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iOS Build an Interactive Story App with Swift 2 Creating the User Interface Programmatically Creating Views Programmatically

Posted by Marc Jones
Marc Jones
3,751 Points

NSLayoutConstraint & ConstraintEqualToAnchor

Hi All,

Apologies if I missed this in the video. I'm going to re-watch in case I did, but when adding the layout constraints, how does it know what values to use? Or am I missing something obvious here?

override func viewWillLayoutSubviews() {
        view.addSubview(artwork)
        //turn off autolayout
        artwork.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activateConstraints([
            artwork.topAnchor.constraintEqualToAnchor(view.topAnchor),
            artwork.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor),
            artwork.rightAnchor.constraintEqualToAnchor(view.rightAnchor),
            artwork.leftAnchor.constraintEqualToAnchor(view.leftAnchor)
        ])


    }

Thanks, Marc

Hemanth C
Hemanth C
8,970 Points

It is not using the values per se. Those 4 constraint added in NSLayoutConstraint basically says that artwork topAnchor/bottomAnchor/rightAnchor/leftAnchor should be equal to main view's topAnchor/bottomAnchor/rightAnchor/leftAnchor respectively.

view's topAnchor is the top edge of the layout guide’s frame.So artwork topAnchor have the same data has view's topAnchor. Similarly for others as well.

Abdulwahab Alansari
Abdulwahab Alansari
15,151 Points
Abdulwahab Alansari
Abdulwahab Alansari
15,151 Points

Thar raises a question, why the bottomAnchor of artwork does not equal the bottomAnchor of the view, the image didn't stretch to the bottom of the view