iOS Debugging Tips/Tricks/Hints

From SwiftTube Smart Debugging Talk 2018 and Advanced Debugging Techniques

brew install chisel

LLDB View Debugging

po [UIWindow.keyWindow recursiveDescription]
po [UIViewController _printHierarchy]
po [UIView _autolayoutTrace]
po [_UIViewLayoutFeedbackLoopDebugger layoutFeedbackLoopDebugger]

// lldb in Swift context:
expr -l objc++ -O -- [UIWindow.keyWindow recursiveDescription]

.lldbinit

Put this file in your home directory
Add this to the file

command alias alt expr -l objc++ -O --[[UIWindow keyWiindow] _autolayoutTrace]

github.com/facebook/chisel

Command	Explanation
`pa11y <aView>`	Print accessibility labels
`alamborder`	Highlight views with ambiguous layout
`pproperties`	Print the properties of an instance
`slowanim`	Slows down animiations. Sim + Device
`showimage`	Open a UIImage in Preview.app
`pactions <control>`	Print the actions/targets of a control

findinstances

findinstances UIView subviews.@count == 0
findinstances NSArray @count > 100
findinstances UIScrollViewDelegate

findinstances UIView window == nil || hidden == true ||
  alpha == 0 || layer.bounds.#size.width == 0 || layer.bounds.#size.height == 0

CLang Analyzer

Always run the analyzer and make sure you don't have any warnings.

clang -cc1 -analyzer-checker-help

Checker	Explanation
`alpha.core.BoolAssignment`	Warn about assining non-{0, 1} values to Boolean variables
`alpha.clone.CloneChecker`	Reports similar pieces of code
`alpha.cplusplus.MisusedMovedObject`	Method calls on a moved-from object and copying a moved-from object will be reported
`optin.osx.cocoa.localizability.NonLocalizedStringChecker`	Warns about uses of non-localized NSStrings passed to UI methods expecting localized NSStrings

Opt out of warnings for a specific code region

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
...
#pragma clang diagnostic pop

clang Sanitizers

Address Sanitizer
Undefined Behavior Sanitizer
Main Thread Checker
Thread Sanitizer (10-50x slower performance when running)

Advanced options are available NOT in the UI.

Retrain Tracker

This is a small tool that the author created

class_setSuperclass

Advanced Debugging Techniques

In lldb

Try the command register read

Launch Arguments (Launch arguments can be passed at runtime!)

(Where to find a list of them ...?)

UIView / Views

Layout Feedback Loop Debugger
- For more details, check out

What's New in AutoLayout

-UIViewLayoutFeedbackLoopDebuggingThreshold 100 // 50...1000
- Logs to com.apple.UIKit:LayoutLoop
-NSViewLayoutFeedbackLoopDebuggingThreshold 100 // 50...1000
- Logs to com.apple.AppKit:LayoutLoop

Core Data

ConcurrencyDebug
- -com.apple.CoreData.ConcurrencyDebug 1

Core Data Concurrency Debugging

Migrationdebug & SQLDebug
- com.apple.CoreData.MigrationDebug 1
- com.apple.CoreData.SQLDebug 3
NSTraceEvents
- `/Applications/TextEdit.app/Contents/MacOS/TextEdit -NSTraceEvents YES

iOS Debugging Magic

macOS Debugging Magic

Custom Launch Arguments

iOS?

if UserDefaults.standard.bool(forKey: "skip_onboarding") {
    // Load main screen
}

macOS?

if ProcessInfo.processInfo.arguments.contains("skip_onboarding") {
    // Load main screen
}

The above may be for both, either via UserDefaults or arguments.

let environment = ProcessInfo.processInfo.environment

if let serverUrl = environment["server_url"] {
    // Set server URL with the value in the environment
}
else {
    // Set the default one
}

Using Keypaths to inspect even private things

let colorObservation = textFieldBackground.observe(\.backgroundColor, options:[.old, .new]) { (_, change) in 
    log.debug("Color: \(change.oldValue) -> \(change.newValue)")
    log.debug("")

In lldb, if you get an error...

(lldb) po UIApplication.shared
error: property 'shared' not found on object 'UIApplication'

in LLDB, there is an Objective-C context and a Swift context.
- The default is Objective-C

One way to fix this and use the Swift context...

(lldb) expr -l swift -O -- UIApplication.shared

An easier way...

(lldb) expr @import UIKit
(lldb) expr @import Foundation

An even better way...

A symbolic breakpoint on UIApplicationMain! (Move into User Breakpoints)

Then, send 2 "Actions: Debugger command"

expr @import UIKit

and

expr @import Foundation

`-NSKVODeallocateBreak`

On Exception Breakpoint

Print out your first argument
- Break: On Throw
- Debugger Command
  - po $arg1

A Bunch More User Breakpoint Examples...

User Breakpoints in Xcode

LLDB (Low Level Debugger)

Changing Variables at Runtime

Debugging Swift code with LLDB

Extending LLDB

With Python LLDB Python Scripting

Chisel (Facebook Chisel)

A collection of handy lldb commands
Install via brew
- paltrace
  - AUTO LAYOUT TRACEC

How Can We Save Time?

Assertions

Assertions are ONLY there in DEBUG mode, they are ignored in PRODUCTION!

assert()

func transformString(string: String?) -> String {
assert(string != nil, "Invalid parameter")
return string! + "_transform"
}

assertionFailure()

switch flippy {
case foo:
  Do something
case bar:
  Do something else
default:
  assertionFailure("We are not handling a FooBar, add it to the cases!)
}
assertionFailure("Something is wrong, did you fail to configure BLAH?")