How to change the height constraint so that headers collapse to zero?

To collapse a header, you can change the constant of its height constraint to 0.

Example with Auto Layout:

// Assume you have an IBOutlet for the header height constraint
@IBOutlet weak var headerHeightConstraint: NSLayoutConstraint!

// To collapse the header:
headerHeightConstraint.constant = 0.0

// To restore the original height (if known):
// headerHeightConstraint.constant = originalHeaderHeight

// Don't forget to update the layout if you're not using automatic updates
UIView.animate(withDuration: 0.3) {
    self.view.layoutIfNeeded()
}

Example using NSLayoutConstraint.activate/deactivate:

// Assume you have the original height constraint and a zero height constraint
let originalHeaderHeightConstraint: NSLayoutConstraint!
let zeroHeaderHeightConstraint: NSLayoutConstraint!

// During initialization or setup:
originalHeaderHeightConstraint = headerView.heightAnchor.constraint(/* ... original height ... */)
zeroHeaderHeightConstraint = headerView.heightAnchor.constraint(equalToConstant: 0.0)
zeroHeaderHeightConstraint.isActive = false // Initially inactive

// To collapse the header:
NSLayoutConstraint.deactivate([originalHeaderHeightConstraint])
NSLayoutConstraint.activate([zeroHeaderHeightConstraint])

// To restore the original height:
// NSLayoutConstraint.deactivate([zeroHeaderHeightConstraint])
// NSLayoutConstraint.activate([originalHeaderHeightConstraint])

// Animate layout update
UIView.animate(withDuration: 0.3) {
    self.view.layoutIfNeeded()
}

The method choice depends on how you've set up your constraints. Using constant is simpler for existing constraints. Activation/deactivation is useful when you need to switch quickly between different constraint states or when multiple constraints define the height.

It's important to call layoutIfNeeded() (preferably inside the animation block) to apply the changes in constants or constraints activation and to update the UI smoothly.