Why does this NSInvocation raise an exception?

Question

I have a real head-scratcher right now. So, an NSTimer object, an NSMethodSignature object, and an NSInvocation object walk into a bar. Here's the rest of the joke:

NSMethodSignature *methodSig = [NSMethodSignature methodSignatureForSelector:@selector(setAlphaValue:)];
NSInvocation *inv = [NSInvocation invocationWithMethodSignature:methodSig];
CGFloat alphaVal = 1.f;

[inv setSelector:@selector(setAlphaValue:)];
[inv setTarget:tabViewItem.view];
[inv setArgument:&alphaVal atIndex:2];

NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:0.5f invocation:inv repeats:NO];

Here's what I'm getting in the debugging console:

+[NSInvocation _invocationWithMethodSignature:frame:]: method signature argument cannot be nil

Edit: I'm not sure why someone thought it was necessary to down-vote my question. Sorry for trying to learn something new. Actually, here's an amendment to my original question: what should I have done differently? Should I have just deleted the post once I figured out what my problem was? I tried to follow all of the Stack Overflow etiquette, and I even took the time to leave an answer in the off chance it could help someone else. Next time, should I just return to my question and leave a response like, "nvm … figured it out. thx?" Or should I just leave it here, unanswered? I sure as hell know I'm tired of clicking on links only to be led to unanswered posts.

score 8 · Accepted Answer · answered Nov 18 '13 at 03:31

8

Oops. I jumped the gun. Xcode's code completion had me thinking that methodSignatureForSelector: was a class method of NSMethodSignature. Whenever I'm dealing with classes I don't usually mess with on a regular basis, I generally start by typing [NSClassImUnfamiliarWith, followed by a space. Doing so brings up Xcode's code completion popup, which generally has all of the methods one can call. I accidentally chose an inherited (?) method from NSObject thinking it was a class method. I guess this would be an example of polymorphism. Anyway, I solved my issue by replacing NSMethodSignature with my object: tabViewItem.view. So all together it looks like this:

NSMethodSignature *methodSig = [tabViewItem.view methodSignatureForSelector:@selector(setAlphaValue:)];

I don't know if this will help anyone else, but by golly, I'm gonna post this just in case. Good luck!

answered Nov 18 '13 at 03:31

Ben Stock

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Same problem here a year later. Thanks for posting! – Max Chuquimia Sep 21 '14 at 06:48
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@Jugale: I told myself last November that if my failures could save just one person from blowing a hole in their computer, I'd consider it a win. You just got me the W! I'm glad I could help. Good luck! – Ben Stock Sep 22 '14 at 07:23
Solved a bad bug for me. Thanks! – M-P May 26 '15 at 23:37
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@MichaelPatzer That's awesome. Good to hear. – Ben Stock May 30 '15 at 00:37

Why does this NSInvocation raise an exception?

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