Why isn't UIButton returning correct constraints?

Question

In my code below: I have 5 buttons added into a vertical scrollView. Each button is constrained to the scrollViews's top + 20 ,leading, trailing edges and its height. I have created a b1HeightConstraint variable. It's there to hold the heightConstraint of the b1 button.

In a button click, I'm trying to remove this constraint. Yet I'm facing an odd issue:

When I log the constraints I only see 2 constraints, even though I've added 4 constraints to it. My the view debug hierarchy is like below:

import UIKit
import Foundation

class ViewController: UIViewController {
    var filterView: UIView!
    var scrollView: UIScrollView!
    var containerView: UIView!

    override func loadView() {
        filterView = UIView()
        view = filterView
        view.backgroundColor = #colorLiteral(red: 0.909803926944733, green: 0.47843137383461, blue: 0.643137276172638, alpha: 1.0)

        scrollView = UIScrollView()
        scrollView.backgroundColor = #colorLiteral(red: 0.474509805440903, green: 0.839215695858002, blue: 0.976470589637756, alpha: 1.0)
        view.addSubview(scrollView)
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        scrollView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
        scrollView.isScrollEnabled = true

        containerView = UIView()
        containerView.backgroundColor = #colorLiteral(red: 0.176470592617989, green: 0.498039215803146, blue: 0.756862759590149, alpha: 1.0)
        scrollView.addSubview(containerView)
        containerView.translatesAutoresizingMaskIntoConstraints = false

        // This is key:  connect all four edges of the containerView to
        // to the edges of the scrollView
        containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
        containerView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
        containerView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor).isActive = true
        containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true

        // Making containerView and scrollView the same height means the
        // content will not scroll vertically
        containerView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
    }



    let b1 = Buttons(titleText: "one")
    let b2 = Buttons(titleText: "two")
    let b3 = Buttons(titleText: "three")
    let b4 = Buttons(titleText: "four")
    let b5 = Buttons(titleText: "five")
    var b1HeightConstraint : NSLayoutConstraint?

    override func viewDidLoad() {
        super.viewDidLoad()


        let buttonArray = [b1, b2, b3, b4, b5]

        b1.button.addTarget(self, action: #selector(ViewController.shrink(_:)), for: .touchUpInside)

        var startPoint = containerView.topAnchor

        for btn in buttonArray {
            let theBtn = btn.button
            containerView.addSubview(theBtn)
            theBtn.translatesAutoresizingMaskIntoConstraints = false
            theBtn.topAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
            theBtn.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
            theBtn.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
            theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor).isActive = true

            startPoint = theBtn.bottomAnchor
            let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
            if btn == b1{
                b1HeightConstraint = btnHeight
            }
        }

        containerView.bottomAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true

    }

    @objc func shrink(_ sender: Any){
        guard let btn = sender as? UIButton else{
            return
        }
        print("count is: \(btn.constraints.count)")

        btn.removeConstraint(b1HeightConstraint!)
        containerView.removeConstraint(b1HeightConstraint!)
        print("count is: \(btn.constraints.count)")
        containerView.updateConstraintsIfNeeded()
        containerView.updateConstraints()
        scrollView.updateConstraintsIfNeeded()
        scrollView.updateConstraints()
    }
}

class Buttons : NSObject {
    let button = UIButton()
    init(titleText: String) {
        button.backgroundColor = #colorLiteral(red: 0.976470589637756, green: 0.850980401039124, blue: 0.549019634723663, alpha: 1.0)
        button.setTitle(titleText, for: .normal)
    }
}

_{The code is ready to just be dumpped in the ViewController class. Works out of the box. My code is a spinoff of the code written here}

Answer 1

Here are several comments about your code:

1. You never added any constraints to any views, so you shouldn't be removing them. iOS (CocoaTouch) added those constraints to those views, so please don't touch them. (In other words: don't call removeConstraint when you didn't call addConstraint). Your control over constraints is activating and deactivating them. Leave the adding and removing to iOS.
2. When you activate a constraint, a constraint is added (by iOS) to the most common ancestor of the two items mentioned in the constraint. So if the two views are siblings, it will be added to the parent. If the two views are parent and child, the constraint will be added to the parent. If the two views are grandparent and grandchild, it will be added to the grandparent. If the two views are first cousins, the constraint will be added to their common grandparent.

3. These lines of code:

   let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
   if btn == b1{
       b1HeightConstraint = btnHeight
   }
   

   are creating a new constraint and assigning it to b1HeightConstraint, but you never activated this constraint, so it hasn't have been added to any view at all. So trying to remove it was never going to work, because that constraint exists only in your b1HeightConstraint property. Since it was never activated, it isn't actually constraining anything.

4. If you want to shrink a button, you need to do one of these: a) modify the constant property of its height constraint OR b) set its height constraint's isActive property to false and then give it a new height constraint OR c) modify the priorities of the active constraints to have Auto Layout choose to use different constraints.

