, I don't yet understand enough of the code to grasp the example's point. So, can anyone provide a more basic example of a delegate? My understanding is that it is a way of subclassing; how is this better than traditional subclassing? Why does every cocoa project automatically include appDelegate.m? Can delegates be used for other purposes, not just GUI stuff?
Okay, I think I see. So a delegate is a class that conforms to the protocol of some other class. A protocol is simply a list of methods and variables that have to (or can, if set to optional) be implemented in the delegate class. To make a delegate, you have to use the #interface keyword, then the name of your delegate, then (in < > signs) the protocol name? So if class c1 wants to set itself up as a delegate of class c, class c must first specify a protocol, right? You would then be able to implement everything in c's protocol in c1:
#interface c1;
I feel like I'm missing some bits there, but hopefully I have the concepts right. :) This also explains the mysterious less- and greater-than signs; they declare what interface the delegate implements.
Outlets are similarly always tied to view code. They seem to be some kind of inter-object messaging system. Is that right? Again, an example of a basic outlet that is not mixed in with complex GUI statements would be great.
So outlets are never needed? I know that IBOutlet and IBAction are not needed except for use with Interface Builder, but I thought outlets were more general than that? The docs seemed to indicate that they are not even specifically for interfaces, but could be used for anything.
Thanks in advance for any help.
Update: Delegates don't have to conform to protocols. Protocols just make it easier to require some classes to have methods. It allows you to know for certain an object one has set as a delegate implements a certain method so you can call it safely, and allows the compiler to verify that method is indeed implemented (if you declare a delegate instance variable as id<SomeProtocol> delegate, the compiler will give a warning or error if you try to set delegate to an object of a class that doesn't conform to SomeProtocol.
Protocols help ensure safety, but they're not strictly necessary. A class can have a delegate (or multiple!), and they don't have to conform to any protocols at all.
As for outlets, no, they're specifically and only used with Interface Builder. The IBOutlet and IBAction keywords have no effect on code (they're even stripped out before compile time) - they're only markers for Interface Builder to look for so it knows which properties and methods should be accessible within the interface. The term 'outlet' is a direct reference to something marked as an IBOutlet, and is really not used in any other context that I can tell.
Again, it's okay if you don't understand this right away. Think it over a bit, and at some point, it'll just 'click'. I was caught up on delegates for a long time, just like this, before one day, I realized that delegates really aren't any special. They're regular objects referenced by other objects - it's just that this design pattern has a special name (delegation), and these objects are only called delegates. They could just as easily be called gyros or falafels, and the net effect would be the same. :P
You don't need to name an object delegate for it to be a delegate; it's just a convention.
About delegates: the first thing to understand, and this got me for a while until I had the proper "Aha!" moment, is that there is nothing special about a "delegate". The word "delegate" is just a title for a type of object that another class depends on, very often for content or decision-making. A developer will use a delegate when they don't want to (or can't) tie one of their classes to another class by name - it's an Object-Oriented way of decoupling and making classes more generic.
Now, very often, classes will require delegates to have specific methods they rely on, and one way to ensure that is with a protocol (more commonly known as an interface in Java). Protocols define a list of methods; classes "conform" to a protocol if they declare they do in their interface (e.g. #interface IFObject : NSObject <SomeProtocol> { ... }) and if they implement all the methods they're required to. Protocols can have optional methods as well.
This model is used often with view controllers, views, and the GUI in general because many AppKit and UIKit classes are written to be as generic as possible. NSTableView, for instance, implements the most basic behavior it can possibly implement without requiring any implementation-specific information; for the rest, it relies on other objects, ones that conform to the NSTableViewDelegate and NSTableViewDataSource protocols. Any object can conform to the protocols, as long as they implement the right methods (and in this case, a controller class will usually implement methods from both protocols, but it doesn't have to be so). In fact, one easy way to understand this topic better is to take a look at NSTableView - it's got a delegate property and a dataSource property, but in effect, they're no different. delegate could be called monkeyButt, and the concept would still work. The key is to not treat delegates as a black box - there's nothing special about them.
