I created a simple vertical stack view containing 3 subview with different width.
I pin the stack view to top left of the parent and did not specify width on purpose.

I expect the stack view to use the width of the largest subview as its width. As stated from apple doc: https://developer.apple.com/documentation/uikit/uistackview

Quote:

Perpendicular to the stack view’s axis, its fitting size is equal to the size of the largest arranged view.

but the result looks like the stackview uses the width of the smallest subview.

I don’t understand why the code behave this way.
Here is my code:

import Foundation
import UIKit
import SwiftUI
import SnapKit

class TestViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        
        // Create the stack view with a vertical axis
        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.backgroundColor = .gray
        stackView.spacing = 20
        stackView.alignment = .fill  // Change this to .leading to see the effect
        stackView.translatesAutoresizingMaskIntoConstraints = false
        
        // Add the stack view to the view controller's view
        view.addSubview(stackView)
        
        // Constraints for the stack view
        stackView.snp.makeConstraints { make in
            make.top.equalTo(view)
            make.left.equalTo(view)
        }
        
        // Create three views with different widths
        let view1 = UIView()
        view1.translatesAutoresizingMaskIntoConstraints = false
        view1.backgroundColor = .red
        view1.snp.makeConstraints { make in
            make.width.equalTo(100)
            make.height.equalTo(50)
        }
        
        let view2 = UIView()
        view2.translatesAutoresizingMaskIntoConstraints = false
        view2.backgroundColor = .green
        view2.snp.makeConstraints { make in
            make.width.equalTo(200)
            make.height.equalTo(50)
        }
        
        let view3 = UIView()
        view3.translatesAutoresizingMaskIntoConstraints = false
        view3.backgroundColor = .blue
        view3.snp.makeConstraints { make in
            make.width.equalTo(300)
            make.height.equalTo(50)
        }
        
        // Add views to the stack view
        stackView.addArrangedSubview(view2)
        stackView.addArrangedSubview(view1)
        stackView.addArrangedSubview(view3)
    }
    
}



struct TestViewControllerTestView : UIViewControllerRepresentable {
    
    typealias UIViewControllerType = TestViewController
    
    func makeUIViewController(context: Context) -> TestViewController {
        return TestViewController()
    }
    
    func updateUIViewController(_ uiViewController: UIViewControllerType, context: Context) {
        
    }
}


struct TestViewController_Preview : PreviewProvider {
    static var previews: some View {
        TestViewControllerTestView()
    }
}