R – App crash when trying to access a subclass method after the object was initialized ( init ) using the parent initializer

inheritanceobjective c

I have a class that subclasses NSMutableArray.

I init it using:

MyClass class = [MyClass arrayWithContentsOfFile:path];

When i try to access any of my subclass methods the app crashes with this error:

-[NSCFArray loadCards]: unrecognized selector sent to instance 0x454a30
* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[NSCFArray > mymethod]: unrecognized selector sent to instance 0x454a30'
app[32259:20b] Stack: (
2524995915,

I am suspecting this happens because arrayWithContentsOfFile:path returns NSArray and not "MyClass" so it can't response to my selector.

Any ideas?

Best Answer

NSArray is a class cluster. To create a new subclass within a class cluster, you must implement its primitive methods.

If what you want is to inherit the behavior of an array, it's usually a better idea to do that with a has-a relationship, rather than an is-a. That is, to write a class that has an NSArray instance variable, and simply forwards the relevant messages to it.

Or, if you want to add new behavior to NSArray, you should do that by adding methods directly to the NSArray class in a category.

Essentially, you basically only want to subclass if you want to provide an NSArray interface for a different storage mechanism, and for that job you'd need to implement the primitive methods anyway.

Related Solutions

Objective-C: Calling selectors with multiple arguments

In Objective-C, a selector's signature consists of:

The name of the method (in this case it would be 'myTest') (required)
A ':' (colon) following the method name if the method has an input.
A name and ':' for every additional input.

Selectors have no knowledge of:

The input types
The method's return type.

Here's a class implementation where performMethodsViaSelectors method performs the other class methods by way of selectors:

@implementation ClassForSelectors
- (void) fooNoInputs {
    NSLog(@"Does nothing");
}
- (void) fooOneIput:(NSString*) first {
    NSLog(@"Logs %@", first);
}
- (void) fooFirstInput:(NSString*) first secondInput:(NSString*) second {
    NSLog(@"Logs %@ then %@", first, second);
}
- (void) performMethodsViaSelectors {
    [self performSelector:@selector(fooNoInputs)];
    [self performSelector:@selector(fooOneInput:) withObject:@"first"];
    [self performSelector:@selector(fooFirstInput:secondInput:) withObject:@"first" withObject:@"second"];
}
@end

The method you want to create a selector for has a single input, so you would create a selector for it like so:

SEL myTestSelector = @selector(myTest:);

R – Adding a category to NSArray

I've used categories on NSArray many times with no problems, so I'm guessing your problem lies elsewhere (since your category looks correct).

Since you're getting an "unrecognized selector" error, that means the runtime doesn't know about your category, which means it wasn't linked into your binary. I would check and make sure your category's .m file is included in the appropriate target, clean, and build again.

EDIT: for example, this blog post shows how to create a category for NSArray for creating a shuffled copy of the original array.

EDIT #2: Apple's documentation on categories uses the specific example of extending the functionality of NSArray, so I find it difficult to believe that the "recommended" approach would be to wrap the array in a helper object. Citation (search the page for the five "NSArray" references)

Best Answer

Related Solutions

Objective-C: Calling selectors with multiple arguments

R – Adding a category to NSArray

Related Topic