Irbisa · cheatsheetSeptember 13, 2026

UIKit Fundamentals

UIView, UIViewController lifecycle, view hierarchy, touch handling

Junior Developer20 itemscompressed for a skim

01
What is UIKit? How does it relate to SwiftUI in modern iOS apps?
Easy
UIKit is Apple's imperative UI framework for iOS, iPadOS, tvOS and Catalyst — it has powered every iOS app since 2008.
```
import UIKit

class HomeViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .systemBackground
…
```
02
Walk through the UIViewController lifecycle: which callbacks fire once and which fire on every appearance?
Easy
Only viewDidLoad fires once per controller instance; the appear and disappear callbacks fire again every single time the screen comes back.
```
class FeedViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    // One-time: build the view tree, register cells, set delegates
  }
…
```

What is the difference between frame and bounds in UIView?

Easy

A view's frame is measured in its superview's coordinate system, its bounds is the view's own coordinate system, and that difference is exactly what makes scrolling work.

let v = UIView()
v.frame = CGRect(x: 20, y: 100, width: 120, height: 60)
// frame.origin = (20,100), bounds.origin = (0,0), bounds.size = (120,60)

// Subview is positioned in v's coordinate system
let inner = UIView(frame: CGRect(x: 10, y: 10, width: 20, height: 20))
…

When the user taps the screen, how does UIKit decide which view receives the touch?

Medium

Every touch is resolved in two passes: UIKit first hit-tests to find the view under the finger, then delivers the event up the responder chain starting from that view.

class TouchView: UIView {
  override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    print("Touch began on \(self)")
    // Omit super to consume the touch; call super to forward it up the chain
    super.touchesBegan(touches, with: event)
  }
…

How do UITableView and UICollectionView work? What is reuse?

Medium

Both are scroll-based containers backed by a data source and a delegate.

// Modern diffable data source for UICollectionView
class FeedVC: UIViewController {
  enum Section { case main }
  struct Item: Hashable { let id: UUID; let title: String }

  var collectionView: UICollectionView!
…

What is the difference between Storyboards, XIBs, and programmatic UIs?

Medium

Storyboards, XIBs and code are three ways to build the same UIKit screen, and they differ mainly in how well they survive a team, a merge and a refactor.

// Storyboard — load by identifier
let sb = UIStoryboard(name: "Main", bundle: nil)
let vc = sb.instantiateViewController(identifier: "DetailVC") as! DetailVC
navigationController?.pushViewController(vc, animated: true)

// XIB — instantiate a UIView subclass from its nib
…

Since scenes arrived, which lifecycle events belong to AppDelegate and which to SceneDelegate?

Easy

AppDelegate owns process-level events, while each UI instance is a UIWindowScene whose SceneDelegate owns the window and the foreground and background transitions.

@main
final class AppDelegate: UIResponder, UIApplicationDelegate {
  // Process level, once per launch: DI graph, analytics, push registration
  func application(_ app: UIApplication,
                   didFinishLaunchingWithOptions options: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    Services.bootstrap()
…

Every UIView owns a CALayer — what does the layer actually do, and when do you drop down to it?

Medium

The view handles touches, layout and the responder chain, while the layer it owns holds the bitmap that is actually drawn and animated.

final class GradientButton: UIButton {
  // Back the view with the layer type you need instead of syncing frames in layoutSubviews
  override class var layerClass: AnyClass { CAGradientLayer.self }
  private var gradient: CAGradientLayer { layer as! CAGradientLayer }

  override init(frame: CGRect) {
…

What is the difference between pushing a view controller, presenting one, and embedding one as a child?

Medium

Pushing adds a controller to an existing navigation stack, presenting puts one on top modally regardless of any stack, and containment makes one controller a child that lives inside your own view.

// Push: onto an existing navigation stack, with a back button and interactive pop
navigationController?.pushViewController(DetailViewController(id: id), animated: true)
navigationController?.popToRootViewController(animated: true)

// Present: modally, above everything, independent of any stack
let editor = UINavigationController(rootViewController: EditorViewController())
…

10
Why is a delegate property declared weak, and when would you choose a closure or a notification instead?
Easy
A delegate is a one-to-one callback contract, and the property is weak because the delegate almost always owns the object it delegates for — a strong reference back would close a retain cycle and leak both.
```
protocol EditorDelegate: AnyObject {   // class-bound, so the property can be weak
  func editor(_ editor: EditorViewController, didSave draft: Draft)
  func editorDidCancel(_ editor: EditorViewController)
}

final class EditorViewController: UIViewController {
…
```
11
Which UIKit animation API do you reach for, and how do you animate an Auto Layout change?
Medium
Animating a constraint means changing its constant and then calling layoutIfNeeded() inside the animation block, because it is the layout pass — not the assignment — that gets animated.
```
// Animating a constraint: change the constant, then lay out inside the block
heightConstraint.constant = isExpanded ? 240 : 88
UIView.animate(withDuration: 0.3) {
  self.view.layoutIfNeeded()   // the layout pass is what is actually being animated
}
…
```

How do you put a UIKit view inside SwiftUI, and a SwiftUI view inside UIKit?

