New iOS 8 APIs don't allow you to use a custom presentation controller for compact devices, in conjunction with a popover on "normal" size class devices

gistfile1.m

//
//  ViewController.m
//  PopoverPresentationControllerExample
//
//  Created by Bryan Irace on 8/8/14.
//  Copyright (c) 2014 Bryan Irace. All rights reserved.
//

#import "CustomCompactPresentationController.h"
#import "ViewController.h"
#import "ModalViewController.h"

@interface ViewController () <UIViewControllerTransitioningDelegate, UIPopoverPresentationControllerDelegate>
@end

@implementation ViewController

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    /**
     This modal view controller should be displayed in a popover on normal size class devices, but should use a custom 
     presentation controller on devices with a compact size class.
     
     I don't know how to accomplish this, though, since `UIPopoverPresentationControllerDelegate`'s 
     `adaptivePresentationStyleForPresentationController` method doesn't allow you to return `UIModalPresentationCustom`.
     
     One way would be to:
     
     1) Use `UIModalPresentationCustom` instead of `UIModalPresentationPopover`
     
     2) Subclass `UIPopoverPresentationController`, implement this behavior myself, and return an instance of my subclass
     from `presentationControllerForPresentedViewController:presentingViewController:sourceViewController`
     instead of accessing `modal.popoverPresentationController` at all
     
     But since popover presentation controllers are not intended to be instantiated directly, it feels like I'm fighting 
     the framework here.
     */
    ModalViewController *modal = [[ModalViewController alloc] init];
    modal.modalPresentationStyle = UIModalPresentationPopover;
    modal.transitioningDelegate = self;
    modal.popoverPresentationController.sourceView = self.view;
    modal.popoverPresentationController.sourceRect = CGRectZero;
    modal.popoverPresentationController.delegate = self;
    
    [self presentViewController:modal animated:YES completion:nil];
}

#pragma mark - UIPopoverPresentationControllerDelegate

- (UIModalPresentationStyle)adaptivePresentationStyleForPresentationController:(UIPresentationController *)controller {
    /**
     The documentation says this can only return `UIModalPresentationFullScreen` or `UIModalPresentationOverFullScreen`, 
     or none. 
     
     It would be great if returning `UIModalPresentationCustom` here would allow the presenting view controller's 
     transitioning delegate to provide a custom presentation controller instance.
     */
    return UIModalPresentationCustom;
}

#pragma mark - UIViewControllerTransitioningDelegate

- (UIPresentationController *)presentationControllerForPresentedViewController:(UIViewController *)presented
                                                      presentingViewController:(UIViewController *)presenting
                                                          sourceViewController:(UIViewController *)source {
    /**
     This method is never called, because the view controller being presented has modal presentation style 
     `UIModalPresentationPopover` instead of `UIModalPresentationCustom`.
     
     If this method were called, I could provide a custom presentation controller for usage on compact size class 
     devices where popovers are not supported.
     */
    return [[CustomCompactPresentationController alloc] init];
}

@end

UIModalPresentationCustom means you want to use a custom presentation controller. It doesn't make sense in this case to try to adapt to a a different controller when you already are using the popover presentation controller! There is only one ever one presentation controller. The popover controller has a delegate method you can use to easily adapt the views to or create new ones for display in full screen for compact devices.