子类化Objective C类方法

Question

I have a question regarding Subclassing and Class methods.

I have a base class MyBaseClass which has a convenience class method

+ (id)giveMeAClassUsing:(NSString *)someParameter;

MyBaseClass is not a singleton.

Now, I wish to create a subclass of MyBaseClass , let's call it MyChildClass . I wish to have the same class method on MyChildClass as well. Additionally, I also wish to initialize an instance variable on MyChildClass when I do that.

Would doing something like this:

+ (id)giveMeAClassUsing:(NSString *)someParameter {

      MyChildClass *anInstance = [super giveMeAClassUsing:someParameter];
      anInstance.instanceVariable = [[UIImageView alloc] initWithFrame:someFrame];

      return anInstance;
}

be valid?

Thanks for all your help (in advance) and for resolving my confusion and clarifying some concepts!

Cheers!

Answer 1

That will work fine.

Possibly better would be to define your convenience constructor in such a way that you don't need to override it:

 + (id)myClassWithString: (NSString *)string {
     return [[[self alloc] initWithString:string] autorelease];
 }

This will do the right thing no matter which of your superclass or any of its subclasses it is called in.

Then change just the initWithString: method in your subclass to handle the initialization:

- (id)initWithString: (NSString *)string {
    return [self initWithString:string andImageView:[[[UIImageView alloc] initWithFrame:someFrame] autorelease]] ;
}

Answer 2

Absolutely that's valid.

One note though is in the superclass, reference the class itself with self , rather than the superclass by name.

This is bad:

// MySuperClass // BAD :(
+ (id)giveMeAClassUsing:(NSString *)someParameter {
  return [[[MySuperClass alloc] initWithParam:someParameter] autorelease];
}

But this is good!

// MySuperClass // GOOD! :D
+ (id)giveMeAClassUsing:(NSString *)someParameter {
  return [[[self alloc] initWithParam:someParameter] autorelease];
}

Otherwise when you subclass, and then call super you aren't actually initializing the right class. Use of self allows the class being instantiated to vary without overriding the class method.