Subclass Object with pre implemented delegate method

delegates, ios, objective-c, subclass

Solution

What you can do is write an `NSProxy` subclass that implements `respondsToSelector:`. Something like this:

URLConnectionProxyDelegate.h:

#import <Foundation/Foundation.h>

@interface URLConnectionProxyDelegate : NSProxy <NSURLConnectionDelegate>

- (instancetype)initWithDelegate:(id<NSURLConnectionDelegate>)delegate;

@end

URLConnectionProxyDelegate.m:

#import "URLConnectionProxyDelegate.h"

@implementation URLConnectionProxyDelegate
{
    __weak id<NSURLConnectionDelegate> _realDelegate;
}


#pragma mark - Object Lifecycle

- (instancetype)initWithDelegate:(id<NSURLConnectionDelegate>)delegate
{
    if (self) {
        _realDelegate = delegate;
    }
    return self;
}


#pragma mark - NSProxy Overrides

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
{
    return [(id)_realDelegate methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    invocation.target = _realDelegate;
    [invocation invoke];
}


#pragma mark - NSObject Protocol Methods

- (BOOL)respondsToSelector:(SEL)sel
{
    // replace @selector(connection:didFailWithError:) with your actual pre-implemented method's selector
    if (sel == @selector(connection:didFailWithError:)) {
        return YES;
    }

    return [_realDelegate respondsToSelector:sel];
}


#pragma mark - NSURLConnectionDelegate Methods

// Since I don't know which method your pre-implemented method is,
// I just chose connection:didFailWithError: as an example. Replace this
// with your actual pre-implemented method.
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"Connection failed: This gets called only when the proxy delegate is used");
}

@end

And then to use this class in, say, a view controller class of yours, you can do something like this:

SomeViewController.m:

#import "SomeViewController.h"
#import "URLConnectionProxyDelegate.h"

@interface SomeViewController () <NSURLConnectionDelegate>

@end


@implementation SomeViewController

#pragma mark - Button actions

- (IBAction)testSuccessURLWithNormalDelegate:(id)sender
{
    NSURL *url = [NSURL URLWithString:@"http://example.com"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    // Using self as the delegate
    [NSURLConnection connectionWithRequest:request delegate:self];
}

- (IBAction)testFailURLWithNormalDelegate:(id)sender
{
    NSURL *url = [NSURL URLWithString:@"not a real url"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    // Using self as the delegate
    [NSURLConnection connectionWithRequest:request delegate:self];
}

- (IBAction)testSuccessURLWithProxyDelegate:(id)sender
{
    NSURL *url = [NSURL URLWithString:@"http://example.com"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    // Using a proxy delegate, with self as the real delegate
    URLConnectionProxyDelegate *proxy = [[URLConnectionProxyDelegate alloc] initWithDelegate:self];
    [NSURLConnection connectionWithRequest:request delegate:proxy];
}

- (IBAction)testFailURLWithProxyDelegate:(id)sender
{
    NSURL *url = [NSURL URLWithString:@"not a real url"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    // Using a proxy delegate, with self as the real delegate
    URLConnectionProxyDelegate *proxy = [[URLConnectionProxyDelegate alloc] initWithDelegate:self];
    [NSURLConnection connectionWithRequest:request delegate:proxy];
}


#pragma mark - NSURLConnectionDelegate Methods

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"Connection failed: This gets called only when the view controller is used as the delegate");
}

- (BOOL)connectionShouldUseCredentialStorage:(NSURLConnection *)connection
{
    NSLog(@"Connection success: This gets called when the view controller OR the proxy delegate is used as the delegate");

    return YES;
}

@end

The important thing to note about all this is that `URLConnectionProxyDelegate` overrides `respondsToSelector:` and passes it along to its `_realDelegate` object instead of calling `super`, and it also always returns `YES` if the selector is your "pre-implemented" method's selector. This means you don't even have to implement any of the other methods in the `NSURLConnectionDelegate` protocol – you just need to implement the "pre-implemented" one.

You could even have several pre-implemented methods, as well. This is easily done by just adding more checks for the selector in `respondsToSelector:` of the proxy class:

[...]

if (sel == @selector(connection:didFailWithError:)) {
    return YES;
}
if (sel == @selector(connectionShouldUseCredentialStorage:)) {
    return YES;
}

[...]

... and then just making sure to implement all of those methods in the proxy class as well, of course:

[...]

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"pre-implemented connection:didFailWithError:");
}

- (BOOL)connectionShouldUseCredentialStorage:(NSURLConnection *)connection
{
    NSLog(@"pre-implemented connectionShouldUseCredentialStorage:");

    return YES;
}

[...]

Hope that makes sense and is of some help to you.

Problem

Original source