Medium

UIViewRepresentable wraps a UIKit view so SwiftUI can use it, and UIHostingController wraps a SwiftUI view so UIKit can use it.

struct SearchBar: UIViewRepresentable {
  @Binding var text: String

  func makeUIView(context: Context) -> UISearchBar {
    let bar = UISearchBar()
    bar.delegate = context.coordinator     // the delegate lives on the coordinator
…

13
Your button's contentEdgeInsets do nothing and the font you set on titleLabel resets after a tap — what replaced that API?
Easy
Since iOS 15 a UIButton.Configuration describes the whole button as one value, and the button rebuilds itself from that value on every state change — which is why anything you poke directly onto titleLabel is overwritten and the edge-inset properties are deprecated.
```
// iOS 15+: one value describes layout and appearance
var config = UIButton.Configuration.filled()
config.title = "Save"
config.subtitle = "to your library"
config.image = UIImage(systemName: "square.and.arrow.down")
config.imagePlacement = .leading
…
```
14
An avatar comes out squashed and the photo spills past the rounded corners you set — which two view properties are wrong?
Easy
contentMode defaults to .scaleToFill, which stretches the image to the view's aspect ratio, and cornerRadius on its own rounds only the layer's background and border — the contents keep their square edges until clipping is on.
```
// WRONG: the default .scaleToFill distorts, and cornerRadius alone does not cut the photo
let avatar = UIImageView(image: photo)
avatar.layer.cornerRadius = 24
avatar.layer.shadowOpacity = 0.3
avatar.clipsToBounds = true        // ...and now the shadow is clipped away as well
…
```
15
A label is set straight from a URLSession completion handler and the app later crashes somewhere unrelated — what is going on?
Easy
UIKit is not thread-safe and must be driven from the main thread, but a URLSession completion handler runs on the session's own delegate queue — so that assignment mutates the view tree from a background thread and the damage surfaces later, in code that is not guilty.
```
// WRONG: URLSession calls back on its own queue, not the main one
URLSession.shared.dataTask(with: url) { data, _, _ in
  self.titleLabel.text = String(decoding: data!, as: UTF8.self)   // UIKit off-main
}.resume()

// RIGHT, callback style: hop back for the UI work only
…
```
16
A pan on your draggable card fights the scroll view under it, and the single tap fires before the double tap — how do you arbitrate between recognizers?
Medium
Gesture recognizers are little state machines competing for the same touches, and by default only one of them may win: you order them with require(toFail:) and let them coexist with the delegate's shouldRecognizeSimultaneouslyWith.
```
final class CardViewController: UIViewController, UIGestureRecognizerDelegate {
  private let scrollView = UIScrollView()
  private let card = UIView()
  private var dragStart: CGPoint = .zero

  override func viewDidLoad() {
…
```
17
You need a table scrolled to today's row before the screen appears, but bounds are wrong in viewWillAppear and viewDidLayoutSubviews runs repeatedly — where does that code go?
Medium
In viewIsAppearing(_:) — the callback added in iOS 17 and back-deployed to iOS 13 for anything built with Xcode 15 or newer, which runs once per appearance, after the view is in the hierarchy with its real traits and geometry, and before the screen is shown.
```
final class AgendaViewController: UIViewController {
  private let tableView = UITableView()
  private let header = UIView()
  private var didScrollToToday = false
  private var todayIndexPath = IndexPath(row: 0, section: 0)
…
```
18
Your view's border stays the light-mode colour after the user switches to dark, and traitCollectionDidChange is deprecated — what is the fix now?
Medium
A UIColor is a recipe that UIKit re-resolves every time it draws, but .cgColor is a photograph of one appearance taken at the moment you asked — so every layer-level colour has to be resolved again when the trait changes, and since iOS 17 you observe that with registerForTraitChanges(_:handler:).
```
final class BadgeView: UIView {
  override init(frame: CGRect) {
    super.init(frame: frame)

    // WRONG: resolved once, against whichever style happened to be current
    layer.borderColor = UIColor.separator.cgColor
…
```
19
Users swipe your editor sheet down in the middle of typing and lose the draft — how do you take control of an interactive dismissal?
Medium
Set isModalInPresentation = true, or answer presentationControllerShouldDismiss(_:) dynamically, and then handle the refused swipe in presentationControllerDidAttemptToDismiss(_:) — that is where the "Discard draft?" choice belongs.
```
final class InboxViewController: UIViewController, UIAdaptivePresentationControllerDelegate {
  private weak var editor: EditorViewController?

  func showEditor(_ draft: Draft) {
    let editor = EditorViewController(draft: draft)
    let nav = UINavigationController(rootViewController: editor)
…
```
20
VoiceOver reads your custom rating control as "star.fill, button" and never says the score — what do you set on the view?
Medium
The accessibility tree is derived from your view hierarchy, so a control you drew yourself has to describe itself: isAccessibilityElement, a human accessibilityLabel, an accessibilityValue for the part that changes, and the traits that say what kind of control it is.
```
final class RatingControl: UIControl {
  private let stars = UIStackView()
  var rating: Int = 0 {
    didSet {
      updateStars()
      accessibilityValue = "\(rating) out of 5"   // update where the state changes
…
```

