First, placeholder is a better word than proxy here.

Normally when you have an object in a NIB/XIB file it means that loading the NIB file will create that instance. The placeholder objects are objects that will already exist when the NIB file is loaded, and they appear inside of the NIB so that you can make connections between the objects that will be created by loading the NIB and the objects that already exist.

The file's owner, first responder and application are all placeholders.

The file's owner is placeholder for the object that will load the nib. All of the NIB loading methods take an 'owner' parameter. When you make a connection with the File's owner, when it's established at runtime, it will be connected to the owner object passed in to the nib loading method. Many UIKit and AppKit classes invoke the nib loading methods for you. NSApplication, NSViewController, NSWindowController, UIApplication, and UIViewController all load NIB files on your behalf. When they do that they pass self as the owner parameter to the nib loading methods. That's why when you use a view controller or a window controller you set the file's owner to your subclass and make most of the connections between your views and the file's owner.

The NSApplication instance is a simple placeholder for [NSApplication sharedApplication]. That's a global singleton and the icon in Interface Builder represents that global singleton. Loading the NIB file does not create a second NSApplication instance. By contrast, when a NIB file contains a window, if you load it a dozen times, you'll have a dozen window instances, but still one NSApplication instance.

The first responder is unique. Connecting an action to the first responder means that when the action is fired, it should dynamically be sent to the responder chain. The responder chain typically starts with the focused view, and continues up through the view hierarchy and includes some controllers and delegates. Each object in the chain gets a shot at handling the action. Menu items work great with the responder chain. If you had a menu item for "Make Bold" that is supposed to make the currently selected text bold, you might start by hooking that up to an NSApplication subclass, but then you'd have to know all of the situations that "Make Bold" applies, and how to handle them. A text view and an editable web view would probably need different code to handle "make bold" and bottling this all up in one object would get quite complex and wouldn't be very extensible. Instead you could connect the "Make Bold" menu item's action up to a makeBold: action on the First Responder. This would mean that when the menu item was selected, the focused object, or one of its parents that responded to makeBold:, would get the makeBold: message. Now many classes can implement a makeBold: method and respond to this menu item when they're in focus.

Important: This check should always be performed asynchronously. The majority of answers below are synchronous so be careful otherwise you'll freeze up your app.

Swift

1. Install via CocoaPods or Carthage: https://github.com/ashleymills/Reachability.swift

2. Test reachability via closures

   let reachability = Reachability()!
   
   reachability.whenReachable = { reachability in
       if reachability.connection == .wifi {
           print("Reachable via WiFi")
       } else {
           print("Reachable via Cellular")
       }
   }
   
   reachability.whenUnreachable = { _ in
       print("Not reachable")
   }
   
   do {
       try reachability.startNotifier()
   } catch {
       print("Unable to start notifier")
   }
   

Objective-C

1. Add SystemConfiguration framework to the project but don't worry about including it anywhere

2. Add Tony Million's version of Reachability.h and Reachability.m to the project (found here: https://github.com/tonymillion/Reachability)

3. Update the interface section

   #import "Reachability.h"
   
   // Add this to the interface in the .m file of your view controller
   @interface MyViewController ()
   {
       Reachability *internetReachableFoo;
   }
   @end
   

4. Then implement this method in the .m file of your view controller which you can call

   // Checks if we have an internet connection or not
   - (void)testInternetConnection
   {
       internetReachableFoo = [Reachability reachabilityWithHostname:@"www.google.com"];
   
       // Internet is reachable
       internetReachableFoo.reachableBlock = ^(Reachability*reach)
       {
           // Update the UI on the main thread
           dispatch_async(dispatch_get_main_queue(), ^{
               NSLog(@"Yayyy, we have the interwebs!");
           });
       };
   
       // Internet is not reachable
       internetReachableFoo.unreachableBlock = ^(Reachability*reach)
       {
           // Update the UI on the main thread
           dispatch_async(dispatch_get_main_queue(), ^{
               NSLog(@"Someone broke the internet :(");
           });
       };
   
       [internetReachableFoo startNotifier];
   }
   

Important Note: The Reachability class is one of the most used classes in projects so you might run into naming conflicts with other projects. If this happens, you'll have to rename one of the pairs of Reachability.h and Reachability.m files to something else to resolve the issue.

Note: The domain you use doesn't matter. It's just testing for a gateway to any domain.