NSApplication subclass adding a delegate method for opening a URL with your app.

GSTApplication.h

//
//  GSTApplication.h
//  Gistapo
//
//  Created by Brent Royal-Gordon on 7/12/13.
//  Copyright (c) 2013 Architechies. All rights reserved.
//

#import <Cocoa/Cocoa.h>

@protocol GSTApplicationDelegate;

@interface GSTApplication : NSApplication

@property (strong) id <GSTApplicationDelegate> delegate;

@end

@protocol GSTApplicationDelegate <NSApplicationDelegate>

@optional
- (void)gst_application:(GSTApplication *)app openURL:(NSURL*)URL;

@end

GSTApplication.m

//
//  GSTApplication.m
//  Gistapo
//
//  Created by Brent Royal-Gordon on 7/12/13.
//  Copyright (c) 2013 Architechies. All rights reserved.
//

#import "GSTApplication.h"

@implementation GSTApplication

@dynamic delegate;

- (void)didReceiveGetURLEvent:(NSAppleEventDescriptor*)URLEvent replyEvent:(NSAppleEventDescriptor*)replyEvent {
    NSString * URLString = [[URLEvent paramDescriptorForKeyword:keyDirectObject] stringValue];
    NSURL * URL = [NSURL URLWithString:URLString];
    if([self.delegate respondsToSelector:@selector(gst_application:openURL:)]) {
        [self.delegate gst_application:self openURL:URL];
    }
}

- (void)finishLaunching {
    [NSAppleEventManager.sharedAppleEventManager setEventHandler:self andSelector:@selector(didReceiveGetURLEvent:replyEvent:) forEventClass:kInternetEventClass andEventID:kAEGetURL];
    
    [super finishLaunching];
}

@end