iOS &amp; Swift

Add a default header to every URLSession request

Set httpAdditionalHeaders on a URLSessionConfiguration, build the session from it, mention per-request overrides win.

How task cancellation works with async/await

Cancellation is a flag, not a kill; check Task.isCancelled or call checkCancellation, URLSession throws CancellationError automatically.

What is an actor and how does it prevent data races

Actors serialize access to mutable state, reachable only via await; compiler blocks unsafe access. Token-refresh actor coalesces concurrent refreshes.

Implicit .animation() vs explicit withAnimation

.animation(value:) animates a view when a tracked value changes; withAnimation wraps the state mutation so all dependent views animate.

Add shadow and rounded corners to a UIView

CornerRadius needs masksToBounds true, but that clips the shadow; set shadowPath to keep both and stay performant.

Difference between frame, bounds, and center

Frame and center are in the superview's space; bounds is the view's own space, usually origin zero. A non-identity transform makes frame undefined to read.

Build a custom circular progress bar in UIKit

Draw an arc with UIBezierPath, assign it to a CAShapeLayer path, set lineWidth and no fill, animate strokeEnd 0 to 1.

Allow simultaneous pan and pinch gestures

Attach pan and pinch recognizers, set yourself as delegate, return true from shouldRecognizeSimultaneouslyWith, apply translation to center and scale to transform.

Diagnose dropped frames during animation

Use Core Animation and color-debug options to spot off-screen passes, blending, and overdraw; cache, flatten, and set shadowPath.

Build an interruptible custom VC transition

A delegate vends an animator (UIViewControllerAnimatedTransitioning) for the visuals and an interactive controller (UIViewControllerInteractiveTransitioning) driven by a pan to scrub progress and…

Animate a SwiftUI Shape morphing point count

Expose a continuous sides value as animatableData so SwiftUI interpolates it, then compute the path from that fractional value each frame.

Request the user's location one time

Add a usage description to Info.plist, set a CLLocationManager delegate, request when-in-use authorization, call requestLocation, handle didUpdateLocations and didFailWithError.

Play a remote video with AVFoundation

Wrap the URL in AVPlayerItem, feed it to AVPlayer, render with AVPlayerLayer added to the view's layer (or AVPlayerViewController for controls).

Track location continuously in the background

Set desiredAccuracy, distanceFilter, activityType; enable the Location background mode and allowsBackgroundLocationUpdates; request Always authorization.

Run a Core ML image model with Vision

Add the .mlmodel so Xcode generates a class, wrap it in a VNCoreMLModel, run a VNCoreMLRequest via a VNImageRequestHandler, read results off the main queue.

Display thousands of map annotations efficiently

Enable clustering via clusteringIdentifier, reuse views with dequeueReusableAnnotationView, render clusters with MKClusterAnnotation in viewFor.

Apply a CIFilter to a live camera feed

Use AVCaptureSession with AVCaptureVideoDataOutput, receive CMSampleBuffers in captureOutput via the sample buffer delegate on a serial queue, filter with CIImage, drop late frames.