Advanced Debugging in XCode

This deck covers some nice tips and techniques for a better and more detailed debugging workflow in XCode using LLDB commands and customised breakpoints, based on Apple’s debugging engineers presentation during 2018’s WWDC. It was presented in NSBrazil conference in 2019.

Transcript

1. ADVANCED DEBUGGING LARISSA BARRA | MICHEL BUENO IN XCODE
2. WHY THESE TIPS, AND WHY “ADVANCED”?
3. ADVANCED DEBUGGING IN XCODE ▸ Debugging can be a part of the process of building things, not just finding bugs in your code!
 ▸ The more efficiently we debug, the more building time we save. That can make a huge difference in complex applications
 ▸ There’s a lot more than what we’ll show here, we chose the tips that help us the most and are simple enough to be done very easily!
 ▸ Because they’re simple, maybe the term “advanced" sounds a little off. But it refers to the act of debugging, not to the content itself
4. ADVANCED DEBUGGING IN XCODE TOPICS COVERED ▸ Change / inject code during runtime ▸ Symbolic breakpoints ▸ Printing assembly code values ▸ Conditional breakpoints ▸ Printing console message ▸ Chisel
5. WHO ARE WE
6. LARISSA BARRA MOBILE ENGINEER @ THOUGHTWORKS linkedin.com/in/larissabconde lbarra@thoughtworks.com
7. MICHEL BUENO MOBILE ENGINEER @ THOUGHTWORKS linkedin.com/in/michelbueno mbueno@thoughtworks.com
8. WHAT IS LLDB
9. LLDB is the default debugger in Xcode on macOS and supports debugging C, Objective-C and C++ on the desktop and iOS devices and simulator. https://lldb.llvm.org ADVANCED DEBUGGING IN XCODE
10. ON TO DEBUGGING!
11. CHANGE / INJECT CODE DURING RUNTIME
12. ADVANCED DEBUGGING IN XCODE - CHANGE / INJECT CODE DURING RUNTIME IMAGINE THAT… ▸ You’re developing code to show a flight’s status ▸ You get this status from an external data source (database, API, whatever) ▸ Possible status values are: ▸ On time ▸ Delayed ▸ Cancelled
13. ADVANCED DEBUGGING IN XCODE - CHANGE / INJECT CODE DURING RUNTIME
14. ADVANCED DEBUGGING IN XCODE - CHANGE / INJECT CODE DURING RUNTIME 2
15. ADVANCED DEBUGGING IN XCODE - CHANGE / INJECT CODE DURING RUNTIME
16. ADVANCED DEBUGGING IN XCODE - CHANGE / INJECT CODE DURING RUNTIME 2 Right click the breakpoint and choose Edit Breakpoint
17. ADVANCED DEBUGGING IN XCODE - CHANGE / INJECT CODE DURING RUNTIME 2 2 Click Add Action and type expression status = .delayed Mark Automatically continue after evaluating actions Select Debugger Command
18. SYMBOLIC BREAKPOINTS
19. ADVANCED DEBUGGING IN XCODE - SYMBOLIC BREAKPOINTS SYMBOLIC BREAKPOINT: ▸ A breakpoint that hits every time a method is called, for every instance of the particular class in your current debugging session. ▸ Helps you easily track the location of any SDK method call you want (apparently, it doesn’t work for your own code) ▸ You must use Objective-C syntax, even for Swift code. Example: ▸ To see every a time a text is set to any label, use -[UILabel setText:]
20. ADVANCED DEBUGGING IN XCODE - SYMBOLIC BREAKPOINTS Go to Breakpoint Navigator and add a Symbolic Breakpoint The symbol can be any method of any class you want. You must use Objective-C syntax. -[UILabel setText:]
21. ADVANCED DEBUGGING IN XCODE - SYMBOLIC BREAKPOINTS Not very helpful, right ? The breakpoint will hit every time setText is called on any UILabel existent in the current session. Sometimes you'll see assembly frames like this:
22. PRINTING ASSEMBLY CODE VALUES
23. ADVANCED DEBUGGING IN XCODE - PRINTING ASSEMBLY CODE VALUES You can inspect the the assembly frame with po command to see readable values Use po $arg1 to see the which object received this call
24. ADVANCED DEBUGGING IN XCODE - PRINTING ASSEMBLY CODE VALUES You can inspect the the assembly frame with po command to see readable values Use po (SEL)$arg2 to see the method name being called
25. ADVANCED DEBUGGING IN XCODE - PRINTING ASSEMBLY CODE VALUES You can inspect the the assembly frame with po command to see readable values Use po $arg3 to see the parameters
