NSURLSessionTask category method crashes with "Unrecognized selector sent to instance"

Question

I need to add method to NSURLSessionTask. Here's my category which is supposed to do that:

//  NSURLSessionTask+Extras.h

#import <Foundation/Foundation.h>

@interface NSURLSessionTask (Extras)

- (void)helloNSURLSessionTask;

@end

//  NSURLSessionTask+Extras.m

#import "NSURLSessionTask+Extras.h"

@implementation NSURLSessionTask (Extras)

- (void)helloNSURLSessionTask {
    NSLog(@"hello NSURLSessionTask!");
}

@end

Everything compiles well, autocomplete works, but when I call this method, my application crashes:

2014-06-27 12:32:23.916 Test[4333:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFLocalDataTask helloNSURLSessionTask]: unrecognized selector sent to instance 0x109723310'

Same approach works if I add category to NSObject, and I can't seem to understand why it doesn't for NSURLSessionTask.

Here's a test project which reproduces this problem for me: https://dl.dropboxusercontent.com/u/25100182/Test.zip

Venerable

Answer 1

Unfortunately it doesn't seem that NSURLSession can support categories. I've run into a problem like that trying to add an associated object.
See also here.

Alignment

Thanks for the pointer, I'll try what you suggest. By the way, do you think this problem is mentioned somewhere in the docs? – Venerable

Nope, I still didn't understand if it's a bug or a "feature" – Alignment

Answer 2

I had similar issue at NSURLSessionDataTask category, when I reduced the scope to NSURLSessionTask it seems to be working correctly.

@interface NSURLSessionTask (iRemoteWallet)
    @property (nonatomic, weak) id context;
@end

Hope this helps.

Towbin

Answer 3

I believe I know what is the origin of this rather odd (to me at least) behaviour. I will provide an example that I've encountered with NSURLSessionDataTask in iOS 9.2.

When you try to use the method that you've added in a category -myMethod: you will see an exception like this:

[__NSCFLocalDataTask -myMethod]: unrecognised selector sent to instance

I've started digging and discovered that __NSCFLocalDataTask class that hides behind our NSURLSessionDataTask class at runtime is just casted to NSURLSessionDataTask but does not inherit from it. This class inheritance chain is __NSCFLocalDataTask : __NSCFLocalSessionTask : __NSCFURLSessionTask : NSObject. So my category does not apply to this class at all. No wonder it is unrecognised.

One solution to this might be to move your category to NSObject and make a bunch of checking. I did it like this:

if ([self respondsToSelector:@selector(response)]) {
    id response = [self performSelector:@selector(response)];

    if ([response isKindOfClass:[NSHTTPURLResponse class]]) {

        return [NSString stringWithFormat:@"%td", ((NSHTTPURLResponse *)response).statusCode];

    }

}

NSException *e =[NSException exceptionWithName:NSInternalInconsistencyException reason:@"errorCode: method should be only used on classes that implement -(NSHTTPURLResponse *)response: method." userInfo:nil];

@throw e;

Ugly? Yes! But it works for now.

Audiphone