Advanced Debugging with Xcode and LLDB - WWDC 2018

Advanced Debugging Tips and Tricks:

• Injecting Code at runtime
  We can change values of variables at runtime while debugging (here variable_name is a Bool)
  
  expression variable_name = false 
  
  Here we are injecting value at runtime, so no need to run again to see the effect. (Other eg.,  expression animator.delegate = self)
• Under Xcode > Preferences > Behaviours > Running > Pauses > Check Show Tab Named, In order to show new tab while a breakpoint is hit. We can set other behaviours as well.

• We can add symbolic breakpoints on methods in frameworks. like
  
  This is in objective c because UIKit is in objective c.
• In order to find the values/parameter of a function inside framework, we can use
  
  For expression -[UILabel setText:]
  
  po $arg1
  
  <UILabel ****>
  
  po (SEL)$arg2
    "setText:"
  
  po $arg3
  "0 ft"
• We can set symbolic breakpoints in 2 ways
  1. By clicking plus in bottom left and choosing symbolic (Cons: Sets BP in all places)
  2. By adding a breakpoint > Editing it to specify a debugger command.  (Pros: we can set at a desired method, for eg., after completion of some task)
  
  

        Here --one-shot is breakpoint is executed only once. (Dependent Breakpoint)

• Skipping code execution after compilation.
  for example if i want to skip line 10 in editor and continue execution at line after line 12. There are 2 ways.
  
  1. Keep BreakPoint at line 10 and Drag the Thread (Instruction Pointer) to desired line 12.
  2. Add breakpoint at line 10 with debugger command "thread jump --by 2"
  
  Use Case:
  Jump execution of a function at line 10 and execute the same function with different parameters
• Use CustomDebugStringConvertible protocol to add our own format of debug description by implementing debugDescription property and returning desired format.
• po is programatic description of object. p is description of format specified by LLDB.
• watchpoint watchpoint for a variable means, a breakpoint is set for a variable and it is called/execution paused when the value is changed next time.
• To execute swift code in objc in LLDB
  
  Swift code: self.view.recursiveDescription()
  The above code throws error as the function is not strictly defined. so we run same code in objc as it is dynamic language.
  
  For Objc way:
  
  expression -l objc  -O -- [`self.view` recursiveDescription]
  objc = to tell we are giving objc code even though we are in swift lldb frame
  -O = to give debug description
  -- = to tell no more options
  `` = are to inherit variable from swift frame to objc
• Creating shortcuts for lengthy commands.
  
  command alias ALIAS_NAME expression -l objc  -O -- 
  
  The above command creates shortcut, so next time.
  ALIAS_NAME [`self.view` recursiveDescription]
• Printing variable from memory address 2 ways.
  1. po 7x99890s0s
  2. po unsafeBitCast(7x99890s0s, to: CustomViewType.self)
  
  unsafeBitCast is more useful as you can print property names and functions on it, like
  po unsafeBitCast(7x99890s0s, to: CustomViewType.self).frame
• Assigning/Modifying view properties from LLDB
  po unsafeBitCast(7x99890s0s, to: CustomViewType.self).center.y = 100
  
  This gets updated only after "expression CATransaction.flush()" (as we are paused in debugger to run po command, this command tell core animation to update ui)
  
  

View Debugging Tips

• To click through the view hold command while click.
  For eg., to select a view which is beneath other full screen view.
• Windows are shown as multiple root level objects on left hand side of view hierarchy. 
• e is shortcut for expression.
• we can create backtraces for views in views hierarchy, by settings.
  Scheme > Edit Scheme > Diagnostics > Logging > Check - Malloc Stack > Select - All allocations and free history

Try: 

po classification

Creates new simpler context with frame of variables so that we can inspect variable values.


po variable_name
prints : "Mercury"

po frame variable variable_name
prints : "(String) variable_name  = Mercury"