How To Fix Swift Error Unable To Activate Constraint With Anchors Because They Have No Common Ancestor & LayoutConstraints Unable To Simultaneously Satisfy Constraints

When I develop an iOS app, I add a UITextField in the app UI programmatically. And I also add some constraints to the UITextField object in swift source code to make it auto layout. But when I run the app, it throws an exception with the error message ‘NSGenericException’, reason: ‘Unable to activate constraint with anchors, because they have no common ancestor like below.

1. The Detailed Error Messages.

  1. Below are the detailed error messages of this NSGenericException error.
    Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors <NSLayoutYAxisAnchor:0x600000f99340 "UITextField:0x7fd8b7404bd0.top"> and <NSLayoutYAxisAnchor:0x600000f99380 "UIView:0x7fd8b760e140.top"> because they have no common ancestor.  Does the constraint or its anchors reference items in different view hierarchies?  That's illegal.'

2. How To Fix The NSGenericException Error.

  1. After some time investigating, I finally find the method to fix this error.
  2. The error is because I do not add the programmatically created UITextField object into the iOS app root view’s subview hierarchy.
  3. So I need to use the below code to add it as the iOS app root view’s subview self.view.addSubview(urlTextBox).
  4. Below is the full source code.
    //
    //  ViewController.swift
    //  iOSWebViewExample
    //
    //  Created by song zhao on 11/28/19.
    //  Copyright © 2019 dev2qa.com. All rights reserved.
    //
    
    import UIKit
    import WebKit
    
    class ViewController: UIViewController {
        
        // First create an instance of UITextField class. 
        let urlTextBox:UITextField = UITextField()
          
        override func viewDidLoad() {
            super.viewDidLoad()
            
            initUrlTextBox()
        }
        
        func initUrlTextBox(){
            
            /* Add the UITextField as the subview of the app root view. This line of code can avoid 'NSGenericException', reason: 'Unable to activate constraint with anchors because they have no common ancestor error when adding constraints to the UITextField object later. */
            self.view.addSubview(urlTextBox1)
            
            /*  Set the UITextField object's translatesAutoresizingMaskIntoConstraints property's value to false. This can avoid [LayoutConstraints] Unable to simultaneously satisfy constraints error when add constraints to the UITextField object later. */
            urlTextBox.translatesAutoresizingMaskIntoConstraints = false
            
            // Set the UITextField object's text.
            urlTextBox.text = "Hello World!"
            
            // Set the UITextField object's backgroud color.
            urlTextBox.backgroundColor = UIColor.green
            
            // Create and activate top anchor constraint, center X anchor constraint and width anchor constraint. 
            let urlTextBoxTopAnchor = urlTextBox.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 500)
            urlTextBoxTopAnchor.isActive = true
            
            
            let urlTextBoxCenterXAnchor = urlTextBox.centerXAnchor.constraint(equalTo: self.view.centerXAnchor)
            urlTextBoxCenterXAnchor.isActive = true
            
            let urlTextBoxWidthAnchor = urlTextBox.widthAnchor.constraint(equalTo: self.view.widthAnchor, multiplier: 0.8)
            urlTextBoxWidthAnchor.isActive = true
           
        }
        
    }
    
  5. The above source code also fixes another error which has the following error message.
    [LayoutConstraints] Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want.
  6. This error is because I do not set the UITextField’s property translatesAutoresizingMaskIntoConstraints value to false before adding constraints to the UITextField object.
  7. So we should first set the UI component’s property translatesAutoresizingMaskIntoConstraints to false before add constraints to it like this urlTextBox.translatesAutoresizingMaskIntoConstraints = false.

3. How To Fix The Post-update Error: Unable To Activate Constraint With Anchors In Xcode 11.5.

3.1 Question.

  1. My iOS app throws the error Post-Update Error: Unable to activate constraint with anchors when I upgrade my Xcode to version 11.5.
  2. There is a TabBar with 6 tab items in it, each tab item contains a view object, when I click one of the tab views, it shows the below error message.
    Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors <NSLayoutYAxisAnchor:0x60000064b4c0 "UILabel:0x7fb6dd8b8b80'Day 1'.centerY"> and <NSLayoutYAxisAnchor:0x600000649780 "UIView:0x7fb6dd4b89d0.centerY"> because they have no common ancestor. Does the constraint or its anchors reference items in different view hierarchies? That's illegal.'
  3. There is a segControl object in the view and the object is used to update a TableView‘s data by pulling the data from the internet.
  4.  I do not change any code, just upgrade my Xcode to version 11.5. And maybe there are some constraints conflicting between labels and views, but I can not figure them out.
  5. Can anyone tell me how to fix this issue? Thanks a lot.

3.2 Answer1.

  1. The reason for this issue is becaue maybe you have modified the IBOutlet property variable names.
  2. You can read the article How To Rename IBOutlet Variable Or IBAction Function Without Lose Connection In Swift to learn more.

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