Debug

Debugging

Basic Remediation

Have you tried turning it OFF and ON again - The IT Crowd

Glad I can reference that British TV show in my work - hobby - mind map knowledge base.

The number of times an iOS Engineer has to go back to basics things to try while having a build or runtime error from Xcode - Apple Developer Tools.

Manually clearing out Derived Data
Clean Build Folder
Deleting app on simulator
Closing Xcode
Force quitting Xcode
Swift Package Manager - Reset package cache
SPM - resolving packages
Restarting Mac

Finding the culprit

Open the .log file on console.app and make sure to not take the last error stack trace at its face value.
Always back track to a point where you can see the actual error.
When Xcode build command fails on GUI or CLI. CLI is a little bit tricky and can only provide final failure information of below logs.

** BUILD FAILED **
** ARCHIVE FAILED **
** TEST FAILED **

You can solve this problem two ways

Open the file / error project target scheme product file in question on Xcode GUI. After Xcode 14 updates, lots of debug build logs are available in verbose in GUI rather than just a .log file.
Or open the log file and search for error or errors generated. or error generated. in your log file. This will give you which file is failing on CLI.

Usually it happens due to multiple different Xcode versions on GUI and xcode-select path set for CLI.
So it comes down to a making sure that whatever tool you're using is on same versions either GUI or CLI. And sometimes hotfixes or build-fixes are merged in latest develop , main or tag so make sure the branch you're working on is also updated with latest merges from parent branches or protected branches.

Or else you can face a classic It Works on my machine! or Multiple Spiderman pointing at each other meme IRL.

Console Debug

When you have specified the breakpoint and you have lldb debugger in the console.

You could use it

po swift\_variable/ objects

SO link Apple Doc Complete Guide LLDB Debug Advanced LLDB

Breakpoint

In UI Xcode IDE, you can specify breakpoints on the code as well group breakpoints, share them and also add specific type of breakpoints. Like having exception breakpoint, printing debug logs, conditional breakpoints.

Apple dev | Stepping through code and inspecting variables to isolate bugs

Print Memory address

Object Reference types

print(Unmanaged.passUnretained(someVar).toOpaque())

Sequence Value types memory address

let array1 = [1,2,3]
    let array2 = array1
    array1.withUnsafeBufferPointer { (point) in
        print(point)
    }

Structs memory address

struct CustomType { let name = "Hello" }
let firstObject = CustomType()
withUnsafePointer(to: firstObject) { print("\($0)") }

Custom conditions in breakpoints
https://developer.apple.com/documentation/xcode/setting-breakpoints-to-pause-your-running-app

Reset Simulator

You can reset the simulator or just uninstall / delete the app from home screen. It is particularly useful for deleting NSUserDefaults & Persistent data associated with the app.

On Simulator App, navigate to ‘Device’ menu -> ‘Reset Content and Settings’

Link

Clean Build Folder

If you need to clean the project derived data which can sometimes resolve previous invalid cache of the build system. You can find it in

Top menu -> Product -> Clean Build Folder

Assign variables in runtime

e let $data = CustomObject()
po $data

print a more readable Dictionary object via lldb

po print(dictionaryType)

Change variables in runtime

You can execute commands or change variables while in debugger breakpoint mode in LLDB Xcode console command.

Just enter expr or e followed by the expression you want to execute.

Function status with parameter change for different user experience timeline switch.

e NetworkConfigurationManager.shared.setConnectivity(isEnabled: true)

Variables alteration

e isValid = false

https://stackoverflow.com/questions/9907387/how-to-change-variables-value-while-debugging-with-lldb-in-xcode

Debug Prints

Your object can conform to protocol CustomDebugStringConvertible in order to print your custom debug message prints.

Whenever you use debugPrint(_:) or String(reflecting:)

Having it in an extension is a good practice since it improves readability and doesn't clutter up your main implementation layer or business layer.

extension CustomClassOrStruct: CustomDebugStringConvertible {
    var debugDescription: String {
        return "(\(self.property1), \(self.property2))"
    }
}

https://developer.apple.com/documentation/swift/customdebugstringconvertible

View Debugging

Xcode provides us a great way to visually debug a view being presented on the screen.

