A custom iOS segue to fully replace the current UIViewController with a new UIViewController.

FullyReplaceSegue.h

// Objective C
// Display initial screen before final UIViewController loads (e.g. Login credentials)
//
// By Andrew Kingdom
// MIT license
// 
// Instructions:
// 
// On the main storyboard, create your login screen as, say, a subclass of UIViewController. 
// Make sure it is the initial scene (check Is Initial View Controller).
// 
// On the storyboard, create a new segue from your login screen to the final screen (e.g. SplitViewController). 
// Give it an arbitrary identifier, such as 'Load SplitViewController',
// and set the segue custom class name to FullyReplaceSegue.
// 
// Finally, in your login class .m file, add code to be called once the user has logged in:
// 
// [self performSegueWithIdentifier:@"Load SplitViewController" sender:self];
// 

#import <UIKit/UIKit.h>

@protocol FullyReplaceSegueDelegate
@optional
/// The segue will invoke this method on the source UIViewController before removing it from its window. Returned data (or nil) will be passed to didGetDataFromSourceViewController: on the destination UIViewController. Suggestion: return an NSDictionary.
- (NSObject * _Nullable)willPassDataToDestinationViewController;
/// The segue will invoke this method on the destination ViewController after adding it, with any data from willPassDataToDestinationViewController on the outgoing replacement destination ViewController.
- (void)didGetDataFromSourceViewController:(NSObject * _Nullable)data;
@end

@interface FullyReplaceSegue : UIStoryboardSegue

@end

FullyReplaceSegue.m

// Objective C
// Display initial screen before final UIViewController loads (e.g. Login credentials)
//
// By Andrew Kingdom
// MIT license
//
#import "FullyReplaceSegue.h"
#import <UIKit/UIKit.h>

@implementation FullyReplaceSegue


- (void)perform {
    // Add your own animation code here.
 
    UIViewController *source = self.sourceViewController;
    UIViewController *destination = self.destinationViewController;
    NSObject * _Nullable data = nil;
    
    if([self.sourceViewController respondsToSelector:
        @selector(willPassDataToDestinationViewController)]) {
        data = [self.sourceViewController
            performSelector: @selector(willPassDataToDestinationViewController)];
    }
    
    UIWindow *window = source.view.window;
    window.rootViewController = destination;
    
    if([self.sourceViewController respondsToSelector:
        @selector(didGetDataFromSourceViewController:)]) {
        [self.sourceViewController
            performSelector: @selector(didGetDataFromSourceViewController:)
            withObject:data];
    }
}

@end