Selector for class (static) method

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Selector for class (static) method

(14 posts) (5 voices)

Started 2 years ago by Decept
Latest reply from ob1

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Decept
Member

I'm creating an object factory in Obj-C and I really need a selector for a class (static) method. Is it at all possible?

I was thinking about storing a selector to a class method that creates objects of that class. Something like this:

@interface Tank : NSObject
+(id) createObject;
-(void) shoot;
-(void) move;
@end

then have a selector point to the create method: @selector(createObject);

I would then have a NSMutableDictionary with @"Tank" as key and the selector (or a thin wrapper for it) as the object.

The end result would be something like this:
// Add an object type
[GlobalFactory addTypeWithName:@"Tank" selector:@selector(createObject)];
// Create an object
id object = [GlobalFactory create:@"Tank"];

Posted 2 years ago #
Clain
Member

It should be possible. Look for SEL in Xcode's documentation.

Posted 2 years ago #
Codemattic
Moderator
Why do you need the dictionary? If you have a string @"Tank" and want to create it then:
```
mySpriteClass = @"Tank"
...
[NSClassFromString(mySpriteClass) createObject];
```
assuming every class you create has the -createObject.

Unless a class may have several class creation methods which you want to choose between, ex:
```
@interface Tank : NSObject
+(id) createBasicTank;
+(id) createAggressiveTank;
-(void) shoot;
-(void) move;
@end
```
In which case your dictionary isnt going to work either. What you would need then is a dictionary of dictionaries. One dictionary with a key and as its object a dictionary with a class and a method.
Posted 2 years ago #
Lam
Member

@Decept
Codemattic is right you can call the class from NSClassFromString and if you wanted a selector that is doable through NSSelectorFromString.

SEL mySelector = NSSelectorFromString(@"createObject")

So I guess those would help you

Posted 2 years ago #
Decept
Member

Thank you so much guys!

I guess I'm still stuck in my C++ world. NSClassFromString did it and as you said, without a dictonary even.
The NSSelectorFromString is also nice.

My problem is solved, but I'm still wondering about static function selector. SEL only points to a message with no regards to a specific class or instance, right? So what if I needed to pass in a static function selector to another function, for say, a callback.
If it was a callback for a specific object I could send in a target (id) aswell. Is sending the class name as a string the only way, or is there some object that can "point" to a class?

Posted 2 years ago #
ob1
Member

To point to a class you can do something like:

[MyClass class];

I think the stuff there all revolves around your question (I used it to write some factory stuff) :

http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/ObjectiveC/Articles/ocObjectsClasses.html

Posted 2 years ago #
Decept
Member

Thank you ob1, that link explained it all. I think its fantastic that a class is an object, I always wanted that in C++.

Posted 2 years ago #
ob1
Member

Yeah, I think it's based on smalltalk, with the concept of Meta classes and all that stuff.

Posted 2 years ago #
Decept
Member

I just realized something. If I have an object factory, is it really such a good idea to be able to create any class using NSClassFromString? Maybe I should add the "approved" class names to a list and check that list before allowing an object to be created.
The approved classes would in this case be that of the game objects. The negative side of this is of course that you would have to manually add all the approved classes first.

Just thought of something. Maybe I should have a GameObject class that all the game objects inherit from. Then in the object factory's createObject method I could check if the suplied class inherits from GameObject.

What do you think?

Posted 2 years ago #
ob1
Member
If you are just worried you will make typos, I would suggest defining all your strings:
```
#define kTankClass @"Tank"
```
Otherwise you can probably not use strings at all and use something like:
```
+ (id) createObject:(Class) class
{
 return [[class alloc] init];
}

[Factory createObject:[Tank class]];
```
It's what I do.

Your idea works too.
Posted 2 years ago #
Decept
Member

Not so much a typo thing. I'm going to create objects from a level file where each object's type is a string.

Posted 2 years ago #
ob1
Member

Then I don't think it really matters if you check the names. The game will crash in any case if a wrong name is given I guess.

Posted 2 years ago #
Lam
Member

@ob1
That's a pretty good idea. I'm loading up my classes using a plist but I just assumed that I'm the only person that will access and change the data file since it's bundled with the app. Maybe it's not as safe as I think...

Posted 2 years ago #
ob1
Member

The bundle is signed. And if an malicious user has a jailbroken phone, he can do what he wants with your application anyways.

Posted 2 years ago #

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