Crude support for UILayoutGuide in Interface Builder

KBLayoutGuidePlaceholderView.swift

//
//  KBLayoutGuidePlaceholderView.swift
//  Copyright © 2019 Kirill byss Bystrov. All rights reserved.
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.
//

import UIKit.UIView
import UIKit.UILayoutGuide
import UIKit.NSLayoutConstraint

internal final class KBLayoutGuidePlaceholderView: UIView {
	internal override func willMove (toSuperview newSuperview: UIView?) {
		super.willMove (toSuperview: newSuperview);
		
		guard let superview = newSuperview, let placeholderIndex = superview.subviews.firstIndex (of: self) else {
			return;
		}
		
		let layoutGuide = UILayoutGuide ();
		superview.addLayoutGuide (layoutGuide);
		
		let placeholderConstraints = superview.constraints.filter { ($0.firstItem === self) || ($0.secondItem === self) };
		let layoutGuideConstraints = placeholderConstraints.map {
			self.rewriteLayoutConstraint ($0, using: layoutGuide);
		};
		let subviewConstraints = self.constraints.map {
			self.rewriteLayoutConstraint ($0, using: layoutGuide);
		}
		
		let subviews = self.subviews;
		for (index, subview) in subviews.enumerated () {
			superview.insertSubview (subview, at: placeholderIndex + index);
		}
		self.removeFromSuperview ();
		superview.addConstraints (layoutGuideConstraints + subviewConstraints);
	}
	
	private func rewriteLayoutConstraint (_ constraint: NSLayoutConstraint, using layoutGuide: UILayoutGuide) -> NSLayoutConstraint {
		return NSLayoutConstraint (
			item: (constraint.firstItem === self) ? layoutGuide : constraint.firstItem as Any,
			attribute: constraint.firstAttribute,
			relatedBy: constraint.relation,
			toItem: (constraint.secondItem === self) ? layoutGuide : constraint.secondItem,
			attribute: constraint.secondAttribute,
			multiplier: constraint.multiplier,
			constant: constraint.constant
		);
	}
}

Package.swift

// swift-tools-version:5.3

import PackageDescription

private let libraryName = "KBLayoutGuidePlaceholderView";

let package = Package (
	name: libraryName,
	products: [ .library (name: libraryName, targets: [libraryName]) ],
	targets: [ .target (name: libraryName, path: ".") ]
);