Master Swift ViewController Architecture

This skill provides expert guidance on iOS Swift ViewController architecture, lifecycle management, delegation patterns, and modern UIKit best practices. It covers the proper lifecycle sequence and when each method fires - viewDidLoad for one-time setup before view bounds are set, viewWillAppear for per-appearance data refresh, viewDidAppear for starting animations/timers, viewWillDisappear for pausing operations, and viewDidDisappear for stopping expensive work - and requires weak references for delegates and closures (weak var delegate, [weak self] in a child view controller's completion closure) to prevent retain cycles between parent and child view controllers.

Use it when architecting or reviewing iOS UIKit ViewController code: lifecycle handling, navigation and presentation, delegate/closure communication, container view controllers, or modern UIKit features like diffable data sources and state restoration. The stated goal throughout is clarity, maintainability, and proper resource management in ViewController implementations.

Navigation patterns cover pushing onto a navigation controller versus modal presentation via a wrapping UINavigationController, and dismissal logic that checks whether a navigation controller context exists before choosing popViewController versus dismiss. Layout guidance sets translatesAutoresizingMaskIntoConstraints = false on a programmatically added subview and anchors it to view.safeAreaLayoutGuide (top and bottom) and view (leading/trailing) via NSLayoutConstraint.activate. Communication patterns cover a protocol-delegate approach (a weak delegate property calling back through an AnyObject-constrained protocol) and a closure-based alternative, shown on an alert view controller with an onDismiss: ((AlertResult) -> Void)? property invoked with a .confirmed result before dismissing itself. Container view controller composition is shown via a switchToViewController implementation that correctly sequences willMove(toParent:), view removal, addChild, didMove(toParent:). Modern UIKit practices cover UICollectionViewDiffableDataSource configuration and snapshot-based updates (appendSections, appendItems, apply(snapshot, animatingDifferences:)), and state restoration via restorationIdentifier/restorationClass plus encodeRestorableState/decodeRestorableState.

A custom TabContainerViewController example demonstrates parent-child containment end to end, tracking currentViewController and swapping it on demand. Built entirely on UIKit: UIViewController lifecycle hooks, UINavigationController, Auto Layout via NSLayoutConstraint, UICollectionViewDiffableDataSource/NSDiffableDataSourceSnapshot, and NSCoder-based state restoration.

iOS developers building or reviewing UIKit-based apps who need concrete, idiomatic patterns for view controller lifecycle, memory-safe delegation, navigation, container composition, and modern data-source APIs. Performance tips include refreshing data in viewWillAppear rather than viewDidLoad, lazy-loading expensive UI components, avoiding heavy main-thread work during transitions, handling safe areas manually rather than relying on automatic scroll-view inset adjustment, implementing proper cell reuse in table/collection views with prepareForReuse() state resets, and using loadViewIfNeeded() in test scenarios - prioritizing clarity, maintainability, and proper resource management throughout.