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iOS Introduction to Auto Layout in iOS Auto Layout in Code Constraints in Code

Greg Kaleka
Greg Kaleka
39,021 Points
Posted by Greg Kaleka
Greg Kaleka
39,021 Points

Adding Constraints in Code

Really not sure what the problem is here. I've created and added all four constraints, but I'm getting the error

Bummer! Make sure you have the correct constraints. One for height, one for width, one for CenterX and one for CenterY
layout.swift 
class MyViewController: UIViewController {

    let sampleView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(sampleView)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        sampleView.translatesAutoresizingMaskIntoConstraints = false

        // Add constraints below
        let sampleViewHeightConstraint  = NSLayoutConstraint(item: sampleView, attribute: .Height, relatedBy: .Equal,
                                          toItem: nil, attribute: .Height, multiplier: 1, constant: 50)
        let sampleViewWidthConstraint   = NSLayoutConstraint(item: sampleView, attribute: .Width, relatedBy: .Equal,
                                          toItem: nil, attribute: .Width, multiplier: 1, constant: 50)
        let sampleViewCenterXConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterX, relatedBy: .Equal,
                                          toItem: view, attribute: .CenterX, multiplier: 1, constant: 0)
        let sampleViewCenterYConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterY, relatedBy: .Equal,
                                          toItem: view, attribute: .CenterY, multiplier: 1, constant: 0)
        view.addConstraints([
          sampleViewHeightConstraint,
          sampleViewWidthConstraint,
          sampleViewCenterXConstraint,
          sampleViewCenterYConstraint
          ])
    }

}
Terrell Robinson
Terrell Robinson
6,957 Points

Hey Greg!

Not sure if you completed this since you posted this question, but the only error that I see is that your multipliers and constant fields aren't doubles. i.e. (multiplier: 1.0, constant: 50.0)

Try that out and it should work!

        let sampleViewCenterXConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1.0, constant: 0)

        let sampleViewCenterYConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1.0, constant: 0)

        let sampleViewHeightConstraint = NSLayoutConstraint(item: sampleView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50.0)

        let sampleViewWidthConstraint = NSLayoutConstraint(item: sampleView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50.0)

        view.addConstraints([
            sampleViewCenterXConstraint,
            sampleViewCenterYConstraint,
            sampleViewHeightConstraint,
            sampleViewWidthConstraint
            ])
    }
Greg Kaleka
Greg Kaleka
39,021 Points
Greg Kaleka
Greg Kaleka
39,021 Points

Thanks Terrell. Unfortunately, that did not fix the problem.

withDoubles.swift 
class MyViewController: UIViewController {

    let sampleView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(sampleView)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        sampleView.translatesAutoresizingMaskIntoConstraints = false

        // Add constraints below
        let sampleViewHeightConstraint  = NSLayoutConstraint(item: sampleView, attribute: .Height, relatedBy: .Equal,
                                          toItem: nil, attribute: .Height, multiplier: 1.0, constant: 50.0)
        let sampleViewWidthConstraint   = NSLayoutConstraint(item: sampleView, attribute: .Width, relatedBy: .Equal,
                                          toItem: nil, attribute: .Width, multiplier: 1.0, constant: 50.0)
        let sampleViewCenterXConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterX, relatedBy: .Equal,
                                          toItem: view, attribute: .CenterX, multiplier: 1.0, constant: 0.0)
        let sampleViewCenterYConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterY, relatedBy: .Equal,
                                          toItem: view, attribute: .CenterY, multiplier: 1.0, constant: 0.0)
        view.addConstraints([
          sampleViewHeightConstraint,
          sampleViewWidthConstraint,
          sampleViewCenterXConstraint,
          sampleViewCenterYConstraint
          ])
    }

}

1 Answer

Chris Adamson
Chris Adamson
132,143 Points
Chris Adamson
Chris Adamson
132,143 Points

This is a tricky one, you were close the attribute is .NotAnAttribute for the 2nd item in the constraint:

class MyViewController: UIViewController {

    let sampleView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(sampleView)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        sampleView.translatesAutoresizingMaskIntoConstraints = false

        // Add constraints below
        let sampleViewHeightConstraint  = NSLayoutConstraint(item: sampleView, attribute: .Height, relatedBy: .Equal,
                                          toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50.0)
        let sampleViewWidthConstraint   = NSLayoutConstraint(item: sampleView, attribute: .Width, relatedBy: .Equal,
                                          toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50.0)
        let sampleViewCenterXConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterX, relatedBy: .Equal,
                                          toItem: view, attribute: .CenterX, multiplier: 1.0, constant: 0.0)
        let sampleViewCenterYConstraint = NSLayoutConstraint(item: sampleView, attribute: .CenterY, relatedBy: .Equal,
                                          toItem: view, attribute: .CenterY, multiplier: 1.0, constant: 0.0)

        view.addConstraints([sampleViewHeightConstraint,sampleViewWidthConstraint,sampleViewCenterXConstraint,sampleViewCenterYConstraint])
    }

}
kols
kols
27,007 Points

Chris Adamson is correct just with the exception that — as of this time (Swift 4) — the member values provided for the 'attribute' parameters (e.g., attribute: .centerX) need to be lowerCamelCase. (Adding full code below for reference / ease.)

class MyViewController: UIViewController {

    let sampleView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(sampleView)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        sampleView.translatesAutoresizingMaskIntoConstraints = false

        // Add constraints below
        setupSampleViewConstraints()
    }

    func setupSampleViewConstraints() {

        let sampleViewCenterXConstraint = NSLayoutConstraint(item: sampleView, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 1.0, constant: 0.0)  
        let sampleViewCenterYConstraint = NSLayoutConstraint(item: sampleView, attribute: .centerY, relatedBy: .equal, toItem: view, attribute: .centerY, multiplier: 1.0, constant: 0.0)
        let sampleViewHeightConstraint = NSLayoutConstraint(item: sampleView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50.0)
        let sampleViewWidthConstraint = NSLayoutConstraint(item: sampleView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 50.0)

        view.addConstraints([sampleViewCenterXConstraint, sampleViewCenterYConstraint, sampleViewHeightConstraint, sampleViewWidthConstraint])
    }
}