UIKit Fundamentals

dequeueReusableCell returns a recycled cell from a pool rather than allocating a new one, so scrolling stays cheap. The cell still holds the previous row's state, so skipping prepareForReuse leaves stale text or images on the wrong row.

viewDidLoad runs once after the view loads — put one-time setup here. viewWillAppear runs before every appearance — refresh data and UI. viewDidAppear runs after the view is on screen — start animations.

Create the data source with a cell-provider closure per Item. Make an NSDiffableDataSourceSnapshot, appendSections([.main]), appendItems(items), then dataSource.apply(snapshot). Unlike reloadData, it animates only the changed rows.

frame is the view's rect in its superview's coordinates — position and size. bounds is the same rect in the view's own coordinates, so its origin is usually (0,0). A scroll view shifts bounds.origin to scroll, which is when it differs.

A recognizer moves through possible, active states for continuous gestures or recognized for discrete, plus failed/cancelled. Two conflict when one touch suits both, like a tap in a pan. require(toFail:) makes one wait for another to fail.

The responder chain is an ordered list of UIResponder objects — view, superviews, controller, window, app. A nil-target action travels up it until one implements the selector. The first responder is the head, usually the focused control.

Storyboards show flow visually and start fast but merge poorly, with git conflicts on a big team. XIBs are smaller per-view files that conflict less and reuse well. Code-only UI avoids merge pain and is most reviewable, but gives no visual preview.

Every UIView is backed by a CALayer that does the drawing, geometry, and animation; the view adds touch handling and the responder chain. 'Layer-backed' means rendering is delegated to that layer, so cornerRadius lives on the layer.

The completion captures the reused cell, recycled to another row before the image arrives, so it paints the wrong avatar. Reset the image in prepareForReuse and, on completion, confirm the cell still maps to that URL, else drop the stale load.

setNeedsLayout marks the view dirty so layoutSubviews runs on the next pass; layoutIfNeeded forces that pass now. layoutSubviews positions subviews and is never called directly. setNeedsDisplay schedules a redraw via draw(_:), a separate pass.

hitTest walks the view tree back-to-front, calling point(inside:); the deepest view whose bounds holds the point wins. A touch outside the parent's bounds is discarded, so to enlarge a tap area override point(inside:) to accept outside points.

Use a container when each screen is a self-contained controller with its own lifecycle, staying decoupled rather than one monolith. Adding a child is addChild, insert child.view, then didMove(toParent:); removing is willMove(toParent: nil), remove the view, removeFromParent. Skip them and appearance and rotation stop forwarding.