Fixes large titles in iOS13 when you still need to compile with Xcode 10 and Swift.

OSThirteenDynamicFixes.h

#import <Foundation/Foundation.h>

@class UINavigationBar;

NS_ASSUME_NONNULL_BEGIN

@interface OSThirteenDynamicFixes: NSObject

/**
 On iOS 13.0+, ensure the background color and title properties are set properly (they are not available in Xcode 10, these are from a future SDK)
 You must set up your `UINavigationBar.appearance()` as per usual before calling this method. Assumes a non-transparent navigation bar.
 */
+ (void)fix:(UINavigationBar *__nullable)bar;
@end

NS_ASSUME_NONNULL_END

OSThirteenDynamicFixes.m

#import "OSThirteenDynamicFixes.h"
#import <UIKit/UIKit.h>

void perform(id __nullable target, NSString  *__nonnull selectorString, id __nullable object);
BOOL isOSThirteen(void);

@implementation OSThirteenDynamicFixes

+ (void)fix:(UINavigationBar *__nullable)bar {

    if (!bar) { return; }
    if (!isOSThirteen()) { return; }

    // Try to get the `UINavigationBarAppearance` class
    id UINavigationBarAppearance = NSClassFromString(@"UINavigationBarAppearance");
    if (!UINavigationBarAppearance) { return; }

    // Instantiate it
    id ios13Appearance = [[UINavigationBarAppearance alloc] init];

    // Set properties on the appearance
    perform(ios13Appearance, @"setBackgroundColor:", [UINavigationBar appearance].barTintColor); // Imitate behaviour for non-transparent bar pre iOS13
    perform(ios13Appearance, @"setTitleTextAttributes:", [UINavigationBar appearance].titleTextAttributes);
    perform(ios13Appearance, @"setLargeTitleTextAttributes:", [UINavigationBar appearance].largeTitleTextAttributes);

    // Unfortunately in Objective C these don't seem to settable on `[UINavigationBar appearance]`
    // It seems that it may listen to messages sent to it and replay them back later...?
    // Hence, set the appearance on the navigation bar directly
    perform(bar, @"setStandardAppearance:", ios13Appearance);
    perform(bar, @"setCompactAppearance:", ios13Appearance);
    perform(bar, @"setScrollEdgeAppearance:", ios13Appearance);
}

@end

/// Performs a selector on an object with an argument - only if possible
/// Assumes the selector takes a single nullable argument
void perform(id __nullable target, NSString  *__nonnull selectorString, id __nullable object) {
    if (!target) { return; }

    SEL selector = NSSelectorFromString(selectorString);

    if ([target respondsToSelector: selector]) {

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        // Using IMP didn't work for me
        [target performSelector:selector withObject:object];
#pragma clang diagnostic pop
    }
}

/// Checks if we are running under iOS13 or later
BOOL isOSThirteen(void) {
    if (@available(iOS 13,*)) {
        return TRUE;
    } else {
        return FALSE;
    }
}