Xcode debugging cheatsheet

This is a cheatsheet - starting point to look for things that you can do.
Avoided complex / 3rd party things as they require some additional setup that may not be allowed in certain projects.

Breakpoints

• Breakpoint is user enabled point in code where the application pauses
• Line and column breakpoints
  • Pauses the program before the statement on line/column is executed
• Conditions
  • Execute walk method just for LadyBug: self.bugType == 1
• Actions: Sound, Log message, Debug command
• Symbolic breakpoints: Add breakpoint on a method without source file [UIImage systemImageNamed:]
  • po $arg1 -> Receiver object
  • po $arg2 -> po (SEL)$arg2 -> Method name
  • po $arg3 -> First argument, and goes
• Watch points: Pauses when the value changes

LLDB commands

• help: Information about commands
• v: Does not compile the expression instead it just dumps value from the stack
• p: Evaluates and prints the output
• po: Evaluates the print debug output
  • description method
  • Use [] syntax if . is failing due to typecasting
• expression: Evaluates and prints, also stores the output in a variable
  • Expressions in other languages: expression -l swift/objc -O -- <code>
• alias: Creates a shortcut for complex commands
  • command alias greet p @"Hello World"

Techniques

• Inject code without building it: expression <sample fix/feature>
  • expr [bugs addObjectsFromArray:[[self makeAntViews:20] mutableCopy]]
• Short circuit / skip code execution:
  • Move the debugger handle
  • thread jump --by 1
• Early return through a method: thread return <return value>
  • NOTE: This does not always work and can be dangerous
• Print the view hierarchy: po [self.view recursiveDescription]
• Highlight a view:
  • (UIColor *)[bug setBackgroundColor:UIColor]
  • (void)[CATransition flush]: Refreshes UI
• Add delay in code: expr [NSThread sleepForTimeInterval:2]
• Auto layout debugging: po [self.view _autolayoutTrace]

References:

1. https://www.wwdcnotes.com/notes/wwdc18/412/
2. Advanced lldb tricks for Swift - Injecting and changing code on the fly
3. https://bryce.co/lldb-method-stubbing/
4. https://www.objc.io/issues/19-debugging/lldb-debugging/
5. https://github.com/facebook/chisel
6. Leveraging Swift in Xcode’s LLDB