Dynamic intrinsicContentSize example

ConstantAreaView.swift

//
//  ViewController.swift
//  ConstantArea
//
//  Created by Gleb Tarasov on 24.05.2020.
//  Copyright © 2020 Gleb Tarasov. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let view1 = ConstantAreaView()
        view1.translatesAutoresizingMaskIntoConstraints = false
        view1.backgroundColor = .green
        let container1 = UIView()
        container1.translatesAutoresizingMaskIntoConstraints = false
        container1.addSubview(view1)
        self.view.addSubview(container1)
        
        let view2 = ConstantAreaLabel()
        view2.translatesAutoresizingMaskIntoConstraints = false
        view2.backgroundColor = .red
        let container2 = UIView()
        container2.translatesAutoresizingMaskIntoConstraints = false
        container2.addSubview(view2)
        self.view.addSubview(container2)
        
        NSLayoutConstraint.activate([
            container1.leadingAnchor.constraint(equalTo: view1.leadingAnchor),
            container1.trailingAnchor.constraint(equalTo: view1.trailingAnchor),
            container1.topAnchor.constraint(equalTo: view1.topAnchor),
            container1.bottomAnchor.constraint(equalTo: view1.bottomAnchor),
            
            container1.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 50),
            container1.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 50),
            
            container2.leadingAnchor.constraint(equalTo: view2.leadingAnchor),
            container2.trailingAnchor.constraint(equalTo: view2.trailingAnchor),
            container2.topAnchor.constraint(equalTo: view2.topAnchor),
            container2.bottomAnchor.constraint(equalTo: view2.bottomAnchor),
            
            container2.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 250),
            container2.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 50),
        ])

        debugPrint("view systemLayoutSize width=500", container1.systemLayoutSizeFitting(CGSize(width: 500, height: CGFloat.greatestFiniteMagnitude), withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel))
        debugPrint("view systemLayoutSize width=200", container1.systemLayoutSizeFitting(CGSize(width: 200, height: CGFloat.greatestFiniteMagnitude), withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel))
        
        debugPrint("label systemLayoutSize width=500", container2.systemLayoutSizeFitting(CGSize(width: 500, height: CGFloat.greatestFiniteMagnitude), withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel))
        debugPrint("label systemLayoutSize width=200", container2.systemLayoutSizeFitting(CGSize(width: 200, height: CGFloat.greatestFiniteMagnitude), withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel))
        
        
        let widths = [
            container1.widthAnchor.constraint(equalToConstant: 500),
            container2.widthAnchor.constraint(equalToConstant: 500),
        ]
        NSLayoutConstraint.activate(widths)
            
        self.view.layoutIfNeeded()
        debugPrint("view realSize width=500", container1.bounds.size)
        debugPrint("label realSize width=500", container2.bounds.size)
        
        widths[0].constant = 200
        widths[1].constant = 200
        
        self.view.layoutIfNeeded()
        debugPrint("view width=200 real size", container1.bounds.size)
        debugPrint("label width=200 real size", container2.bounds.size)
    }
}

class ConstantAreaView: UIView {
    init() {
        super.init(frame: .zero)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    private var preferredMaxLayoutWidth: CGFloat? {
        didSet {
            invalidateIntrinsicContentSize()
            setNeedsLayout()
        }
    }
    
    override var intrinsicContentSize: CGSize {
        return calculateSize(width: preferredMaxLayoutWidth)
    }
    
    override func layoutSubviews() {
        let width = bounds.size.width
        if width == UIView.layoutFittingExpandedSize.width {
            preferredMaxLayoutWidth = calculateSize(width: nil).width
        } else if preferredMaxLayoutWidth != bounds.size.width {
            preferredMaxLayoutWidth = bounds.size.width
        } else {
            // manually layout subviews here
            super.layoutSubviews()
        }
    }
}

class ConstantAreaLabel: UILabel {
    init() {
        super.init(frame: .zero)
        numberOfLines = 0
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func draw(_ rect: CGRect) {
        // do nothing, we are not a label actually
    }
    
    override func textRect(forBounds bounds: CGRect, limitedToNumberOfLines numberOfLines: Int) -> CGRect {
        let resultSize = calculateSize(width: bounds.size.width)
        return CGRect(origin: .zero, size: resultSize)
    }
    
    override func layoutSubviews() {
        // layout subviews manually here
        super.layoutSubviews()
    }
}

func calculateSize(width: CGFloat?) -> CGSize {
    let area: CGFloat = 10000
    let defaultWidth: CGFloat = 100
    
    let width = width ?? defaultWidth
    if width <= 0 {
        return .zero
    }
    return CGSize(width: width, height: area / width)
}