UIKit Fundamentals

What is UIKit? How does it relate to SwiftUI in modern iOS apps?

Walk through the UIViewController lifecycle: which callbacks fire once and which fire on every appearance?

What is the difference between frame and bounds in UIView?

When the user taps the screen, how does UIKit decide which view receives the touch?

How do UITableView and UICollectionView work? What is reuse?

What is the difference between Storyboards, XIBs, and programmatic UIs?

Since scenes arrived, which lifecycle events belong to AppDelegate and which to SceneDelegate?

Every UIView owns a CALayer — what does the layer actually do, and when do you drop down to it?

What is the difference between pushing a view controller, presenting one, and embedding one as a child?

Why is a delegate property declared `weak`, and when would you choose a closure or a notification instead?

Which UIKit animation API do you reach for, and how do you animate an Auto Layout change?

How do you put a UIKit view inside SwiftUI, and a SwiftUI view inside UIKit?

Your button's `contentEdgeInsets` do nothing and the font you set on `titleLabel` resets after a tap — what replaced that API?

An avatar comes out squashed and the photo spills past the rounded corners you set — which two view properties are wrong?

A label is set straight from a URLSession completion handler and the app later crashes somewhere unrelated — what is going on?

A pan on your draggable card fights the scroll view under it, and the single tap fires before the double tap — how do you arbitrate between recognizers?

You need a table scrolled to today's row before the screen appears, but bounds are wrong in viewWillAppear and viewDidLayoutSubviews runs repeatedly — where does that code go?

Your view's border stays the light-mode colour after the user switches to dark, and traitCollectionDidChange is deprecated — what is the fix now?

Users swipe your editor sheet down in the middle of typing and lose the draft — how do you take control of an interactive dismissal?

VoiceOver reads your custom rating control as "star.fill, button" and never says the score — what do you set on the view?

What is UIKit? How does it relate to SwiftUI in modern iOS apps?

Walk through the UIViewController lifecycle: which callbacks fire once and which fire on every appearance?

What is the difference between frame and bounds in UIView?

When the user taps the screen, how does UIKit decide which view receives the touch?

How do UITableView and UICollectionView work? What is reuse?

What is the difference between Storyboards, XIBs, and programmatic UIs?

Since scenes arrived, which lifecycle events belong to AppDelegate and which to SceneDelegate?

Every UIView owns a CALayer — what does the layer actually do, and when do you drop down to it?

What is the difference between pushing a view controller, presenting one, and embedding one as a child?

Why is a delegate property declared weak, and when would you choose a closure or a notification instead?

Which UIKit animation API do you reach for, and how do you animate an Auto Layout change?

How do you put a UIKit view inside SwiftUI, and a SwiftUI view inside UIKit?

Your button's contentEdgeInsets do nothing and the font you set on titleLabel resets after a tap — what replaced that API?

An avatar comes out squashed and the photo spills past the rounded corners you set — which two view properties are wrong?

A label is set straight from a URLSession completion handler and the app later crashes somewhere unrelated — what is going on?

A pan on your draggable card fights the scroll view under it, and the single tap fires before the double tap — how do you arbitrate between recognizers?

You need a table scrolled to today's row before the screen appears, but bounds are wrong in viewWillAppear and viewDidLayoutSubviews runs repeatedly — where does that code go?

Your view's border stays the light-mode colour after the user switches to dark, and traitCollectionDidChange is deprecated — what is the fix now?

Users swipe your editor sheet down in the middle of typing and lose the draft — how do you take control of an interactive dismissal?

VoiceOver reads your custom rating control as "star.fill, button" and never says the score — what do you set on the view?

Why is a delegate property declared `weak`, and when would you choose a closure or a notification instead?

Your button's `contentEdgeInsets` do nothing and the font you set on `titleLabel` resets after a tap — what replaced that API?