Subclassing NSURLConnection gives error: unrecognized selector sent to instance

Question

i'm trying to make a subclass of NSURLConnection where i have an additional property (in this case "connectionName") to help me distinguish between 2 different connections.

i created the subclass, named it CustomURLConnection and gave it the property "connectionName".

then in my file ImagesViewController.m (which is an UICollectionView) i import the header CustomURLConnection and try to give the connections a name and retrieve it afterwards, but it doesn't work, as soon as i enter this collection view the app crashes and gives me the following error:

-[NSURLConnection setConnectionName:]: unrecognized selector sent to instance 0x1090a40f0

Here is some code: (if you want, here's a CLEARER IMAGE)

CustomURLConnection.h

#import <Foundation/Foundation.h>

@interface CustomURLConnection : NSURLConnection

@property (strong, nonatomic) NSString *connectionName;

@end

ImagesViewController.h

#import <UIKit/UIKit.h>

@interface ImagesViewController : UICollectionViewController<NSURLConnectionDelegate>

@property (strong, nonatomic) UIImageView *imageView;

@end

ImagesViewController.m

...
#import "CustomURLConnection.h"

@interface ImagesViewController (){
    NSArray *contentStrings;
    NSMutableData *contentData; // Holds data from the initial load
    NSMutableData *contentImageData; // Holds data when the user scrolls up/down in the collection view
}
@end

...

-(void)loadInitialData{ // Loads data from page
    NSString *hostStr = @"http://www.website.com/example";
    NSURL *dataURL = [NSURL URLWithString:hostStr];
    NSURLRequest *request = [NSURLRequest requestWithURL:dataURL];
    CustomURLConnection *connectionData = (CustomURLConnection *)[NSURLConnection connectionWithRequest:request delegate:self]; // Make connection
    connectionData.connectionName = @"InitialData"; // Give it a name
}

...

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
    // Do some stuff

    NSString *hostStr = @"http://www.website.com/example2";

    _imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0,0,100,100)];
    [imageCell addSubview:_imageView]; // Adds an image view to each collection cell

    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:hostStr]];
    CustomURLConnection *connectionImg = (CustomURLConnection *)[NSURLConnection connectionWithRequest:request delegate:self]; // Make connection
    connectionImg.connectionName = @"ImageData"; // Give it a different name than before

    // Do some stuff
    return imageCell;
}

...

// Here are the main methods for the connections
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    if([((CustomURLConnection *)connection).connectionName isEqualToString:@"InitialData"]){
        contentData = [[NSMutableData alloc] init];
    }
    else{
        contentImageData = [[NSMutableData alloc] init];
    }
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    if([((CustomURLConnection *)connection).connectionName isEqualToString:@"InitialData"]){
        [contentData appendData:data];
    }
    else{
        [contentImageData appendData:data];
    }
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection{
    if([((CustomURLConnection *)connection).connectionName isEqualToString:@"InitialData"]){
        // Do some stuff
    }
    else{
        UIImage *image = [[UIImage alloc] initWithData:contentImageData];
        _imageView.image = image;
    }
}

am i missing something? i came across this error many times before but the causes are never the same and this time i can't seem to find a solution on my own.

hopefully you can see what i'm doing wrong and help me :) thanks.

EDIT: turns out there is a better way to achieve my goal, have a look here
Thank again to everyone for the help :)

Answer 1

In your delegate methods change NSURLConnection by CustomURLConnection, for instance :

- (void)connection:(CustomURLConnection *)connection didReceiveResponse:(NSURLResponse *)response

and when you create it just do :

CustomURLConnection *connectionImg = [[CustomURLConnection alloc] initWithRequest:request delegate:self];
connectionImg.connectionName = @"ImageData"; // Give it a different name than before

Answer 2

CustomURLConnection *connectionImg = (CustomURLConnection *)[NSURLConnection connectionWithRequest:request delegate:self]; // Make connection

creates an NSURLConnection object. Casting to CustomURLConnection does not change the class of this object. Replace that line with

CustomURLConnection *connectionImg = [CustomURLConnection connectionWithRequest:request delegate:self]; // Make connection

to create an instance of your subclass.

Answer 3

In this line:

CustomURLConnection *connectionData = (CustomURLConnection *)[NSURLConnection connectionWithRequest:request delegate:self];

you are creating an instance of NSURLConnection, not CustomURLConnection. So, when you cast the result to CustomURLConnection * you are lying to the compiler.

Later, at runtime, when you try to use a feature of CustomURLConnection you get an exception because your connection is the wrong class type and doesn't implement the method.

You need to instantiate CustomURLConnection, not NSURLConnection.

Answer 4

Adding to the other good answers here, your CustomURLConnection class should override +connectionWithRequest:delegate: to return an instance of CustomURLConnection, like this:

+(CustomURLConnection*)connectionWithRequest:(NSURLRequest*)request delegate:(id)delegate
{
    return [[CustomURLConnection alloc] initWithRequest:request delegate:delegate];
}

That lets you use the same style you had:

CustomURLConnection *connectionData = [CustomURLConnection connectionWithRequest:request delegate:self]; // Make connection

More importantly, a user of your code (most likely the future you) might assume that sending +connectionWithRequest:delegate: to CustomURLConnection would return an instance of CustomURLConnection. Without the override, they'll get an instance of NSURLConnection instead, and that's a difficult bug to spot.