Hooking into didFinishLaunching from a library

Observer.h

@interface Observer : NSObject
+ (id)sharedInstance;
- (void)didFinishLaunching:(NSNotification *)notification;
@end

Observer.m

#import <Foundation/Foundation.h>

@implementation Observer

+ (id)sharedInstance {
    static Observer *bridgeInstance = nil;
    @synchronized(self) {
        if (bridgeInstance == nil)
            bridgeInstance = [[self alloc] init];
    }
    return bridgeInstance;
}

- (void) didFinishLaunching: (NSNotification*) n
{
    UIWindow *appWindow = [[[UIApplication sharedApplication] delegate] window];
    NSLog( @"application: %@", [UIApplication sharedApplication] );
    NSLog( @"delegate:    %@", [[UIApplication sharedApplication] delegate] );
}
@end

UIResponder+MyCategory.h

#import <UIKit/UIKit.h>
#import "Observer.h"

NS_ASSUME_NONNULL_BEGIN

@interface UIResponder (MyCategory) 
@end

NS_ASSUME_NONNULL_END

UIResponder+MyCategory.m

@implementation UIResponder (MyCategory)


+ (void) load
{
    [[NSNotificationCenter defaultCenter] addObserver: [Observer sharedInstance]
                                             selector: @selector(didFinishLaunching:) name: UIApplicationDidFinishLaunchingNotification object: nil];
}

@end

Using a Category to observe didFinishLaunching and then triggering any set up needed in a singleton, all without modifying AppDelegate directly.
Useful for making libraries/Cocoapods that might need to respond to didFinishLaunching but without having to add them to AppDelegate

Turns out this is a bad idea