What changes need to be made to constraints before calling layout?

Changes to constraints should be made before calling layoutIfNeeded() or before the end of the update cycle, during which the system automatically calls layout. The most common way is within the updateConstraints() method of a custom UIView or UIViewController.updateViewConstraints().

Example of changing constraints in updateConstraints():

override func updateConstraints() {
    // Check if the constraint is not yet created or needs to be changed
    if myConstraint == nil {
        myConstraint = viewA.bottomAnchor.constraint(equalTo: viewB.topAnchor, constant: -8)
        myConstraint?.isActive = true // Activate the constraint
    }

    // Change the constraint's value if necessary
    myConstraint?.constant = someNewValue

    // Always call super
    super.updateConstraints()
}

Changes can also be made directly before calling layoutIfNeeded(), for example, in response to a user action or a change in state:

// Change the value of an existing constraint
someConstraint.constant = newValue

// Call layoutIfNeeded() for immediate application of changes
view.layoutIfNeeded()

The main thing is that constraint changes are completed before Auto Layout begins calculating new frames.