17 ways to debug UIKit

Replace print() the moment a bug survives one rebuild. Logs persist on-device and are filterable by category, even without Xcode attached.

import OSLog
let log = Logger(subsystem: "dev.mafham.app", category: "checkout")
log.error("Payment failed: \(error.localizedDescription)")

v reads straight from debug info. Faster and more reliable than p when you just need a value.

(lldb) v myArray.count
(lldb) po myViewController.view

Turn any breakpoint into a live print statement you can edit while the app keeps running. The single most underused feature in Xcode.

// Breakpoint → Add Action → Log Message, then check "Automatically continue":
count is @count@, user is @user.id@

Add this once per project, on day one. Stops at the line that threw, not the useless main.swift crash you get by default.

// Breakpoint navigator → + → Exception Breakpoint → All, Objective-C

Stop only on the one row, the one user ID, the one edge case, out of thousands of calls.

// Right-click any breakpoint → Edit Breakpoint → Condition:
indexPath.row == 42 && items[indexPath.row].isExpired

Debug bar's three-layer icon. Pauses the app and gives you an exploded 3D model of every view on screen. The fastest answer to "is my view actually there?"

Test a visual fix before writing a line of code. CATransaction.flush() forces the redraw while still paused.

(lldb) e label.textColor = .systemRed
(lldb) e CATransaction.flush()

Fastest way to confirm a memory leak before reaching for heavier tools.

deinit { print("💀 ProfileVC gone") }
// push, pop. No skull printed = confirmed leak.

Applies to escaping closures, delegates, and timers, in that order of frequency.

self.onUpdate = { [weak self] in self?.refresh() }
weak var delegate: SomeDelegate?
// Timers retain their target through the run loop. Invalidate in viewDidDisappear:
timer?.invalidate()

Select it in Debug View Hierarchy, grab its address, ask it the three questions that matter.

(lldb) e let $b = unsafeBitCast(0x7fa2c8d09e30, to: UIButton.self)
(lldb) p $b.isUserInteractionEnabled
(lldb) p $b.alpha
(lldb) p $b.superview!.bounds.contains($b.frame.origin)

The sneakiest cause: everything above checks out, but a parent's tap recognizer swallows the touch first.

(lldb) po $b.window?.gestureRecognizers
// then walk each ancestor's .gestureRecognizers the same way

Finds ambiguous layout, views that could be in more than one valid position, without touching any UI.

(lldb) expr -l objc -O -- [[UIWindow keyWindow] _autolayoutTrace]
// flags every view: "AMBIGUOUS LAYOUT" if it applies

Dump the entire view tree as text when the visual debugger is overkill for a quick check.

(lldb) expr -l objc -O -- [[UIWindow keyWindow] recursiveDescription]

Break inside Apple's own code, without the source. The dedicated tool for constraint conflicts.

// Breakpoint navigator → + → Symbolic Breakpoint → Symbol:
UIViewAlertForUnsatisfiableConstraints

When the deinit tripwire confirms a leak but you don't know what's holding it. The three-circles icon in the debug bar draws every strong reference as a graph. Cycles render as a closed ring.

See your own code's cost inside an Instruments trace, named and labeled, not guessed from a stack sample.

let signposter = OSSignposter(subsystem: "dev.mafham.app", category: "feed")
let state = signposter.beginInterval("decode-page")
decodePage()
signposter.endInterval("decode-page", state)

When the automated pipeline is down mid-incident and you have a raw address and a dSYM. Staff-level, but it ends incidents fast when nothing else can.

atos -arch arm64 \
  -o MyApp.app.dSYM/Contents/Resources/DWARF/MyApp \
  -l 0x100e34000 0x100e5b3c4
// → CheckoutViewModel.applyCoupon(_:) (CheckoutViewModel.swift:214)