Add constraint programmatically to Views

Question

Here is my simple code to add constraint:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let contentView = UIView()
        contentView.backgroundColor = UIColor.greenColor()
        contentView.frame.size.width = 250
        contentView.frame.size.height = 100
        contentView.center = view.center
        view.addSubview(contentView)

        let userNameLabel = UILabel()
        userNameLabel.translatesAutoresizingMaskIntoConstraints = false
        userNameLabel.text = "Username:"
        contentView.addSubview(userNameLabel)

        let passwordLabel = UILabel()
        passwordLabel.translatesAutoresizingMaskIntoConstraints = false
        passwordLabel.text = "Password:"
        contentView.addSubview(passwordLabel)

        let usernameTextField = UITextField()
        usernameTextField.translatesAutoresizingMaskIntoConstraints = false
        usernameTextField.borderStyle = .RoundedRect
        contentView.addSubview(usernameTextField)

        let passwordTextField = UITextField()
        passwordTextField.translatesAutoresizingMaskIntoConstraints = false
        passwordTextField.borderStyle = .RoundedRect
        contentView.addSubview(passwordTextField)

        let submitButton = UIButton()
        submitButton.translatesAutoresizingMaskIntoConstraints = false
        submitButton.setTitle("Submit", forState: .Normal)
        submitButton.setTitleColor(UIColor.redColor(), forState: .Normal)
        submitButton.setTitleColor(UIColor.redColor(), forState: .Highlighted)
        contentView.addSubview(submitButton)

        // For Username
        NSLayoutConstraint(item: userNameLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
        NSLayoutConstraint(item: userNameLabel, attribute: .Top, relatedBy: .Equal, toItem: contentView, attribute: .Top, multiplier: 1, constant: 10).active = true
        NSLayoutConstraint(item: usernameTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
        NSLayoutConstraint(item: usernameTextField, attribute: .Leading, relatedBy: .Equal, toItem: userNameLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
        NSLayoutConstraint(item: userNameLabel, attribute: .Baseline, relatedBy: .Equal, toItem: usernameTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true
        // For Password
        NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
        NSLayoutConstraint(item: passwordLabel, attribute: .Top, relatedBy: .Equal, toItem: userNameLabel, attribute: .Bottom, multiplier: 1, constant: 15).active = true
        NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
        NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
        NSLayoutConstraint(item: passwordLabel, attribute: .Baseline, relatedBy: .Equal, toItem: passwordTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true
        // For Button
        NSLayoutConstraint(item: submitButton, attribute: .Top, relatedBy: .Equal, toItem: passwordLabel, attribute: .Bottom, multiplier: 1.0, constant: 8.0).active = true
        NSLayoutConstraint(item: submitButton, attribute: .CenterX, relatedBy: .Equal, toItem: contentView, attribute: .CenterX, multiplier: 1.0, constant: 0.0).active = true
    }

}

But why it look like this, I want the passWordTextField's Leading is 3 to the Trailing of passWordLabel:

I look at this line and can not find any problems:

NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true

Any helps would be appreciated. Thanks

Robert Gummesson · Accepted Answer · 2016-04-21 13:47:31Z

2

The problem is that the label and textfield have the same hugging priority. You need to define which one to prioritize. Example:

userNameLabel.setContentHuggingPriority(251, forAxis: .Horizontal)
passwordLabel.setContentHuggingPriority(251, forAxis: .Horizontal)

edited Apr 21, 2016 at 13:47

answered Apr 21, 2016 at 13:15

Robert Gummesson

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6 Comments

Twitter khuong291 Over a year ago

It worked. Because I think that UILabel will automatically set it width base on the text inside it. But why the userNameLabel works while the passwordLabel do not work.

Twitter khuong291 Over a year ago

But when I change text on passwordLabel, it looks so bad.

Twitter khuong291 Over a year ago

No, the problem is not that. Please check again.

Robert Gummesson Over a year ago

Ok, I know what the issue is. Let me update my answer.

Robert Gummesson Over a year ago

It works but is randomly deciding which one to prioritize as you can tell by your screenshot. Now that the labels are prioritized, it will behave as expected. You can still add an equal width constraint between the labels to make the textviews start at the same x coordinate.

|

Twitter khuong291 · Accepted Answer · 2016-04-21 13:38:20Z

The problem is that: this line :

NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true

should be appear before with this line:

NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true

the code for password will look like:

 // For Password
 NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
 NSLayoutConstraint(item: passwordLabel, attribute: .Top, relatedBy: .Equal, toItem: userNameLabel, attribute: .Bottom, multiplier: 1, constant: 15).active = true
 NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
 NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
 NSLayoutConstraint(item: passwordLabel, attribute: .Baseline, relatedBy: .Equal, toItem: passwordTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true

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