AppDelegate.h

//
//  AppDelegate.h
//
//  Created by Don Mag on 4/8/19.
//

#import <UIKit/UIKit.h>

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

@end

AppDelegate.m

//
//  AppDelegate.m
//
//  Created by Don Mag on 4/8/19.
//

#import "AppDelegate.h"
#import "TestViewController.h"

@interface AppDelegate ()
@property (strong, nonatomic) TestViewController *testVC;
@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
	// Override point for customization after application launch.

	self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
	_testVC = [TestViewController new];
	_window.rootViewController = _testVC;
	[_window makeKeyAndVisible];

	return YES;
}

@end

TestViewController.h

//
//  TestViewController.h
//
//  Created by Don Mag on 5/6/19.
//

#import <UIKit/UIKit.h>

@interface TestViewController : UIViewController
@end

TestViewController.m

//
//  TestViewController.m
//
//  Created by Don Mag on 5/6/19.
//

#import "TestViewController.h"

@interface TestViewController ()
@end

@implementation TestViewController

- (void)viewDidLoad {
    [super viewDidLoad];

	// make sure we're seeing the correct view filling the window
	self.view.backgroundColor = [UIColor orangeColor];

	// create a button
	UIButton *aButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
	// blue background
	aButton.backgroundColor = [UIColor blueColor];
	// white text
	[aButton setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
	// set the title
	[aButton setTitle:@"Test Button" forState:UIControlStateNormal];
	
	// add the button to the view
	[self.view addSubview:aButton];
	
	// we'll use auto-layout constraints
	[aButton setTranslatesAutoresizingMaskIntoConstraints:NO];
	
	// constrain button centerX and centerY
	[NSLayoutConstraint activateConstraints:
	 @[
	   [aButton.centerXAnchor constraintEqualToAnchor:self.view.centerXAnchor],
	   [aButton.centerYAnchor constraintEqualToAnchor:self.view.centerYAnchor]
	   // or
	   //[aButton.centerXAnchor constraintEqualToAnchor:aButton.superview.centerXAnchor],
	   //[aButton.centerYAnchor constraintEqualToAnchor:aButton.superview.centerYAnchor]
	   ]
	 ];
	
}

@end