UIResponder Informal Protocol

Foo.m

/**
Informal protocol that traverses the responder chain until a concrete class provides
an instance of NSManagedObjectContext. If no responder provides on, UIApplication is extended to forward
the request to it's delegate. We extend the UIApplicationDelegate to add this new behavior.

Concrete responders can provide optional implementations of managedObjectContext. For example, a responder such as
a view may call [self managedObjectContext], which would start walking up the responder chain. When it gets to the view controller that owns the view, the view controller may have a concrete implementation of managedObjectContext. This can be a property, or a method that creates a new context, etc. The instance vended by the view controller method is what the view that called [self managedObjectContext] gets as a response from that method.

Both the responder chain and informal protocols are incredible powerful concepts that are often under utilized by developers outside of Apple.

https://developer.apple.com/library/mac/documentation/general/conceptual/devpedia-cocoacore/Protocol.html

https://developer.apple.com/library/ios/documentation/general/conceptual/Devpedia-CocoaApp/Responder.html

https://developer.apple.com/library/ios/documentation/EventHandling/Conceptual/EventHandlingiPhoneOS/event_delivery_responder_chain/event_delivery_responder_chain.html#//apple_ref/doc/uid/TP40009541-CH4-SW3

**/


@implementation UIResponder (CoreData)

- (NSManagedObjectContext *) managedObjectContext {
    NSManagedObjectContext    *result = nil;
    
    // Move up the responder chain.
    if ([self nextResponder] != nil){
        if ([[self nextResponder] respondsToSelector:@selector(managedObjectContext)]){
            result = [[self nextResponder] managedObjectContext];
        }
    }
    return result;
}

@end

@implementation UIApplication (CoreData)

- (NSManagedObjectContext *) managedObjectContext {
    NSManagedObjectContext    *result = nil;
    
    // If we have gotten this far, forward to the delegate.    
    if ([[self delegate] respondsToSelector:@selector(managedObjectContext)]){
        result = [(id)[self delegate] managedObjectContext];
    }
    
    return result;
}

@end

@protocol UIApplicationDelegateCoreData <UIApplicationDelegate>

// Extend UIApplicationDelegate to add the new behavior.
@optional
- (NSManagedObjectContext *) managedObjectContext;

@end