--- name: uikit description: Expert UIKit assistance covering UIViewController lifecycles, Auto Layout programmatically, UITableView/UICollectionView architectures, delegates, and view hierarchies. Use when maintaining production iOS applications, writing custom UIKit components, or bridging UIKit with SwiftUI. --- # UIKit UIKit is the traditional, imperative framework for building iOS user interfaces. While SwiftUI is the future, UIKit remains essential for maintaining existing apps and unrestricted access to the OS. ## When to Use - **Production iOS Codebase Maintenance**: Maintaining and extending established enterprise iOS applications built on UIKit. - **Fine-Grained Touch & Gesture Control**: Implementing bespoke drag-and-drop, gesture recognizers, and custom touch tracking. - **Complex UICollectionView Layouts**: Building high-performance custom layouts with `UICollectionViewCompositionalLayout` and Diffable Data Sources. - **SwiftUI Interoperability**: Hosting complex UIKit view controllers inside SwiftUI via `UIViewControllerRepresentable`. ## Quick Start ```swift // Programmatic UIKit (No Storyboards) in SceneDelegate func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options: UIScene.ConnectionOptions) { guard let windowScene = (scene as? UIWindowScene) else { return } window = UIWindow(windowScene: windowScene) window?.rootViewController = UINavigationController(rootViewController: HomeViewController()) window?.makeKeyAndVisible() } class HomeViewController: UIViewController { override func viewDidLoad() { super.viewDidLoad() view.backgroundColor = .systemBackground title = "UIKit Home" let button = UIButton(type: .system) button.setTitle("Tap Me", for: .normal) button.translatesAutoresizingMaskIntoConstraints = false view.addSubview(button) NSLayoutConstraint.activate([ button.centerXAnchor.constraint(equalTo: view.centerXAnchor), button.centerYAnchor.constraint(equalTo: view.centerYAnchor) ]) } } ``` ## Core Concepts ### UICollectionView Diffable Data Sources & Compositional Layout Eliminates index-path calculation bugs by managing list state through unique hashable identifiers and snapshots: ```swift final class FeedViewController: UIViewController { enum Section { case main } struct Post: Hashable { let id: UUID; let title: String } private var collectionView: UICollectionView! private var dataSource: UICollectionViewDiffableDataSource! override func viewDidLoad() { super.viewDidLoad() configureLayout() configureDataSource() } private func configureLayout() { var config = UICollectionLayoutListConfiguration(appearance: .insetGrouped) collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: UICollectionViewCompositionalLayout.list(using: config)) view.addSubview(collectionView) } private func configureDataSource() { let registration = UICollectionView.CellRegistration { cell, _, item in var content = cell.defaultContentConfiguration() content.text = item.title cell.contentConfiguration = content } dataSource = UICollectionViewDiffableDataSource(collectionView: collectionView) { cv, ip, item in cv.dequeueConfiguredReusableCell(using: registration, for: ip, item: item) } } } ``` ### UIViewController Lifecycle Flow Strictly isolates setup, layout, and appearance phases to ensure optimal memory and rendering performance: ```swift class OrderDetailViewController: UIViewController { override func loadView() { // Instantiate and assign custom root view (bypassing storyboard) self.view = OrderDetailRootView() } override func viewDidLoad() { super.viewDidLoad() // One-time data binding, notification registration } override func viewWillAppear(_ animated: Bool) { super.viewWillAppear(animated) // Refresh lightweight view state } } ``` ### Programmatic Layout Anchor Constraints Constructs responsive layouts programmatically without external dependencies: ```swift NSLayoutConstraint.activate([ headerView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor), headerView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 16), headerView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -16), headerView.heightAnchor.constraint(equalToConstant: 64) ]) ``` ## Common Patterns ### Programmatic Auto Layout with NSLayoutConstraint **Problem**: Storyboard merge conflicts and fragile constraint debugging in team repositories. **Solution**: Construct view hierarchies and constraints purely in Swift. ```swift final class ProfileViewController: UIViewController { private let titleLabel: UILabel = { let label = UILabel() label.translatesAutoresizingMaskIntoConstraints = false label.font = .systemFont(ofSize: 20, weight: .bold) return label }() override func viewDidLoad() { super.viewDidLoad() view.backgroundColor = .systemBackground view.addSubview(titleLabel) NSLayoutConstraint.activate([ titleLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor), titleLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor) ]) } } ``` ## Best Practices **Do**: - Adopt Diffable Data Sources: Replace error-prone `reloadData()` and `performBatchUpdates()` with atomic `NSDiffableDataSourceSnapshot`. - Use `translatesAutoresizingMaskIntoConstraints = false`: Always disable autotranslation on programmatically created views before activating constraints. - Bridge with SwiftUI: Wrap new feature views in `UIHostingController` rather than rebuilding everything in UIKit. - Audit Retain Cycles in Closures: Capture `[weak self]` in completion handlers and event closures to prevent ViewController leaks. **Don't**: - Trigger expensive layout inside `draw(_:)`: Keep custom core graphics rendering strictly inside draw methods; avoid layout passes. - Hardcode frame coordinates: Avoid explicit `CGRect(x: ..., y: ...)` math; use Auto Layout or safe area layout guides. - Block the main thread: Perform data decoding and persistence asynchronously and update UIKit views on `MainActor`. ## Troubleshooting | Error | Cause | Solution | | :-------------------------- | :----------------------------- | :------------------------------------------------------------------------- | | `Unsatisfiable Constraints` | Conflicting Auto Layout rules. | Check console log for breaking constraint IDs; simplify rules. | | `TableView Updates Crash` | Inconsistent data vs rows. | Use Diffable Data Source or ensure data model updates before `insertRows`. | | `Retain Cycle` | Strong reference loop. | Use Memory Graph Debugger; verify `[weak self]`. | ## References - [Apple UIKit Documentation](https://developer.apple.com/documentation/uikit) - [Modern UIKit Patterns](https://developer.apple.com/videos/play/wwdc2020/10097/) --- Source attribution: G1Joshi Original project: https://github.com/G1Joshi/Agent-Skills/blob/6a74cf03a8a86dc7d993ead3091b41123dbd0181/skills/mobile/uikit/SKILL.md License: MIT License information: https://opensource.org/license/mit Original upstream instructions; downloaded from Dave AI. ## Upstream license notice MIT License Copyright (c) 2026 G1Joshi Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Dave AI resource metadata Creator / publisher: G1Joshi Original project: https://github.com/G1Joshi/Agent-Skills/blob/6a74cf03a8a86dc7d993ead3091b41123dbd0181/skills/mobile/uikit/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Frontend Developer, Mobile Developer Specialties: Cross Platform, Frontend, Mobile Tools: Tool not specified Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 88ade9a93d068d811895608b1928f593f110bcc243d79a97046111181581a809