Re: Implementing delegation

FROM : Fritz Anderson
DATE : Tue Nov 23 21:15:23 2004

On 19 Nov 2004, at 1:51 PM, Bruce Toback wrote:

> Is there some developer information somewhere on the best way to
> implement delegation in my own frameworks?
>
> Currently, when one of my objects with a delegate intends to call a
> delegate method, it does something like this:
>
>    if ([delegate
> respondsToSelector:@selector(myObject:wantsToTellYou:)])
>      [delegate myObject: self wantsToTellYou: something];
>
> There are three things that seem to be wrong with this:
>
> 1. I'm testing to see if the delegate implements a method on every
>    message to the delegate. Obviously, I could cache the results of
>    the test the first time through. Is that worth the effort for
>    methods that aren't called in tight loops or something?


Assuming myObject:wantsToTellYou: is a method that is mandatory for
delegates of your class, you could put into the setDelegate: method
   NSAssert1([proposedDelegate
respondsToSelector:@selector(myObject:wantsToTellYou:)],
           @"%@ isn't a legitimate delegate", proposedDelegate);
If the delegate could be set in a NIB, do the same thing, checking that
the delegate member is non-nil first. Apple's practice, I believe, is
to cache the type-checking of delegates in this way. You really need to
detect this only once, as only a logical error in your code would
trigger the assertion.

   -- F

  --
Fritz Anderson
Consulting Programmer                Chicago, Illinois
http://resume.manoverboard.org/