when to use respondsToSelector in objective-c

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You use it pretty much just when you think you need to: to check to see if an object implements the method you are about to call. Usually this is done when you have an optional methods or an informal protocol.

I’ve only ever used respondsToSelector when I’m writing code that must communicate with a delegate object. 

if ([self.delegate respondsToSelector:@selector(engineDidStartRunning:)]) {
        [self.delegate engineDidStartRunning:self];
    }

You sometimes would want to use respondsToSelector on any method that returns and id or generic NSObject where you aren’t sure what the class of the returned object is.

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