@optional is a compile-time directive. It means that objects that conform to your protocol don't *have* to implement the method when the code is compiled. However, it has nothing to do with runtime-verification. That is up to you (so yes, you should be using respondsToSelector:).
If you didn't have the @optional directive, the call to respondsToSelector: would be unnecessary. Dave On Jan 5, 2010, at 11:53 AM, John Michael Zorko wrote: > > Hello, all ... > > After [re-]reading the Apple documentation, i'm still not clear as to what > @optional really does in a @protocol. I thought that declaring certain > messages as @optional would make the app not crash if a certain message > didn't have an implementation in a class that adopts the protocol, however > i'm finding that this isn't the case: > > 2010-01-05 10:40:58.698 VoxityNowPlaying[14413:207] *** Terminating app due > to uncaught exception 'NSInvalidArgumentException', reason: '*** > -[VoxityNowPlayingViewController playerURLFinished:]: unrecognized selector > sent to instance 0x39138f0' > > ... yet my protocol is defined thusly: > > @protocol VoxityPlayerDelegate <NSObject> > > @optional > > - (void)playerURLFinished:(NSURL *)url; > > . > . > . > > @end > > Do I need to wrap code that sends messages to delegates with > -respondsToSelector? If so, what is the point of @optional in the protocol? > > Regards, > > John
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