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 onc…

Accepted Answer

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!

Why does this NSInvocation raise an exception?

Solution

Problem