iOS: NSURLSession category methods not being found at runtime

Question

I wanted to create a category on NSURLSession. The app compiles ok, but when I attempt to call the category methods I get

-[__NSCFURLSession doSomething]: unrecognized selector sent to instance 0xbf6b0f0
<unknown>:0: error: -[NSURLSession_UPnPTests testCategory] : -[__NSCFURLSession doSomething]: unrecognized selector sent to instance 0xbf6b0f0

Very strange, here is a test class I built to show the issue. The category on the NSString works fine, but the category on NSURLSession fails to find the method at runtime. I suspect this is something internal.

Opinions please :-)

#import <XCTest/XCTest.h>

@interface NSString (test)
-(void) doSomethingHere;
@end

@implementation NSString (test)
-(void) doSomethingHere {
    NSLog(@"Hello string");
}
@end

@interface NSURLSession (test)
-(void) doSomething;
@end

@implementation NSURLSession (test)
-(void) doSomething {
    NSLog(@"Hello!!!!");
}
@end

@interface NSURLSession_UPnPTests : XCTestCase
@end

@implementation NSURLSession_UPnPTests
-(void) testCategory {
    [@"abc" doSomethingHere];
    NSURLSession *session = [NSURLSession sharedSession];
    [session doSomething];
}
@end

Answer 1

I've had similar results with backgrounded NSURLSessionUploadTasks, which get deserialized as __NSCFURLSessionUploadTasks during the -URLSession:task:didCompleteWithError: delegate callback.

If I were you, I'd give up on that approach, and use composition (i.e. make the NSURLSession an ivar in another object). If you need to store some info with the NSURLSession, you could stuff a JSON-encoded dictionary into the .sessionDescription.

Here's the code that I used to do that for a task:

#pragma mark - Storing an NSDictionary in NSURLSessionTask.description

// This lets us attach some arbitrary information to a NSURLSessionTask by JSON-encoding
// an NSDictionary, and storing it in the .description field.
//
// Attempts at creating subclasses or categories on NSURLSessionTask have not worked out,
// because the –URLSession:task:didCompleteWithError: callback passes an
// __NSCFURLSessionUploadTask as the task argument. This is the best solution I could
// come up with to store arbitray info with a task.

- (void)storeDictionary:(NSDictionary *)dict inDescriptionOfTask:(NSURLSessionTask *)task
{
    NSData *data = [NSJSONSerialization dataWithJSONObject:dict options:0 error:nil];
    NSString *stringRepresentation = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    [task setTaskDescription:stringRepresentation];
    [stringRepresentation release];
}

- (NSDictionary *)retrieveDictionaryFromDescriptionOfTask:(NSURLSessionTask *)task
{
    NSString *desc = [task taskDescription];
    if (![desc length]) {
        DDLogError(@"No description for %@", task);
        return nil;
    }
    NSData *data = [desc dataUsingEncoding:NSUTF8StringEncoding];
    NSDictionary *dict = (data ? (id)[NSJSONSerialization JSONObjectWithData:data options:0 error:nil] : nil);
    if (!dict) {
        DDLogError(@"Could not parse dictionary from task %@, description\n%@", task, desc);
    }
    return dict;
}