5. In your view debug hierarchy, all the constraints shown are active constraints (meaning they are available to be used by Auto Layout). The grayed out ones are the ones Auto Layout chose not to use because a higher priority constraint had precedence over it. This causes no conflict. The self.height = 34 (content size) constraint is added by the system to account for content compression and content hugging. UIButtons resist compression with priority 750 and resist expansion with priority 250. The self.height = 34 (content size) constraint is grayed out because content hugging has a priority of 250 and another higher priority constraint was used instead (the constraint which sets the button's height equal to the scrollView's height has priority 1000).

---

Updated Code:

Here is your modified code. I changed two things:

1. I made sure b1HeightConstraint was an activated constraint.
2. I changed the shrink method to deactivate the old height constraint and then create and activate a new one.

Updated code

import UIKit
import Foundation

class ViewController: UIViewController {
    var filterView: UIView!
    var scrollView: UIScrollView!
    var containerView: UIView!

    override func loadView() {
        filterView = UIView()
        view = filterView
        view.backgroundColor = #colorLiteral(red: 0.909803926944733, green: 0.47843137383461, blue: 0.643137276172638, alpha: 1.0)

        scrollView = UIScrollView()
        scrollView.backgroundColor = #colorLiteral(red: 0.474509805440903, green: 0.839215695858002, blue: 0.976470589637756, alpha: 1.0)
        view.addSubview(scrollView)
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0).isActive = true
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        scrollView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1).isActive = true
        scrollView.isScrollEnabled = true

        containerView = UIView()
        containerView.backgroundColor = #colorLiteral(red: 0.176470592617989, green: 0.498039215803146, blue: 0.756862759590149, alpha: 1.0)
        scrollView.addSubview(containerView)
        containerView.translatesAutoresizingMaskIntoConstraints = false

        // This is key:  connect all four edges of the containerView to
        // to the edges of the scrollView
        containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
        containerView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor).isActive = true
        containerView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor).isActive = true
        containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true

        // Making containerView and scrollView the same height means the
        // content will not scroll vertically
        containerView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
    }

    let b1 = Buttons(titleText: "one")
    let b2 = Buttons(titleText: "two")
    let b3 = Buttons(titleText: "three")
    let b4 = Buttons(titleText: "four")
    let b5 = Buttons(titleText: "five")
    var b1HeightConstraint : NSLayoutConstraint?

    override func viewDidLoad() {
        super.viewDidLoad()

        let buttonArray = [b1, b2, b3, b4, b5]

        b1.button.addTarget(self, action: #selector(ViewController.shrink(_:)), for: .touchUpInside)

        var startPoint = containerView.topAnchor

        for btn in buttonArray {
            let theBtn = btn.button
            containerView.addSubview(theBtn)
            theBtn.translatesAutoresizingMaskIntoConstraints = false
            theBtn.topAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true
            theBtn.leadingAnchor.constraint(equalTo: containerView.leadingAnchor).isActive = true
            theBtn.trailingAnchor.constraint(equalTo: containerView.trailingAnchor).isActive = true
            //theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor).isActive = true

            startPoint = theBtn.bottomAnchor
            let btnHeight = theBtn.heightAnchor.constraint(equalTo: scrollView.heightAnchor)
            btnHeight.isActive = true
            if btn == b1{
                b1HeightConstraint = btnHeight
            }
        }

        containerView.bottomAnchor.constraint(equalTo: startPoint, constant: 20).isActive = true

    }

    @objc func shrink(_ sender: UIButton) {
        b1HeightConstraint?.isActive = false
        b1HeightConstraint = sender.heightAnchor.constraint(equalToConstant: 20)
        b1HeightConstraint?.isActive = true
    }
}

class Buttons : NSObject {
    let button = UIButton()
    init(titleText: String) {
        button.backgroundColor = #colorLiteral(red: 0.976470589637756, green: 0.850980401039124, blue: 0.549019634723663, alpha: 1.0)
        button.setTitle(titleText, for: .normal)
    }
}

---

Options for shrinking the button's height

1. Setting the constant property of the height constraint

   // shrink button's height by 200 points
   b1HeightConstraint?.constant -= 200
   

2. Deactivate the old constraint and create and activate a new one

   // make button height 20 points
   b1HeightConstraint?.isActive = false
   b1HeightConstraint = sender.heightAnchor.constraint(equalToConstant: 20)
   b1HeightConstraint?.isActive = true
   

3. Change the priority of the height constraint

   // Set b1HeightConstraint's priority to less than 250, and the
   // *content hugging* with priority 250 will take over and resize
   // the button to its intrinsic height
   b1HeightConstraint?.priority = UILayoutPriority(rawValue: 100)
   

   Note: For this to work, you have to give the height constraint an initial priority less than 1000 (999 works nicely) because Auto Layout will not let you change the priority of an active constraint if it is required (priority 1000).

   OR

   // Just deactivate the buttonHeight constraint and the *content
   // hugging* will take over and it will set the button's height
   // to its intrinsic height
   b1HeightConstraint?.isActive = false
   

Answer 2

This is because a constraint between a view and its superView is added to the superView , you only see height/width constraint if they are static added to the UIButton , look to this diagram from Vandad IOS Book

see this Demo