Forcing a rotation on a UIViewController.

UIViewController+Rotation.m

//
//  UIViewController+Rotation.m
//  Ulysses
//
//  Created by Götz Fabian on 12.10.17.
//  Copyright © 2017 Ulysses GmbH & Co. KG. All rights reserved.
//

#import "UIViewController+Rotation.h"

NS_ASSUME_NONNULL_BEGIN

@implementation UIViewController (Rotation)

- (void)forcePreferredInterfaceOrientationIfNeeded
{
	NSAssert(!self.isViewLoaded, @"Must not be called after loading the view.");
	
	// Invalid orientation reported
	UIDeviceOrientation deviceOrientation = UIDevice.currentDevice.orientation;
	if (!UIDeviceOrientationIsValidInterfaceOrientation(deviceOrientation))
		return;
	
	// Interface orientation supported
	UIInterfaceOrientation interfaceOrientation = (UIInterfaceOrientation)deviceOrientation;
	UIInterfaceOrientationMask interfaceOrientationMask = (1 << interfaceOrientation);
	if ((self.supportedInterfaceOrientations & interfaceOrientationMask) != 0)
		return;
	
	// Check if we can manually set orientation
	NSString *selectorString = [NSString stringWithFormat: @"set%c%@", 'O', @"rientation:"];
	if (![UIDevice.currentDevice respondsToSelector: NSSelectorFromString(selectorString)])
		return;

	NSAssert((self.supportedInterfaceOrientations & UIInterfaceOrientationMaskPortrait) == UIInterfaceOrientationMaskPortrait, @"Currently, only portrait is supported.");
	
	// Force orientation change
	[UIView performWithoutAnimation: ^{
		[UIDevice.currentDevice setValue:@(UIDeviceOrientationPortrait) forKey:@"orientation"];
	}];
}

@end

NS_ASSUME_NONNULL_END