UIKit

Traditional iOS UI framework with imperative patterns.

When to Use

• Legacy iOS app maintenance
• Complex custom animations
• Low-level UI control
• Bridging with SwiftUI

Quick Start

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let label = UILabel()
        label.text = "Hello UIKit!"
        label.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(label)

        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
    }
}

Core Concepts

View Controllers

class ProfileViewController: UIViewController {
    private let nameLabel = UILabel()
    private let avatarImageView = UIImageView()

    private let user: User

    init(user: User) {
        self.user = user
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
        configure(with: user)
    }

    private func setupUI() {
        view.backgroundColor = .systemBackground
        // Add subviews and constraints
    }

    private func configure(with user: User) {
        nameLabel.text = user.name
    }
}

Table Views

class UsersViewController: UITableViewController {
    private var users: [User] = []

    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.register(UserCell.self, forCellReuseIdentifier: "UserCell")
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        users.count
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "UserCell", for: indexPath) as! UserCell
        cell.configure(with: users[indexPath.row])
        return cell
    }
}

Common Patterns

Navigation

// Push
navigationController?.pushViewController(detailVC, animated: true)

// Present
present(modalVC, animated: true)

// Coordinator pattern
protocol Coordinator {
    var navigationController: UINavigationController { get set }
    func start()
}

class AppCoordinator: Coordinator {
    var navigationController: UINavigationController

    func start() {
        let vc = HomeViewController()
        vc.onUserSelected = { [weak self] user in
            self?.showProfile(for: user)
        }
        navigationController.pushViewController(vc, animated: false)
    }
}

Auto Layout

NSLayoutConstraint.activate([
    stackView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 16),
    stackView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 16),
    stackView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -16),
])

Best Practices

Do:

• Use programmatic Auto Layout
• Implement Coordinator pattern
• Use composition over inheritance
• Support Dynamic Type

Don't:

• Force unwrap UI elements
• Block main thread
• Ignore retain cycles
• Mix storyboard and code

Troubleshooting

Issue	Cause	Solution
Constraint conflict	Ambiguous layout	Check constraint priorities
Memory leak	Retain cycle	Use weak references
UI freeze	Main thread blocked	Move work to background

References

• UIKit Documentation
• Human Interface Guidelines