NRDelegateProxy is an Objective-C proxy class used for intercepting delegate callbacks.

NRDelegateProxy

// NRDelegateProxy is a base class for proxy objects that are used as intercepting
// delegates in other objects.
//
// NRDelegateProxy is subclassed to customize delegate behavior by selectively responding
// to some delegate methods while passing on other methods. It can thus be used to implement
// a chain of delegates where each delegate is responsible for some specific task.
//
// Below is an example using a UITextField. It sets up a chain of delegates that looks as follows:
//
// UITextField -> NRTextFilterProxy -> NRKeyboardHiderProxy -> ViewController
//
// Each of the three delegates takes over some specific part in text input handling and passes
// other messages along in direction of the custom view controller which sits at the end of the
// chain.


#import <objc/runtime.h>

@interface NRDelegateProxy : NSProxy

@property (nonatomic, weak) id delegate;

@end


@implementation NRDelegateProxy

- (BOOL)respondsToSelector:(SEL)selector
{
	// Check if the proxy (self) responds to the selector.
	BOOL selfResponds = class_getInstanceMethod([self class], selector) != NULL;

	// If not, pass on to delegate, if any.
	return selfResponds || [_delegate respondsToSelector:selector];
}

// Fast path forwarding, circumventing expensive invocation handling
- (id)forwardingTargetForSelector:(SEL)selector
{
	return _delegate;
}

@end



// Example delegate proxy that filters text input, simulating a broken f key.

@interface NRTextFilterProxy : NRDelegateProxy <UITextFieldDelegate>
@end

@implementation NRTextFilterProxy

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
	return ! [string isEqualToString:@"f"];
}

@end



// Example delegate proxy that hides the keyboard when pressing enter.

@interface NRKeyboardHiderProxy : NRDelegateProxy <UITextFieldDelegate>
@end

@implementation NRKeyboardHiderProxy

- (BOOL)textFieldShouldReturn:(UITextField *)textField
{
	return [textField endEditing:NO];
}

@end



// Example view controller that sets up a delegate chain on its textField outlet.

@interface ViewController ()

@property (weak, nonatomic) IBOutlet UITextField *textField;

@end


@implementation ViewController
{
	NRTextFilterProxy *_textFilter;
	NRKeyboardHiderProxy *_keyboardHider;
}

- (void)viewDidLoad
{
	if (_textFilter == nil) {

		_keyboardHider = [NRKeyboardHiderProxy alloc];
		_keyboardHider.delegate = self;

		_textFilter = [NRTextFilterProxy alloc];
		_textFilter.delegate = _keyboardHider;

		_textField.delegate = _textFilter;
	}

	[super viewDidLoad];
}

- (BOOL)textFieldShouldBeginEditing:(UITextField *)textField
{
	NSLog(@"textFieldShouldBeginEditing");

	return YES;
}

@end