Delegates can be also used for non-GUI purposes; one concrete example, as you mention, is the app delegate. NSApplication sends a delegate notifications to let it know when the application has been launched (among other things), so it can set up shop. Again, any object can be a delegate to any other object, for any purpose. It's simply a convention.
Briefly about outlets: as others have mentioned, outlets are simply connections between an interface defined in an XIB and your code. They're a way of letting Xcode link up the interface to the appropriate elements so that when your application loads the interface file, it can load up the right pieces of code or execute them.
They're generally an easier way of setting up an interface - they're not strictly necessary (you can create an interface programmatically, without using an XIB file), but if you do decide to go the XIB route, they're how you relate your interface to your code.
A delegate is an object that is delegated some task by an object it declares itself the delegate of. Let's say an object does some task and then needs to notify the object that "owns" it, so to speak. In order to allow the object to work under any conditions, it cannot be allowed to know what type of object it should contact, so we set its delegate and adhere to its protocol. Setting the object's delegate is like saying "Here's the object I want you to use contact with the messages declared in your protocol. I promise the object actually implements the methods in the protocol." For example, you might see this.
#protocol SpriteDelegateProtocol
#required
- (void) projectionMatrix;
#optional
- (void) animation:(int)animationIndex willCompleteFrameNumber:(int)frame forSprite:(id)sender;
#end
#interface Sprite
#property (nonatomic, assign) id<SpriteDelegateProtocol> delegate;
#end
The interface for our sprite object declares that it has a property called a delegate, which is of type id. It can be any object, but it must adhere to the protocol in the triangle brackets. If it says it adheres to the protocol, it has to implement the methods declared under the #required and can implement the ones listed under the #optional. It's up to us to determine whether or not our delegate actually implements the optional methods using something like respondsToSelector:. We might even store the return value if we had a lot of methods to do that for.
The application delegate is special in that it is the delegate to our UIApplication object. It receives messages on application state, such as applicationDidEnterBackground:. You can see what methods are in the protocol adhered to by our application delegate here.
Delegates can be used with any object. Any object can say it has a delegate property to set, and that it must have the following methods as shown above. The protocol is essentially a portable interface that can be used to tell another object what we need it to implement so that we can call said methods to delegate some portion of functionality out to it. We can notify our delegate when a task has completed, ask it to give us information (typically called a data source instead of a delegate so that they can be different objects if you want) or ask it whether or not we should do some task at all.
An outlet is a way of connecting a view instantiated in a NIB or storyboard to a property in its controller. So, if you place a UIViewController into your main storyboard and change its class to MyGreatControllerSubclass then proceed to place a button in that controller's view, you might want to hook that button up to an "outlet" (property) in the controller's interface so that you can access the button from the controller later.
No delegates are not a way of subclassing. I also came from Java and have been doing O-C for almost 5 years now.
Delegates generally conform to Protocols, which are the equivalent of Interfaces. What's great about that is it leaves you a lot of flexibility in how you implement the things that are covered by the delegate. So for instance, you will see that Table Views can be implemented by extending UITableViewController or implementing the delegate. That way, if you have a composed view where a user is selecting items from one table and putting them into another, you can do a single controller and have it perform the delegation for both views.
The best way to think about delegates is as examples of message/event oriented callbacks. Have you ever heard the saying that what makes a Framework different from ordinary programming is, with a Framework, it tells you what it wants you to do to ensure the results you seek? That is what delegation is. I was doing a Collection View last night and could not get the edit menu to come up. Just provide delegation of three methods, know nothing about how the menu is invoked, where the Long Press gesture handler is, etc.
Note that Delegates are the glue that makes Dependency Injection unneeded in the O-C world. I prefer them.
Delegates are the common design pattern (http://en.wikipedia.org/wiki/Delegation_pattern), they are not objective-c specific.
Outlets make possibly the binding of Controller and View (GUI). May be you know the MVC paradigm (model-view-controller)? So you build your GUI (the View from MVC) with Interface Builder and bind the elements of this GUI such as Buttons, Text fields and so on to your Obj-C code (the Controller from MVC) using outlets.
Take a look at this post: Conforming Objective-C’s Protocols in C# Codes
Don't worry about the word "C#" in the title, you could just ignore that part. The main part of this post talks about protocols and delegates in Objective-C. This might be helpful.

