[iOS Issue] UIView.animate에서 constraint 변경 동작 안함

How do I animate constraint changes?

 

아래와 같이 Constraint의 constant 값을 수정해서 UIView의 크기를 조절하는 애니메이션 함수를 만들었다.

//
//  ViewController.swift
//  Practice
//
//

import UIKit

class ViewController: UIViewController {
    
    @IBOutlet weak var redView: UIView!
    @IBOutlet weak var redViewWidth: NSLayoutConstraint!
    @IBOutlet weak var redViewHeight: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        
    }
    
    @IBAction func show(_ sender: Any) {
        UIView.animate(withDuration: 0.2, delay: 0, options: .curveLinear, animations: {
            self.redViewWidth.constant = 240
        }, completion: nil)
    }
    
    @IBAction func close(_ sender: Any) {
        UIView.animate(withDuration: 0.2, delay: 0, options: .curveLinear, animations: {
            self.redViewWidth.constant = 0
        }, completion: nil)
    }
}

하지만 뭔가 내가 예상했던대로 동작하지 않는다.

이 때 animate안에 view.layoutIfNeeded를 호출해주면 정상 동작한다.

 

//
//  ViewController.swift
//  Practice
//

import UIKit

class ViewController: UIViewController {
    
    @IBOutlet weak var redView: UIView!
    @IBOutlet weak var redViewWidth: NSLayoutConstraint!
    @IBOutlet weak var redViewHeight: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        
    }
    
    @IBAction func show(_ sender: Any) {
        UIView.animate(withDuration: 0.2, delay: 0, options: .curveLinear, animations: {
            self.redViewWidth.constant = 240
            self.view.layoutIfNeeded()
        }, completion: nil)
    }
    
    @IBAction func close(_ sender: Any) {
        UIView.animate(withDuration: 0.2, delay: 0, options: .curveLinear, animations: {
            self.redViewWidth.constant = 0
            self.view.layoutIfNeeded()
        }, completion: nil)
    }
}