Constraints programmatically for custom UIView in Dynamic ViewController

Question

I'm creating new UIViewController dynamycally using this code

@IBAction func newVCBtnPressed(_ sender: Any) {
            let controller = DynamicVC()
            show(controller, sender: sender)
    }

In the new UIViewController I'm using this code for creation of the new UIView:

override func loadView() {

        view = UIView()
        view.backgroundColor = .lightGray
}

In result I have view with .lightGray backgroundcolor.

I want to add custom UIView and setup the constraints programmatically, and in result i want UIView with following constraints:

top: 0

bottom:(view.frame.height*0.9)

leading:0

trailing:(view.frame.width*0.15)

width:(view.frame.width*0.85)

height:(view.frame.height*0.1)

Example:

Here is my code:

topMenuView = UIView()
        topMenuView.backgroundColor = .red

        view.addSubview(topMenuView)
        topMenuView.translatesAutoresizingMaskIntoConstraints = false
         setupConstraints(item: topMenuView, topC: 0, topToItem: view, bottomC: (view.frame.height*0.9), bottomToItem: view, widthC: (view.frame.width*0.85), heightC: (view.frame.height*0.1), leadingCon: 0, trailingCon: (view.frame.width*0.15))

I'm using this constructed function for constraints:

 func setupConstraints(item:UIView, topC:CGFloat, topToItem:UIView, bottomC:CGFloat, bottomToItem:UIView, widthC:CGFloat, heightC:CGFloat, leadingCon:CGFloat, trailingCon:CGFloat) {

            let topConstraint = NSLayoutConstraint(item: item, attribute: .top, relatedBy: .equal, toItem: topToItem, attribute: .bottom, multiplier: 1, constant: topC)
            let bottomConstraint = NSLayoutConstraint(item: item, attribute: .bottom, relatedBy: .equal, toItem: bottomToItem, attribute: .top, multiplier: 1, constant: bottomC)
            let widthConstraint = NSLayoutConstraint(item: item, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: widthC)
            let heightConstraint = NSLayoutConstraint(item: item, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: heightC)

            let leading = NSLayoutConstraint(item: item,attribute: .leading,relatedBy: .equal, toItem: view, attribute: .leadingMargin, multiplier: 1.0, constant: leadingCon)

            let trailing = NSLayoutConstraint(item: item,attribute: .trailing, relatedBy: .equal, toItem: view, attribute: .trailingMargin,multiplier: 1.0,constant: trailingCon)

            view?.addConstraints([topConstraint, bottomConstraint, widthConstraint, heightConstraint, leading, trailing])

            NSLayoutConstraint.activate([topConstraint, bottomConstraint, widthConstraint, heightConstraint, leading, trailing])
        }

But in the result i receive only UIView with gray background, the new UIView with red background doesn't appears.

What I'm doing wrong???

One thing: only activate the constraints. You should not add them to the view. — vacawama
– vacawama, Commented Nov 22, 2017 at 12:53
It is also odd that you are relating .top of one view to .bottom of another and vice versa. Why not .top to .top and .bottom to .bottom? — vacawama
– vacawama, Commented Nov 22, 2017 at 12:54
Also, ,call super.loadView() in your overridden loadView method. — Puneet Sharma
– Puneet Sharma, Commented Nov 22, 2017 at 12:57

Josh Homann · Accepted Answer · 2017-11-22 12:50:11Z

2

You should only specify bottom OR height and width OR trailing, otherwise you are going to get conflicts here.

see playground:

import PlaygroundSupport
import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let red = UIView()
        red.backgroundColor = .red
        view.addSubview(red)
        red.translatesAutoresizingMaskIntoConstraints = false
        red.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        red.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        red.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.85).isActive = true
        red.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.1).isActive = true
    }
}

PlaygroundPage.current.liveView = ViewController()

answered Nov 22, 2017 at 12:50

Josh Homann

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