26. CONDITIONAL BREAKPOINTS
27. ADVANCED DEBUGGING IN XCODE - CONDITIONAL BREAKPOINTS IMAGINE THAT… ▸ You’re iterating values of an array using a for loop ▸ You want to inspect values for an specific index ▸ You're too lazy to be hitting continue until the pointer reaches the index you want
28. ADVANCED DEBUGGING IN XCODE - CONDITIONAL BREAKPOINTS Right click the breakpoint and choose Edit Breakpoint
29. ADVANCED DEBUGGING IN XCODE - CONDITIONAL BREAKPOINTS thread jump —by 1 36 Enter any expression that results in a boolean. You can use any variable of this context. In this case, we are using i == 4
30. ADVANCED DEBUGGING IN XCODE - CONDITIONAL BREAKPOINTS The breakpoint will hit only when the condition you defined is satisfied:
31. PRINTING CONSOLE MESSAGE
32. ADVANCED DEBUGGING IN XCODE - PRINT CONSOLE MESSAGE Right click the breakpoint and choose Edit Breakpoint
33. ADVANCED DEBUGGING IN XCODE - PRINT CONSOLE MESSAGE 36 Select Log Message Put variables between @@ to print its value Mark Automatically continue after evaluating actions
34. ADVANCED DEBUGGING IN XCODE - PRINT CONSOLE MESSAGE No more using print() in your code to debug!
35. CHISEL OTHER USEFUL LLDB COMMANDS WITH
36. ADVANCED DEBUGGING IN XCODE - OTHER USEFUL LLDB COMMANDS WITH CHISEL WHAT IS CHISEL? ▸ A collection of LLDB commands created to help you during debugging in Xcode. (by Facebook) ▸ Works for both iOS and MacOS apps. Some commands are platform exclusive ▸ Pretty easy to install and use ▸ Available on: https://github.com/facebook/chisel
37. ADVANCED DEBUGGING IN XCODE - OTHER USEFUL LLDB COMMANDS WITH CHISEL MOST HANDY COMMANDS pviews Print the recursive view description for the key window pvc Print the recursive view controller description for the key window visualize Generates an image an opens it in Preview.app. Accepts UIImage, CGImageRef, UIView, or CALayer types. pdata Print the contents of NSData object as string pcurl Print the NSURLRequest (HTTP) as curl command For the complete list of available commands, visit: https://github.com/facebook/chisel/wiki
38. ADVANCED DEBUGGING IN XCODE - OTHER USEFUL LLDB COMMANDS WITH CHISEL MOST HANDY COMMANDS For the complete list of available commands, visit: https://github.com/facebook/chisel/wiki show/hide Show or hide the given view or layer. You don't even have to continue the process to see the changes. mask/unmask Overlay a view or layer with a transparent rectangle to visualize where it is border/ unborder Add a border to a view or layer to visualize where it is pmethods Print the class and instance methods of a class pjson Print JSON representation of NSDictionary or NSArray object
39. DEMO
40. RECAP
41. ADVANCED DEBUGGING IN XCODE - RECAP RECAP ▸ You can save time avoiding to change code and recompile every time by using LLDB commands; ▸ LLDB allows you to change variable values at debugging time. ▸ Breakpoints can be customised to: ▸ Inject code; ▸ Only stop under a specific condition; ▸ Print a console message; ▸ A ton of other stuff!
42. ADVANCED DEBUGGING IN XCODE - RECAP RECAP ▸ Symbolic breakpoints can help you track the location of any SDK method call; ▸ Use po command with $arg1, $arg2 and $arg3 to transform assembly frames into readable text; ▸ Use expression command to change variable values; ▸ You can create your own aliases or use some collection from the community, such as Chisel; ▸ LLDB goes far beyond this presentation. Explore it!
43. ADVANCED DEBUGGING IN XCODE - RECAP RECOMMENDED READING: ▸ 2018's WWCD Debugging session: https://developer.apple.com/videos/play/wwdc2018/412/ ▸ Dancing in the Debugger — A Waltz with LLDB https://www.objc.io/issues/19-debugging/lldb-debugging/
44. THATS ALL FOLKS! Larissa Barra linkedin.com/in/larissabconde lbarra@thoughtworks.com Michel Bueno linkedin.com/in/michelbueno mbueno@thoughtworks.com We are hiring!