IOS NSMethodSignature stays nil

Question

I recently started to develop for IOS and struggle with the Invocation Object.

What i have is a Class "Location", derived from NSManagedObject (it's part of the Coredata model)

@interface Location (CoreDataGeneratedAccessors)

- (void)addHasLocationInfoObject:(Info *)value;
...
@end

I also have other Classes that have similar signatures (addHasWorkorderInfoObject,...).

These InfoObjects have constraints with their "parent objects" in this case "Location" has several "LocationInfo" Objects, which i retrieve from a Database and want to add to the Location. Same thing should happen to all Objects that have InfoObjects assigned.

I am now trying to create a method that will work for any Object that sticks to the naming-conventions fixed by the project-documentation (Location -> addHasLocationInfoObject, XY -> addHasXYInfoObject ...).

My Approach for adding Infos to Objects now is:

-(void)setInfoForObject:(NSManagedObject *)managedObject withClass:(NSString *)className 
NSString *noteRefName = [[NSString alloc]init];
noteRefName = [NSString stringWithFormat:@"%@Info", className];
NSString *addInfoSelectorName = [[NSString alloc]init];
addInfoSelectorName = [NSString stringWithFormat:@"addHas%@Object::", infoClassName];

SEL addInfoPropertySelector = NSSelectorFromString(addInfoSelectorName);
NSMethodSignature *signature  = [[managedObject class] methodSignatureForSelector:addNotePropertySelector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];

[invocation setTarget:managedObject];
[invocation setSelector:addInfoPropertySelector];
[invocation setArgument:&note   atIndex:2];

However this gives me an Error, because the Signature object gets set to nil. I tried searching for the Problem and it seems to be related to the ":" in the Selectors' name.

But I do not understand how many and where and why i have to set these? I also seem to fail to find the Docs page that tells me how to do it properly.

Anny help appreciated, Thanks in advance!

PS. I Logged the SelectorName and the Classname and they both are spelled correctly.

not sure i understand why you put two "::" in a method name, while there's only one in a method signature? — kovpas, Aug 08 '13 at 11:47
because it neither worked without : nor with one :, so i gave it a shot^^ — billdoor, Aug 08 '13 at 11:49
NSLog the generated selector name. ("addHas(null)Object:" is not a valid selector name, eg.) — Hot Licks, Aug 08 '13 at 11:53
"Location - addHasLocationInfoObject:" if i log className - selectorName — billdoor, Aug 08 '13 at 12:13
Why not use `-performSelector:withObject:` using `NSSelectorFromString`? — Nate Chandler, Aug 08 '13 at 12:45
You should also send `methodSignatureForSelector:` to `managedObject` rather than to its `class`. — Nate Chandler, Aug 08 '13 at 12:49
performSelector and clearing the core data stuff did the Trick, if you put it in an answer i can give you the credit for it^^ — billdoor, Aug 08 '13 at 13:22

score 2 · Answer 1 · edited May 23 '17 at 12:28

2

Ok, I see multiple problems in your code. First of all, you allocate empty string instances and then immediately re-write them with new instances:

NSString *noteRefName = [[NSString alloc]init];
noteRefName = [NSString stringWithFormat:@"%@Info", className];

This is wrong. Correct way is:

NSString *noteRefName = [NSString stringWithFormat:@"%@Info", className];

There are two types of methods in ObjC: class methods and instance methods. If you don't know the difference, read about it here. So, the second problem is that you are trying to get a class method signature with methodSignatureForSelector: instead of getting instance method signature with instanceMethodSignatureForSelector:.

So, as far as I can tell, the correct way of re-writing this piece of code could be:

NSString *addInfoSelectorName = [NSString stringWithFormat:@"addHas%@Object:", infoClassName];
...
NSMethodSignature *signature = [[managedObject class] instanceMethodSignatureForSelector:addNotePropertySelector];

edited May 23 '17 at 12:28

Community

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answered Aug 08 '13 at 11:55

kovpas

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i corrected the instanciating empty objects, but your fix leads to the same error – billdoor Aug 08 '13 at 12:12
then, I guess, managedObject is wrong. I've just created a test project and it works fine for me. – kovpas Aug 08 '13 at 12:24
NSLog([NSString stringWithFormat:@"%@ - %@", [managedObject class], addNoteSelectorName]); Logs correct values "Location - addHasLocationInfoObject" – billdoor Aug 08 '13 at 12:28
add the following log: NSLog(@"%d", [managedObject respondsToSelector:addInfoPropertySelector]); – kovpas Aug 08 '13 at 12:29
respondsToSelector Logs: "1" – billdoor Aug 08 '13 at 12:38
why "addHasLocationInfoObject" in your output is without a colon? – kovpas Aug 08 '13 at 13:14
You should just write `[managedObject methodSignatureForSelector:addNotePropertySelector]` instead of `[[managedObject class] instanceMethodSignatureForSelector:addNotePropertySelector]` – newacct Aug 08 '13 at 22:55

IOS NSMethodSignature stays nil

1 Answers1