What is Auto Layout and how does it work?

Auto Layout is UIKit's constraint-based layout system that allows views to automatically calculate their size and position based on relationships (constraints) between views. It adapts to different screen sizes, orientations, and dynamic content. Constraints define: a view's width, height, position relative to other views or the superview. Each constraint is a linear equation: view1.attribute = multiplier × view2.attribute + constant. Anchors API (programmatic): // Add subview: view.addSubview(label) label.translatesAutoresizingMaskIntoConstraints = false // REQUIRED NSLayoutConstraint.activate([ label.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 16), label.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 16), label.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -16) ]). UIStackView: container that arranges subviews automatically: let stack = UIStackView(arrangedSubviews: [label, button]) stack.axis = .vertical stack.spacing = 8 stack.distribution = .fill. Visual Format Language: string-based constraints (less common now): NSLayoutConstraint.constraints(withVisualFormat: "H:|-16-[view]-16-|", ...). Compression resistance and content hugging: priorities that control how views behave when there's extra or insufficient space. Safe area layout guide: accounts for notch, home indicator, status bar. Use safeAreaLayoutGuide instead of direct superview anchors. Intrinsic content size: some views (UILabel, UIButton) have a natural size based on content — Auto Layout respects this without explicit width/height constraints.