Update the Constant Property of a Constraint Programmatically in Swift

How to update the constant height constraint of a UIView programmatically?

Select the height constraint from the Interface builder and take an outlet of it. So, when you want to change the height of the view you can use the below code.

yourHeightConstraintOutlet.constant = someValue
yourView.layoutIfNeeded()

Method updateConstraints() is an instance method of UIView. It is helpful when you are setting the constraints programmatically. It updates constraints for the view. For more detail click here.

Update the constant property of a constraint programmatically in Swift?

In order to declare an animation, you cannot re-define the constraint and call updateConstraints. You are supposed to change the constant of your constraint and follow the format below:

self.view.layoutIfNeeded()
UIView.animate(withDuration: 1) {
    self.sampleConstraint.constant = 20
    self.view.layoutIfNeeded()
}

Modify constraint programmatically Swift

So this is the way I achieved this :

• Create a constraint programmatically (height in my case) :

  // Drawing height property
  var drawingHeightConstraint: NSLayoutConstraint?
  

• Deactivate old constraint and set the new one if needed

Note: heightContraints is an array of NSLayoutConstraint which contains my outlets

        for (index, (drawing, ratio)) in drawingElements.enumerate() {
        drawingViews[index].image = UIImage(named: drawing)

        // update height constraint if ratio is different than defaut ratio of 1/2
        if ratio != 0.5 {
            heightConstraints[index].active = false
            drawingHeightConstraint = NSLayoutConstraint(item: drawingViews[index], attribute: .Height, relatedBy: .Equal, toItem: drawingView, attribute: .Height, multiplier: CGFloat(ratio), constant: 0)
            drawingHeightConstraint!.active = true
        }

    }

• Call layoutIfNeeded() right after (note: not sure when to call it)

Update height constraint programmatically

Instead of adding a new constraint, you need to modify the constant on your existing constraint.

Use an IBOutlet to connect to your constraint in Interface Builder:

@property (nonatomic, weak) NSLayoutConstraint *heightConstraint;

Then, when you need to set it programmatically, simply set the constant property on the constraint:

heightConstraint.constant = 100;

OR

If you can't access the nib in Interface Builder, find the constraint in code:

NSLayoutConstraint *heightConstraint;
for (NSLayoutConstraint *constraint in myView.constraints) {
    if (constraint.firstAttribute == NSLayoutAttributeHeight) {
        heightConstraint = constraint;
        break;
    }
}
heightConstraint.constant = 100;

And in Swift:

if let constraint = (myView.constraints.filter{$0.firstAttribute == .width}.first) {
            constraint.constant = 100.0
        }

Updating constraints in swift 5

first declare the variable, after set start constraints variable to true and after call a function that active and dis active constraints, try with my example:

under controller class declare a button and a view do you want to move:

let dummyView = UIView()
let button = UIButton(type: .system)

after declare variable to move your view with constraint:

var up: NSLayoutConstraint?
var down: NSLayoutConstraint?

in ViewDiLoad set the view, the button and the constraints like this:

dummyView.backgroundColor = .yellow
    dummyView.translatesAutoresizingMaskIntoConstraints = false

    button.setTitle("viewDiwn", for: .normal)
    button.backgroundColor = .red
    button.setTitleColor(.white, for: .normal)
    button.titleLabel?.font = .systemFont(ofSize: 16)
    button.addTarget(self, action: #selector(handletapButton), for: .touchUpInside)// button action that call handletapButton func
    button.translatesAutoresizingMaskIntoConstraints = false

    view.addSubview(dummyView)
    up = dummyView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor)
    up?.isActive = true // start constraint active
    down = dummyView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 100) // constraint to activate for move the view

    dummyView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
    dummyView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
    dummyView.heightAnchor.constraint(equalToConstant: 50).isActive = true

    view.addSubview(button)
    button.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
    button.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
    button.heightAnchor.constraint(equalToConstant: 50).isActive = true
    button.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true

now write the function to move the view by constraint animation:

@objc func handletapButton() {
    UIView.animate(withDuration: 0.5, delay: 0, options: .curveEaseOut, animations: {
        self.up?.isActive = false
        self.down?.isActive = true
        self.view.layoutIfNeeded()
    }, completion: nil)
}

this is the result

changing constraints at runtime in swift

just make the outlet of height of both UIView and use constant property to change the height constraints like...

if conditionfails{
     heightconstOfView1.constant = 0
}
else{
    heightconstOfView2.constant = 0
}

can't change constraint.constant programmatically in viewDidLoad:

See that wR hR option under Constant? That was the problem. Looks like constraints that use size classes cant be set in viewDidLoad.

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