Auto Layout · priorities · UIStackView · self-sizing cells

How it works

• translatesAutoresizingMaskIntoConstraints = false
  on every code-created view — else UIKit turns the autoresizing mask into constraints that fight yours (wrong place / zero size / conflict log). IB sets it for you. Then NSLayoutConstraint.activate([…]).
• Anchors: leading/trailing (flip for RTL — prefer them over left/right), top/bottom, centerX/Y, width/height, firstBaseline. Trailing/bottom constants are negative (-16).
• Intrinsic content size: UILabel, UIButton, UIImageView, UISwitch know their natural size, so position constraints alone suffice. Plain UIView has none. Custom: override intrinsicContentSize, call invalidateIntrinsicContentSize() when it changes.
• Hugging (default 250) = resist growing beyond intrinsic size. Compression resistance (default 750) = resist shrinking below it. Per axis. Lower value loses: stretches first / truncates first.
• UILayoutPriority: .required = 1000, .defaultHigh = 750, .defaultLow = 250, .fittingSizeLevel = 50. <1000 = optional: broken silently, lowest first. Tie at 750 vs 750 → arbitrary; nudge one to 749 / 251 to make it deterministic.
• Unsatisfiable (“Unable to simultaneously satisfy constraints”) = ≥2 required constraints contradict (leading 0 + width 300 + trailing 0 in 320 pt). The engine breaks one — “Will attempt to recover by breaking…” — which one is arbitrary. Fix: drop one below 1000. Debug: symbolic breakpoint UIViewAlertForUnsatisfiableConstraints, the View Debugger, hasAmbiguousLayout.
• Safe area: pin content to view.safeAreaLayoutGuide. Superview edges put text under the notch / Dynamic Island and buttons under the home indicator (the system gesture eats the tap). Full-bleed backgrounds may go to the real edges.
• Size classes: two axes (horizontal, vertical) × two values (.compact, .regular). iPhone portrait = wC hR; iPhone landscape = wC hC (Max/Plus: wR hC); iPad full screen = wR hR; iPad narrow split = wC. Read traitCollection.horizontalSizeClass; react on trait change (swap constraints / stack axis).
• Animate a constraint: change .constant, then UIView.animate { superview.layoutIfNeeded() }.

Example — a title + price row

let row = UIStackView(arrangedSubviews: [title, price])
row.axis = .horizontal; row.spacing = 8  // direction, gap
row.distribution = .fill          // sizing ALONG the axis
row.alignment = .firstBaseline    // position ACROSS the axis
row.translatesAutoresizingMaskIntoConstraints = false // #1 bug
view.addSubview(row)
let m = view.layoutMarginsGuide   // inset, inside the safe area
NSLayoutConstraint.activate([     // height: intrinsic size
  row.topAnchor.constraint(equalTo: m.topAnchor),
  row.leadingAnchor.constraint(equalTo: m.leadingAnchor),
  row.trailingAnchor.constraint(equalTo: m.trailingAnchor)])
title.setContentHuggingPriority(.defaultLow - 1, for: .horizontal)
title.setContentCompressionResistancePriority(.defaultHigh - 1,
                                              for: .horizontal)

Result: title (249 / 749) stretches to fill and truncates first; price (250 / 750) stays compact and readable. .fill picks the stretcher by lowest hugging.

Picture — the priority scale

Picture — hugging vs compression

Picture — stack view · self-sizing cell

Cell: tableView.rowHeight = UITableView.automaticDimension + tableView.estimatedRowHeight = 80 (non-zero) + the chain; no heightForRowAt. Stack: add with addArrangedSubview (not addSubview); isHidden collapses a child.

Interview traps

• Priority numbers (Q25, blank): 1000 / 750 / 250, and why 749: a strict loser beats a random tie.
• UIStackView (Q19, blank): axis, distribution, alignment, spacing. Distribution = along, alignment = across.
• Self-sizing cells (Q23): not the delegate — automaticDimension + estimatedRowHeight + chain.
• Hugging is about growing past content, not about “edges” (Q8).
• Wrong place / zero size in code → first suspect translates… (Q7).
• Size classes = compact/regular per axis, not device sizes (Q24).

Remember

“A thousand is law, 750 guards the content, 250 lets it grow.” Stack: A-D-A-S — Axis, Distribution, Alignment, Spacing. Cell: auto + estimate + chain.

Likely questions

1. Four facts per view? — x, y, width, height (constraints or intrinsic size).
2. Why translates…= false? — autoresizing constraints clash with yours.
3. Label truncates instead of its neighbour? — lower its compression resistance.
4. Required conflict vs optional? — warning + arbitrary break vs silent, lowest loses.