Question

I would like the view controller to be able to access dog in the app delegate.

I would like the app delegate to be able to access mouse in the view controller.

---

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
{
    int mouse;  // <----------------
}
@end

---

- (void)viewDidLoad
{
    [super viewDidLoad];

    mouse = 12;  // <-------------------

    NSLog(@"viewDidLoad %d", dog); // <---------------
}

---

#import <UIKit/UIKit.h>

@class ViewController;

@interface AppDelegate : UIResponder <UIApplicationDelegate>
{
    int dog;  // <---------------
}
@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) ViewController *viewController;

@end

---

- (void)applicationWillResignActive:(UIApplication *)application
{
    NSLog(@"applicationWillResignActive %d", mouse); // <--------------
}

---

 - (void)applicationDidBecomeActive:(UIApplication *)application
{
    dog = 77; // <---------------------

    NSLog(@"applicationDidBecomeActive");
}

Answer 1

Part 1:
In the ViewController.h:

-(int)mouse;  //add this before the @end

In the ViewController.m, add this method:

-(int)mouse
{
    return mouse;
}

To access mouse from AppDelegate, use self.viewController.mouse
For example;

NSLog(@"ViewController mouse: %i", self.viewController.mouse);

Part2:

In the AppDelegate.h:

-(int)dog;  //add this before the @end

In the AppDelegate.m, add this method:

-(int)dog
{
    return dog;
}

In the ViewController.m:

#import "AppDelegate.h"

To access dog from ViewController, use this:

AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
NSLog(@"dog from AppDelegate: %i", [appDelegate dog]);  //etc.