Learn to Debug Like a Pro

If your idea of debugging is breakpoints and print() calls, this session will help show you more advanced techniques to catch bugs faster. You’ll learn about watch points, assertions, conditional breakpoints, and more – all with the goal of helping you find and fix bugs faster.

Presented at Appdevcon 2018, Amsterdam.

Transcript

1. PAUL HUDSON — @twostraws LEARN TO DEBUG LIKE A PRO https://www.hackingwithswift.com
2. HACKING WITH SWIFT
6. ▸ @twostraws on Twitter ▸ twostraws on GitHub ▸ twostraws on Reddit ▸ twostraws on StackOverflow ▸ paul@hackingwithswift.com GET IN TOUCH!
7. DEBUGGING The process of finding and removing the mistakes we put there in the first place
10. “I’m sorry, I can’t hear you over the sound of how awesome I am.”
11. Learning a new language while learning a new language ELLEN SHAPIRO
12. ▸ Basics and reading values ▸ Breakpoints and watchpoints ▸ Layouts ▸ Memory THE PLAN
16. BASICS
17. print() print() print() print() print() print() print() print() print() print() print() print() print() print() 50% 50%
23. github.com/twostraws/sharpshooter SHARPSHOOTER
29. assert()
30. assert(someCode())
33. if _isDebugAssertConfiguration() { if !_branchHint(condition(), expected: true) { _assertionFailure("Assertion failed", message()) } }
34. if _isDebugAssertConfiguration() { if !_branchHint(condition(), expected: true) { _assertionFailure("Assertion failed", message()) } }
35. assert(users.count == 5)
36. assert(slowCode() == true)
38. If you do nothing else after
 listening to this talk… Count how many calls to assert() you already have, then work to double that figure
39. USE ASSERT()
40. precondition()
41. precondition(someCode())
45. Empty removeLast() Reversed ranges: 10…1 Division by zero Reading out of array ranges
46. fatalError()
48. public func fatalError( _ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line ) -> Never
49. public func fatalError( _ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line ) -> Never
50. ▸ Dequeue cells. ▸ Instantiate from storyboards. ▸ Read the app bundle. HANDLING LOGIC ERRORS Trying to recover at runtime just masks the problem
53. assert() precondition() fatalError()
55. Best debugging is
 no debugging” PETER STEINBERGER “
57. READING VALUES
58. (lldb)
59. (lldb) p someInt
60. (lldb) p someInt (Int) $R0 = 10 (lldb)
61. (lldb) p someInt (Int) $R0 = 10 (lldb) po someObject
62. (lldb) p someInt (Int) $R0 = 10 (lldb) po someObject <App.ExampleObj: 0x7ff4f0014260> (lldb)
63. <App.ExampleObj: 0x7ff4f0014260> (lldb) (lldb) p someObject (App.ExampleObj) $R1 = 0x00007fdd10817200 { username = “twostraws” favoriteSongs = 25 values { [0] = “Shake it Off”
65. frame variable “Show me the value of this local variable, without changing anything”
66. (lldb) fr v username (String) username = “twostraws”
67. expr “Change this variable to something else”
68. WTF?
73. po “Print the debug description of this thing.”
74. override var debugDescription: String { return “Username is \(username)” }
75. @objc func debugQuickLookObject() -> Any? { return "Hello world!" }
80. ▸ Basics and reading values ▸ Breakpoints and watchpoints ▸ Layouts ▸ Memory THE PLAN
81. BREAKPOINTS
91. enum BreakpointManager { static let firstRun = 0 static let userCreate = 0 static let userLogin = 0 static let subscribe = 0 }
93. enum BreakpointManager { static let firstRun = 0 static let userCreate = 0 static let userLogin = 0 static let subscribe = 0 }
94. enum BreakpointManager { static let firstRun = 0 static let userCreate = 1 static let userLogin = 0 static let subscribe = 0 }
101. let vc = storyboard?.instantiateViewController ( withIdentifier: “Detail” ) as? DetailViewController
102. let vc = storyboard?.instantiateViewController ( withIdentifier: “Detail” ) as? DetailViewController
103. UIApplicationMain + 159 23 Project38 0x0000000102a52b47 main + 55 24 libdyld.dylib 0x000000010879dd81 start + 1 ) libc++abi.dylib: terminating with uncaught exception of type NSException (lldb)
104. 2018-03-14 08:16:53.226319+0000 Project38[82247:4984913] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Storyboard (<UIStoryboard: 0x60800006f6c0>) doesn't contain a view controller with identifier 'Detail'' *** First throw call stack: ( 0 CoreFoundation
105. 2018-03-14 08:16:53.226319+0000 Project38[82247:4984913] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Storyboard (<UIStoryboard: 0x60800006f6c0>) doesn't contain a view controller with identifier 'Detail'' *** First throw call stack: ( 0 CoreFoundation
114. WATCHPOINTS
115. What’s making this variable change? Let’s find out… [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over] [Step Over]