Sending messages to objects?

I have done this successfully many times before, but this time it just won't work.
The code in my .h file:
#interface Images : NSView {
}
- (void) draw;
#end
The .m file (of another object):
- (IBAction) go: (id) sender; {
[Images draw:nil];
}
When I try to compile this, it says the following:
'*' may not respond to '*
Images may not respond to '+draw'
This has me quite confused. Shouldn't this be working?
Please Note: Yes, I have seen the other questions about messages, but none of those answers was even partially helpful for me. After reading them, I was even more confused.
Your draw method is an instance method: it can only be called on instances of the Images class. In your go: method you're trying to call it as a class method—if this is what you want, change it to:
#interface Images : NSView
+ (void)draw;
#end
I think a review of some of the basic concepts of object-oriented programming is in order; namely the difference between a class and an object or instance.
A class, in the general sense, is a collection of data and the functions which act upon it. A class defines the interface that one uses to access and manipulate data that is logically grouped together, and serves as a blueprint for creating objects or instances. See http://en.wikipedia.org/wiki/Class_(computer_programming)
Instances of a class (objects) are the typical things you manipulate in an object-oriented program, and they are created from the class "blueprint" and follow the behavior as specified by the class.
A typical example would be a fruit- take apples for example. An imaginary Apple class would represent all apples in the general sense and would model properties such as color and size and actions such as wash and eat. An instance would represent one, single physical apple- a Granny Smith or Pippin or whatever variety.
Just as it doesn't make sense to wash or eat apples in the general sense of the word (the concept of apples, not the agglomeration), typically it doesn't make sense to tell a class what to do. You tell objects (individual apples) what to do.
The code you present above defines the class Images. The "-" in front of -(void)draw indicates that the draw method only exists for specific objects. It is an instance method in typical OO parlance.
Of course, in Obj-C it is also possible to send a message to a class without requiring an object. This is denoted by a "+" in front of the method name, as other answers indicate. This is called a static method and it typically used to control some shared behavior or aspect of all objects of that particular class.
The problem with your code is that you are declaring -(void)draw as an instance method but calling it as a static method. Which way you want to do things is up to you, and it's difficult to determine from your code what the intent of the Images class is.

How does inheriting from NSObject work?

There are a couple of things about Objective-C that are confusing to me:
Firstly, in the objective-c guide, it is very clear that each class needs to call the init method of its subclass. It's a little bit unclear about whether or not a class that inherits directly from NSObject needs to call its init method. Is this the case? And if so, why is that?
Secondly, in the section about NSObject, there's this warning:
A class that doesn’t need to inherit any special behavior from another class should nevertheless be made a subclass of the NSObject class. Instances of the class must at least have the ability to behave like Objective-C objects at runtime. Inheriting this ability from the NSObject class is much simpler and much more reliable than reinventing it in a new class definition.
Does this mean that I need to specify that all objects inherit from NSObject explicitly? Or is this like Java/Python/C# where all classes are subtypes of NSObject? If not, is there any reason to make a root class other than NSObject?
1) Any time an object is allocated in Objective-C its memory is zeroed out, and must be initialized by a call to init. Subclasses of NSObject may have their own specialized init routines, and at the beginning of such they should call their superclass' init routine something like so:
self = [super init];
The idea being that all init routines eventually trickle up to NSObject's init.
2) You need to be explicit about the inheritance:
#instance myClass : NSObject { /*...*/ } #end
There is no reason to have a root class other than NSObject -- a lot of Objective-C relies heavily on this class, so trying to circumvent it will result in you needlessly shooting yourself in the foot.
Since it is possible to inherit from different root base classes, yes you must explicitly declare you inherit from NSObject when you make any new class (unless of course you are subclassing something else already, which itself in turn probably subclasses NSObject).
Almost never is there a need to make your own base class, nor would it be easy to do so.
Objective-C can have multiple root classes, so you need to be explicit about inheritance. IIRC NSProxy is another root class. You'll likely never want or need to create your own root class, but they do exist.
As for calling NSObject's init, it's part custom and part safety. NSObject's init may not do anything now, that's no guarantee that future behaviour won't change. Call init to be safe.
You need to call [super init] because there is code behind initializing that you dont have to write because it is written for you in NSObjects init, such as probably actual memory allocation etc.

Objective C subclass that overrides a method in the superclass

In Objective C, if you are subclassing something, and are planning to override a method on the superclass, should you re-declare the superclass method in your subclass #interface?
For example, if you are subclassing UIViewController (e.g. MyViewController), and you are planning to override "viewDidLoad" should you include that method in your MyViewController #interface declaration, or just implement it in MyViewController.m?
In examples I've come across, I've seen it done both ways (re-declaring the method in your subclass interface, or not re-declaring the method). There may not be any functional difference, but what is the best practice?
I often declare methods that I plan to override in either the public header or at least in a private category. The benefit to this is that you'll get an incomplete class definition warning if you forget to actually override the method... which comes in handy from time to time.
As for when to place it in the public header, that's pretty subjective and probably up to you/your team's coding styles. I usually only redeclare a method in the public header if I plan to radically change what the method is going to do or if I plan not to invoke the super class's version of the method.
People often use the header as documentation for the class (and tools like AutoDoc support this). Obviously, if you're following that convention, the only sensible choice is to include redefined methods so you can explain what you've done with them. Otherwise your docs for the class are either incomplete or scattered to the four corners of the earth.
But if we're just copy-pasting the declaration, I don't personally like to redeclare methods. It's not DRY and it bloats your header unnecessarily. Less code is better code.