View controller that can override the content size category of a single child via the standard size change edit actions.

StandardSizeChangeViaTraitCollectionViewController.swift

import UIKit

public final class StandardSizeChangeViaTraitCollectionViewController<Base>:
    UIViewController
where
    Base: UIViewController
{
    public let base: Base
    
    // MARK: Public Initialization
    
    public init(_ base: Base) {
        self.base = base
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    // MARK: UIViewController Implementation
    
    override public func viewDidLoad() {
        super.viewDidLoad()
        
        base.view.autoresizingMask = [.flexibleHeight, .flexibleWidth]
        
        addChild(base)
        view.addSubview(base.view)
        base.view.frame = view.bounds
        base.didMove(toParent: self)
    }
    
    // MARK: UIResponderStandardEditActions Implementation
    
    override public func decreaseSize(_ sender: Any?) {
        setOverrideTraitCollection(
            UITraitCollection(preferredContentSizeCategory: base.traitCollection.preferredContentSizeCategory.previous),
            forChild: base
        )
    }
    
    override public func increaseSize(_ sender: Any?) {
        setOverrideTraitCollection(
            UITraitCollection(preferredContentSizeCategory: base.traitCollection.preferredContentSizeCategory.next),
            forChild: base
        )
    }
}

// MARK: - Extension for UIContentSizeCategory

extension UIContentSizeCategory {
    // MARK: Internal Instance Interface
    
    var next: UIContentSizeCategory {
        switch self {
        case .extraSmall:
            return .small
        case .small:
            return .medium
        case .medium:
            return .large
        case .large:
            return .extraLarge
        case .extraLarge:
            return .extraExtraLarge
        case .extraExtraLarge:
            return .extraExtraExtraLarge
        case .extraExtraExtraLarge:
            return .accessibilityMedium
        case .accessibilityMedium:
            return .accessibilityLarge
        case .accessibilityLarge:
            return .accessibilityExtraLarge
        case .accessibilityExtraLarge:
            return .accessibilityExtraExtraLarge
        case .accessibilityExtraExtraLarge:
            return .accessibilityExtraExtraExtraLarge
        case .accessibilityExtraExtraExtraLarge:
            return self
        case .unspecified:
            return self
        default:
            return self
        }
    }
    
    var previous: UIContentSizeCategory {
        switch self {
        case .extraSmall:
            return self
        case .small:
            return .extraSmall
        case .medium:
            return .small
        case .large:
            return .medium
        case .extraLarge:
            return .large
        case .extraExtraLarge:
            return .extraLarge
        case .extraExtraExtraLarge:
            return .extraExtraLarge
        case .accessibilityMedium:
            return .extraExtraExtraLarge
        case .accessibilityLarge:
            return .accessibilityMedium
        case .accessibilityExtraLarge:
            return .accessibilityLarge
        case .accessibilityExtraExtraLarge:
            return .accessibilityExtraLarge
        case .accessibilityExtraExtraExtraLarge:
            return .accessibilityExtraExtraLarge
        case .unspecified:
            return self
        default:
            return self
        }
    }
}