But to top it off with the icing on the cake you can actually change constraints or values of the paused debugged view.
lldb is pretty powerful in most cases to let us change values on the fly and get more testing while in the current state and saves us a lot of time to recreate that current state again by relaunching the app. Which I would admit that's how I usually do my UI testing previously with constraints. But with lldb you can just change specific constraints values by executing expression constraint_memory_pointer.value = newValue

// 1 Change UI element with new value
expression constraint_memory_pointer.value = newValue 

// 2 Retrigger UI Draw Cycle / Refresh UI
expr CATransaction.flush()

// 3 Executing in swift environment
expression -l Swift -O --

If you get errors of unresolved identifier you may have to import UIKit, Appkit or any UI dependent Cocoa library of apple ecosystem you're debugging. Refer this below article for more insights.

https://itnext.io/leveraging-swift-in-xcodes-lldb-d75e2adc5741

Objective C variant of this code
https://medium.com/ios-os-x-development/dynamically-modify-ui-via-lldb-expression-1b354254e1dd

You can definie lldb alias'es just like how you add them for your zShell or bash shell. It is located at ~/.lldbinit directory of the root of OS.

A shared lldb alias config file from the above sourced link

command alias objc expression -l objc -O --

command regex swift 's#(.+)#expression -l Swift -O -- defer { CATransaction.flush() }; %1#'

breakpoint set -n AppDelegate.application --one-shot true --auto-continue true

breakpoint command add

swift import Foundation

swift import UIKit

swift import MyApp_tvOS

swift import MyApp_iOS

swift func $printSubviews(of view: UIView) { print(view.perform("recursiveDescription")!) }

swift func $findViews<C: UIView>(_ c: C.Type, in t: UIView = UIApplication.shared.keyWindow!) -> [C] { var r: [C] = []; t.subviews.forEach({ if let v = $0 as? C { r.append(v) }; r.append(contentsOf: $findViews(C.self, in: $0)) }); return r }

swift func $flush() { CATransaction.flush() }

swift func $view<T>(_ x: T) -> UIView { unsafeBitCast(x, to: UIView.self) }

swift func $vc<T>(_ x: T) -> UIViewController { unsafeBitCast(x, to: UIViewController.self) }

swift func $nav<T>(_ x: T) -> UINavigationController { unsafeBitCast(x, to: UINavigationController.self) }

swift extension NSObject { static func $from(_ addr: UInt) -> Self { unsafeBitCast(addr, to: Self.self) }}

DONE

git gist

Crash

apple dev | Diagnosing issues using crash reports and device logs

Method_Swizzling

You can reverse engineer certain libraries / frameworks of apple UIKit , AppKit and utilize objective-C runtime methods to quickly replace our own implementation of the method we want to override.
Sometimes the library we are consuming won't expose all the options to tweak or change its behavior so method swizzling basically comes into the picture where you just hack / capture the memory and replace it with our own custom function / method. To get more custom behavior and just understand the underlying working architecture of the abstracted library.

Haven't heard anyone explicit mention this most of the time but one senior engineer from Objective - C did gave me a nudge to check it out in 2020 just because I mentioned I came from Turbo-C , DosBox and C++ compilers era.

access hidden func inits

Utilizing Objective-C runtime classes to have a work around since some APIs don't allow to subclass things to create test mocks

static func loader() -> MockAssetResourceLoader { 
	let allocated = MockAssetResourceLoader
		.perform(NSSelectorFromString("alloc")) 
	return allocated?
		.takeRetainedValue() as! MockAssetResourceLoader 
}

Custom Debug String

Enforcing a struct with protocol CustomDebugStringConvertible and use debugDescription computed property to provide more information for getting customized console prints.

Definition

struct TrackCodecs: CustomDebugStringConvertible {
    var audioCodec: [String]
    var textCodec: [String]
    var closedCaptionCodec: [String]
    var subtitleCodec: [String]
    
    var debugDescription: String {
        """
          Current Media Asset Info
          Type: Audio | Codec: \(audioCodec)
          Type: Text | Codec: \(textCodec)
          Type: Audio | Codec: \(closedCaptionCodec)
          Type: Subtitle | Codec: \(subtitleCodec)
          ------------------------
        """
    }
}

Usage

let codec = TrackCodecs(...)
print(codec.debugDescription)

Convert data to String

Could be helpful for converting String data into String object with its default encoding provider.

if let accessLog: AVPlayerItemAccessLog = playerItem?.accessLog(),
   let errorLog: AVPlayerItemErrorLog = playerItem?.errorLog() {
	print("$$$ avFoundationFailure")
	if let dataError = errorLog.extendedLogData() {
		let convertedString = NSString(data: dataError, encoding: errorLog.extendedLogDataStringEncoding)
		
		print("ASCII string: \(convertedString ?? "Conversion failed.")")
		print(" ------------ ")
		print(accessLog)
	}
}

the-ultimate-guide-to-converting-swift-data-to-string

Framework swapping

Debugging framework post

SO | how-to-debug-framework-source-from-main-project

SO | post debugger not showing variable names objc

SO | swift-debugger-does-not-show-variable-values-when-importing-objc-framework