ios-debugging

name	ios-debugging
description	iOS debugging expert for troubleshooting and profiling. Use when working with Instruments, Time Profiler, Memory Graph, LLDB, breakpoints, console logging, or performance analysis.

iOS Debugging

Expert guidance for debugging and profiling iOS applications.

Console Logging

os_log (Recommended)

import os.log

let logger = Logger(subsystem: "com.myapp", category: "networking")

// Log levels
logger.debug("Debug message")       // Not shown in release
logger.info("Info message")
logger.notice("Notice message")
logger.warning("Warning message")
logger.error("Error message")
logger.critical("Critical message")

// With interpolation
logger.info("User \(userID, privacy: .private) logged in")
logger.debug("Processing \(items.count) items")

Structured Logging

extension Logger {
    static let network = Logger(subsystem: "com.myapp", category: "network")
    static let database = Logger(subsystem: "com.myapp", category: "database")
    static let ui = Logger(subsystem: "com.myapp", category: "ui")
}

// Usage
Logger.network.info("Request started: \(url)")
Logger.database.debug("Query executed in \(duration)ms")

Privacy in Logs

// Hide sensitive data
logger.info("Email: \(email, privacy: .private)")
logger.info("Token: \(token, privacy: .sensitive)")

// Show in debug, hide in release
logger.info("UserID: \(userID, privacy: .auto)")

// Always show (public data only)
logger.info("Item count: \(count, privacy: .public)")

LLDB Debugger

Common Commands

# Print variable
po myVariable
p myVariable

# Print with format
p/x myInt        # Hexadecimal
p/t myInt        # Binary
p/d myInt        # Decimal

# Expression evaluation
expr myVariable = newValue
expr print(myObject.description)

# View memory
memory read myPointer
x/4xw myPointer  # 4 words in hex

# Backtrace
bt               # Full backtrace
bt 5             # Last 5 frames

# Thread info
thread list
thread select 2

# Continue execution
c                # Continue
n                # Next (step over)
s                # Step (step into)
finish           # Step out

Breakpoint Commands

# Set breakpoint
breakpoint set -n functionName
breakpoint set -f ViewController.swift -l 42

# Conditional breakpoint
breakpoint set -n myFunction -c "count > 10"

# Breakpoint action
breakpoint command add 1
> po self.items
> continue
> DONE

# Watchpoint (break on value change)
watchpoint set variable myVariable

Swift-Specific

// In LLDB
expr import UIKit
expr let view = unsafeBitCast(0x12345678, to: UIView.self)
expr view.backgroundColor = .red

// Print SwiftUI state
po _state

Breakpoints in Xcode

Symbolic Breakpoint

Symbol: -[UIViewController viewDidLoad]
Module: UIKitCore
Action: Log Message "VC loaded: @(id)@"

Exception Breakpoint

• Add Exception Breakpoint
• Break on: All Exceptions
• Exception: Objective-C (or All)

Swift Error Breakpoint

• Add Swift Error Breakpoint
• Catches all thrown errors

Conditional Breakpoint

// Condition
item.id == "problem-id"

// Ignore count
5  // Skip first 5 hits

Instruments

Time Profiler

1. Product > Profile (Cmd+I)
2. Select Time Profiler
3. Record and interact with app
4. Analyze call tree

// Add signposts for custom intervals
import os.signpost

let log = OSLog(subsystem: "com.myapp", category: .pointsOfInterest)

os_signpost(.begin, log: log, name: "DataProcessing")
processData()
os_signpost(.end, log: log, name: "DataProcessing")

Allocations

1. Profile > Allocations
2. Look for:
   • Memory growth over time
   • Transient allocations
   • Persistent allocations

Leaks

1. Profile > Leaks
2. Run app and perform actions
3. Check for leaked objects

Network

1. Profile > Network
2. Monitor:
   • Request/response timing
   • Data transferred
   • Connection issues

Memory Debugging

Memory Graph Debugger

1. Run app
2. Debug Navigator > Memory
3. Click "Debug Memory Graph" button

Finding Retain Cycles

// Use weak/unowned to break cycles
class ViewModel {
    var onComplete: (() -> Void)?

    func setup(view: View) {
        // Capture self weakly
        onComplete = { [weak self] in
            self?.handleComplete()
        }
    }
}

Debug Memory Issues

// Track deallocations
deinit {
    print("ViewModel deinitialized")
}

// In LLDB
expr import Foundation
expr CFGetRetainCount(myObject)

View Debugging

View Hierarchy Debugger

1. Run app
2. Click "Debug View Hierarchy" button
3. Inspect 3D view hierarchy

Runtime View Modifications

# In LLDB
expr view.backgroundColor = UIColor.red
expr CATransaction.flush()

Print View Hierarchy

po view.value(forKey: "recursiveDescription")

Network Debugging

URLSession Logging

// Environment variable
CFNETWORK_DIAGNOSTICS = 3

// Or in code
URLSession.shared.configuration.urlCache?.removeAllCachedResponses()

Charles Proxy / Proxyman

1. Install proxy app
2. Configure iOS Simulator to use proxy
3. Install root certificate

Crash Analysis

Crash Logs

# Location on device
Settings > Privacy > Analytics > Analytics Data

# Symbolicate in Xcode
Window > Devices and Simulators > View Device Logs

Analyzing Crash

Exception Type: EXC_CRASH (SIGABRT)

Thread 0 Crashed:
0   libsystem_kernel.dylib     __pthread_kill
1   libsystem_c.dylib          abort
2   MyApp                      ViewController.buttonTapped + 123

Performance Debugging

Measure Code Performance

import os.signpost

let signpostID = OSSignpostID(log: log)

os_signpost(.begin, log: log, name: "Operation", signpostID: signpostID)
performOperation()
os_signpost(.end, log: log, name: "Operation", signpostID: signpostID)

// Or use CFAbsoluteTimeGetCurrent
let start = CFAbsoluteTimeGetCurrent()
performOperation()
let duration = CFAbsoluteTimeGetCurrent() - start
print("Duration: \(duration * 1000)ms")

Main Thread Checker

Edit Scheme > Run > Diagnostics
✓ Main Thread Checker

Thread Sanitizer

Edit Scheme > Run > Diagnostics
✓ Thread Sanitizer

Address Sanitizer

Edit Scheme > Run > Diagnostics
✓ Address Sanitizer

Debug-Only Code

#if DEBUG
print("Debug only message")
#endif

// Assert (debug only)
assert(condition, "Message")

// Precondition (always checked)
precondition(condition, "Message")

// Fatal error (always crashes)
fatalError("Unimplemented")

Common Issues

UI Not Updating

// Ensure main thread
DispatchQueue.main.async {
    self.label.text = newValue
}

// Or with MainActor
await MainActor.run {
    self.updateUI()
}

Optional Unwrapping Crash

// Use safe unwrapping
if let value = optionalValue {
    use(value)
}

// Guard for early exit
guard let value = optionalValue else {
    return
}

Debugging SwiftUI

// Print on body evaluation
let _ = print("Body evaluated")

// Self inspection
let _ = Self._printChanges()

Apple Documentation

• Debugging
• Instruments
• LLDB
• os_log
• Memory Management