Re: [iPhone] Why can't a UITextField be its own delegate?
Re: [iPhone] Why can't a UITextField be its own delegate?
- Subject: Re: [iPhone] Why can't a UITextField be its own delegate?
- From: Kyle Sluder <email@hidden>
- Date: Sat, 25 Jul 2009 15:23:21 -0700
On Sat, Jul 25, 2009 at 3:00 PM, WT<email@hidden> wrote:
> What if the delegates are already subclassed? Since multiple inheritance is
> not an option, the code that limits the number of characters would have to
> be repeated in several places.
You always have categories, but that's not necessarily a good idea.
If you find yourself in this situation (class C needs behavior from
both class B and class A), you can always collapse the class hierarchy
and turn A and B into class AB, with options to configure its
behavior. That's how we wind up with things like NSController on the
desktop side, which can exist as an object controller or an entity
controller.
> This custom text field is still a text field in every way.
Then it should remain a UITextField.
> There is no reason for any outside entity to want to refer to
> super.delegate. In fact, unless I'm wrong, I don't think it's even possible
> to retrieve super.delegate from the custom instance.
Ooh, I think we've found your bug.
Internally, UITextField is going to use self.delegate to get its
delegate, following the correct accessor behavior. You've gone and
replaced -delegate to return self. But the delegate pattern says that
messages which this object does not understand should be forwarded to
the delegate. This means that a class with a delegate needs to
implement -respondsToSelector: this way:
- (BOOL)respondsToSelector:(SEL)aSelector {
return [super respondsToSelector:aSelector]
|| [self.delegate respondsToSelector:aSelector];
}
Since -delegate is going to return self, -respondsToSelector: is going
to result in infinite recursion.
So It's Never Safe To Make An Object Its Own Delegate.
--Kyle Sluder
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