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How to activate multiple Auto Layout constraints using activate()

Paul Hudson       @twostraws

Using Auto Layout is the best way to create complex layouts that automatically adapt to their environment, but sometimes adding and removing lots of constraints can cause performance problems.

As an example, here’s a simple UIView with some color so you can see it on-screen:

let vw = UIView()
vw.translatesAutoresizingMaskIntoConstraints = false
vw.backgroundColor = .red
view.addSubview(vw)

We could use Auto Layout anchors to give that constraints: stay 20 points from the leading and trailing edges of its superview, be fixed at 100 points in height, and center itself on-screen:

vw.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 20).isActive = true
vw.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -20).isActive = true
vw.heightAnchor.constraint(equalToConstant: 100).isActive = true
vw.centerYAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerYAnchor).isActive = true

However, while that approach is easy to read – and perfectly fine while you’re learning or if you don’t have complex layouts – there is a more efficient way. NSLayoutConstraint has a class method called activate() that activates multiple constraints at once, which should allow Auto Layout to update its entire layout as a single batch.

The code for this is straightforward: just pass in an array of constraints to the activate() method, like this:

NSLayoutConstraint.activate([
    vw.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 20),
    vw.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -20),
    vw.heightAnchor.constraint(equalToConstant: 100),
    vw.centerYAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerYAnchor)
])

If you need to deactivate constraints, there’s an equivalent deactivate() method that is used the same way.

Note: Auto Layout is smart enough to bulk actual layout changes even with the isActive approach – i.e., it will only call layoutSubviews() once per view even if you change four constraints – but Apple says that using activate() is definitely more efficient.

Available from iOS 8.0

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