Explain the layout cycle. What is the difference between setNeedsLayout and layoutIfNeeded?

#TL;DR

UIKit does three passes, top to bottom, before it renders a frame: update constraints, layout, display. Each pass has a "mark as dirty" method and a method that does the work. setNeedsLayout() marks the view dirty. Layout happens later, once, at the end of the run loop. layoutIfNeeded() runs layout now, if something is dirty.

#The three passes

Never call layoutSubviews(), updateConstraints() or draw(_:) directly. Ask UIKit to do it.

#Why "later" is good

Many changes in one run loop pass become one layout pass.

titleLabel.text = "New title"
subtitleLabel.text = "New subtitle"
badge.isHidden = false
// Each change calls setNeedsLayout() inside UIKit.
// Layout runs once at the end, not three times.

This batching is why UIKit is fast.

#When you need layoutIfNeeded()

#1. Animating constraints

topConstraint.constant = 0
UIView.animate(withDuration: 0.25) {
    self.view.layoutIfNeeded() // layout inside the animation block = animated frames
}

Call it on the superview that contains the changing views. Some people also call layoutIfNeeded() before the animation. This flushes old pending changes so they do not animate too.

#2. Reading a correct frame right away

stackView.addArrangedSubview(newView)
stackView.layoutIfNeeded()
print(newView.frame) // now correct

Without layoutIfNeeded(), the frame would still be .zero.

#Writing layoutSubviews

final class AvatarView: UIView {
    private let imageView = UIImageView()

    override func layoutSubviews() {
        super.layoutSubviews()
        imageView.frame = bounds
        layer.cornerRadius = bounds.width / 2
    }
}

Rules:

• Call super first. With Auto Layout, super applies the constraint results.
• Only set frames of subviews. Do not change your own frame.
• Do not change constraints here. It can cause an infinite layout loop.
• Keep it fast. It can run many times.

#setNeedsDisplay is different

It is only for custom drawing in draw(_:). Changing a frame does not redraw content by default. Changing a label's text calls it for you.

var progress: CGFloat = 0 {
    didSet { setNeedsDisplay() }
}

#updateConstraints

Apple recommends changing constraints right where the change happens. Use updateConstraints() only when many changes happen together and batching gives a real speed win. If you override it, call super last.

#Common follow-up questions

• What triggers layout automatically? Changing the view size, adding or removing subviews, rotation, and constraint changes.
• Can layoutSubviews cause an infinite loop? Yes, if it changes something that marks the same view dirty again.
• Why is my frame zero in viewDidLoad? Layout has not run yet. Use viewDidLayoutSubviews or viewIsAppearing.
• What is the SwiftUI version? A custom Layout with sizeThatFits and placeSubviews. SwiftUI also batches updates per frame.