Advanced Debugging in XCode

ADVANCED DEBUGGING
LARISSA BARRA | MICHEL BUENO
IN XCODE

WHY THESE TIPS, AND
WHY “ADVANCED”?

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

TOPICS COVERED
▸ Change / inject code during runtime
▸ Symbolic breakpoints
▸ Printing assembly code values
▸ Conditional breakpoints
▸ Printing console message
▸ Chisel

LARISSA BARRA
MOBILE ENGINEER @ THOUGHTWORKS
linkedin.com/in/larissabconde
lbarra@thoughtworks.com

MICHEL BUENO
MOBILE ENGINEER @ THOUGHTWORKS
linkedin.com/in/michelbueno
mbueno@thoughtworks.com

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

CHANGE / INJECT CODE
DURING RUNTIME

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

2

2
Right click the
breakpoint and
choose Edit
Breakpoint

2
2
Click Add Action and type
expression status = .delayed
Mark Automatically continue
after evaluating actions
Select
Debugger
Command

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:]

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:]

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:

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

Use po (SEL)$arg2 to see the method name being called

Use po $arg3 to see the parameters

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

Right click the breakpoint
and choose Edit Breakpoint

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

The breakpoint will hit only when the condition you deﬁned is satisﬁed:

ADVANCED DEBUGGING IN XCODE - PRINT CONSOLE MESSAGE
Right click the
breakpoint and
choose Edit
Breakpoint

36
Select
Log Message Put variables between
@@ to print its value
Mark Automatically continue
after evaluating actions

No more using print() in your code to debug!

CHISEL
OTHER USEFUL LLDB COMMANDS WITH

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

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

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

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!

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!

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/

THATS ALL FOLKS!
Larissa Barra
linkedin.com/in/larissabconde
lbarra@thoughtworks.com
Michel Bueno
linkedin.com/in/michelbueno
mbueno@thoughtworks.com
We are hiring!

Advanced Debugging in XCode

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