117. BREAKPOINTS Pause when a location is reached
 or when an event happens Pause when a value is changed by
 any part of your code WATCHPOINTS
118. var subscriptionsLoaded = false override func viewDidLoad() { authenticateUser() downloadSubscriptions() showWelcome() }
121. Watchpoint 1 hit: old value: false new value: true (lldb)
122. Watchpoint 1 hit: old value: true new value: false (lldb)
127. ▸ Basics and reading values ▸ Breakpoints and watchpoints ▸ Layouts ▸ Memory THE PLAN
128. LAYOUTS
131. ▸ The view debugger ▸ Recursive descriptions ▸ Identifying constraints ▸ Exercising ambiguity ▸ Layout feedback loops ▸ Pseudolanguages DEBUGGING LAYOUTS
138. <UIView: 0x7fd501f044b0; frame = (0 0; 736 414); autoresize = W+H; layer = <CALayer: 0x604000034b20>> | <UIView: 0x7fd501f04690; frame = (0 64; 375 470); autoresize = RM+BM; layer = <CALayer: 0x604000034b60>> | | <UIImageView: 0x7fd501f04250; frame = (10 10; 355 450); autoresize = RM+BM;
139. po view.value(forKey: "recursiveDescription")!
140. VIEW DEBUGGER LLDB Fancy 3D effects Includes VCs Shows constraints Filters Plain text Shows all values Great for diffs
142. let leadingAnchor =
 promptView.leadingAnchor.constraint (
 equalTo: view.leadingAnchor ) leadingAnchor.isActive = true
143. let leadingAnchor =
 promptView.leadingAnchor.constraint (
 equalTo: view.leadingAnchor ) leadingAnchor.identifier = "Prompt Leading" leadingAnchor.isActive = true
145. "<NSLayoutConstraint:0x6040000912b0 ‘Prompt Leading' H:|-(0)-[UIView:0x7fd5cdc0a6b0] (active, names: '|':UIView:0x7fd5cdc0a8b0 )>", "<NSLayoutConstraint:0x6040000913a0 ‘Prompt Trailing' H:[UIView:0x7fd5cdc0a8b0]-(0)- [UIView:0x7fd5cdc0a6b0] (active)>", "<NSLayoutConstraint:0x608000090cc0 'UIView- Encapsulated-Layout-Width' UIView: 0x7fd5cdc0a8b0.width == 375 (active)>”
146. "<NSLayoutConstraint:0x6040000912b0 ‘Prompt Leading' H:|-(0)-[UIView:0x7fd5cdc0a6b0] (active, names: '|':UIView:0x7fd5cdc0a8b0 )>", "<NSLayoutConstraint:0x6040000913a0 ‘Prompt Trailing' H:[UIView:0x7fd5cdc0a8b0]-(0)- [UIView:0x7fd5cdc0a6b0] (active)>", "<NSLayoutConstraint:0x608000090cc0 'UIView- Encapsulated-Layout-Width' UIView: 0x7fd5cdc0a8b0.width == 375 (active)>”
147. WHY THE FAILURE, AUTOLAYOUT? wtfautolayout.com
155. override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) { box.exerciseAmbiguityInLayout() }
156. print(vw.hasAmbiguousLayout)
157. *UIView:0x7fad2d7039e0- AMBIGUOUS LAYOUT for UIView:0x7fad2d7039e0.minX{id: 45} LEGEND * - is laid out with auto layout + - is laid out manually, but is represented in the layout engine because translatesAutoresizingMaskIntoConstraints = YES • - layout engine host
158. *UIView:0x7fad2d7039e0- AMBIGUOUS LAYOUT for UIView:0x7fad2d7039e0.minX{id: 45} LEGEND * - is laid out with auto layout + - is laid out manually, but is represented in the layout engine because translatesAutoresizingMaskIntoConstraints = YES • - layout engine host
160. A B C
161. ALL ABOARD THE FAILBOAT
162. Layout Feedback
 Loop Debugger
163. ▸ Disabled by default. ▸ When enabled, allows views to run layout X times. ▸ You choose the value of X. ▸ It runs a little more to collect info, then throws an error. FEEDBACK LOOP DEBUGGER
165. –UIViewLayoutFeedbackLoopDebuggingThreshold 100
166. override func viewDidLayoutSubviews() { view.setNeedsLayout() }
167. 500+
168. 1. The call stack each time layout took place. 2. Which views were laid out and in which order. 3. Whether ambiguous layout was part of the problem.
177. Hottentottensoldatententententoonstellingsbouwterrein
178. ▸ Basics and reading values ▸ Breakpoints and watchpoints ▸ Layouts ▸ Memory THE PLAN
179. MEMORY
189. ▸ Basics and reading values ▸ Breakpoints and watchpoints ▸ Layouts ▸ Memory THE PLAN
190. SUMMARY
191. ▸ print() is fine for basic work, but there are better alternatives ▸ All codebases need more assert()s ▸ fatalError() makes cleaner code ▸ Know when to use p, po, expr, and frame variable THE BASICS
192. ▸ Stop adding and deleting the same breakpoints: conditions! ▸ Play sounds to help you track invisible operation state ▸ Always use exception breakpoints ▸ Watchpoints will save you time BREAKPOINTS AND WATCHPOINTS
193. ▸ You’re not alone: no one likes debugging layouts ▸ Filter the view debugger ▸ Always use constraint identifiers ▸ Exercise ambiguity! ▸ Make Xcode detect layout loops LAYOUTS
194. ▸ This is best done in Instruments, but Xcode can help ▸ The basic memory graph shows allocations and loops ▸ Enable Malloc Stack and Malloc Scribble to make it work harder! MEMORY
197. Optional is Not a Failure PHIL NASH
198. @twostraws
199. PAUL HUDSON — @twostraws LEARN TO DEBUG LIKE A PRO https://www.hackingwithswift.com