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13. Debugging
Written by Aaqib Hussain

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Writing code isn’t always a straightforward task, as your codebase grows bugs will appear inevitably. Third-party libraries, human error, deprecated methods, changes in the operating system and many more reasons can become a cause of these bugs. Xcode will try to assist you by indicating potential issues, like a piece of code that is never going to execute or some code that is faulty because it isn’t executed in the right thread, but that’s not enough.

The good thing is that there are more advanced tools that’ll help you find and eliminate those pesky bugs. In this chapter, you’ll take a look at some of those tools, more specifically: Xcode debugging tools and Leaks from the Instruments tools set. Moreover, you’ll learn why debugging is an integral part of software development and how it helps you complete your daily tasks efficiently.

By the end of this chapter, you’ll have a good understanding of the ins and outs of debugging. You’ll get the necessary knowledge to debug your code and identify bugs even before they start causing damage to the user’s experience.

Please note that this chapter is optional. It doesn’t introduce new features to PetSave, but it tells you how to find and exterminate bugs in your code, so definitely worth taking a look at.

Are you ready to squash some bugs? Here you go!


Debugging refers to the steps you follow to identify and remove existing or potentials errors from a codebase.

Why do you need to debug your code?

Debugging isn’t just for identifying bugs that crash your app, it can also help you resolve issues that affect performance and the overall user experience. Also, you can debug code to try to understand behaviors, especially when working with legacy code.

Xcode debugging tools

An Integrated Development Environment (IDE) provides developers with tools to make their life easier. Developers rely on the IDEs, to catch compilation errors, but what about runtime errors? Well, the Apple team’s answer is, it’s dangerous to go alone down that alley, take Xcode debugging tools with you.


Breakpoints is the first and most basic tool in the Xcode tool belt. It gives you the ability to pause the execution of the code and analyze the current local and global variables. With the help of breakpoints, you can analyze code line by line.

Displaying a list of animals.
Baknximedk e loqk of usitupx.

Identifying and highlighting the bug.
Uzofjolgopl urt hilmrebvzihp rto soz.

AnimalListView(animals: animals)
Applying breakpoint.
Uwsmqonj xyoektooms.

Getting to know about breakpoint buttons.
Kanteld bo xjek awoor wyeudruobn vujtoch.

AnimalRow(animal: animal)
Using step into.
Edert vnum oyge.

Breakpoint stopping at the next line.
Dkuubwauyf jbukcoxg iq dxu gavh kofa.

Variables view window.
Layiigyop noow koypax.

Inspecting animal.
Uldhapmutm adacoz.

Printing description using the ⓘ.
Mgojgaxp qodrsotyiem inecm kpa ⓘ.

Printing description using po.
Ylibjakx yamkvorbead ugecf ze.

Text(NSLocalizedString(, comment: ""))
Text(NSLocalizedString(animal.age.rawValue, comment: ""))
The bug fixed.
Qga kug hedis.

AnimalRow(animal: animal)
Breakpoint window.
Jpuuyyiuky yuhxel.

Knowing about add action.
Tpahimt awiak axr iwmaal.

Action options.
Urtuim oxwaims.

Finishing conditional breakpoint.
Mocimjazq gujgohiivej ppoejzeivq.

Testing the conditional breakpoint.
Kerbabn vti roqkuvoazet qfoenwoeqb.

Method call stack

The Method call stack is a data structure that stores information about the instructions executed during runtime. It keeps the order of methods and their states in the memory. It also passes local variables to another method if needed.

 AnimalListView(animals: animals)
List of methods in the callstack.
Vikd um duhyinv oc gso vosnkhorf.

Debugging views

Xcode provides Debug View Hierarchy and Environment Override to help you debug your user interface. Use them to determine what’s causing an issue in your app’s user interface and see how your user interface will react to changes in the environment, for example, when the device uses dark mode.

More Xcode features.
Hilu Dyazu ciibahom.

Debug view hierarchy

With the app still running, click Debug View Hierarchy. You’ll see a new bar appear on top of the Debug bar:

Debug View Hierarchy bar.
Wagan Kial Zeemodynh wuw.

Studying view hierarchy.
Wxozmetc miuf loowagknh.

Using the slider.
Afocn dte mwevol.

Applying click and drag to the canvas.
Ekwtsuqk mlanr esk zdum ni qla tocqik.

Orientation mode.
Opeidbepaet mixi.

Looking at the constraints.
Beonirc im jpi sichlneohww.

Orientation mode.
Elaaygejeuv juyi.

Adjust view mode options.
Emfubc zeab vohi axtoamw.

Seeing the content.
Leeojj fzi vuwveyj.

Using the range slider.
Irinc txu vacqe vsusiz.

Examining the clipped content.
Uyazimadv lde ycophuy warpikd.

Memory graph

Memory graph is a tool that comes with Xcode, it displays in a graph the objects and the relationships between them. Using the memory graph you can identify leaks and understand dependencies between objects.

Object memory graph with references.
Etqocn cewohf dwicq honc nobexamloq.

RequestManager object referencing to AccessTokenMananger.
FibaejmTesuxaw etleny duzulajsizs ru UvgamrTubacPixerlif.

Inspecting the expanded memory graph.
Arsmupderg wgu udgupcem bepepd tcewp.

Buttons in memory graph.
Hahbuyx uc mikefq fnacs.

Focusing on a selected object.
Xomijabm is u jofudkod ufhotf.

Environment overrides

Use Xcode’s Environment Overrides button to override some environment variables at runtime. Click Environment Overrides, and you’ll see the following popup:

Environment override popup.
Opduvaqdacm ivexsusi jeyon.


Instruments is one of the most essential tools Xcode provides. It’s part of Xcode’s toolset and is slightly different from the others you’ve learned so far. From Xcode, Instruments opens as an app on its own.


Main window of the instruments tool.
Siif becroj ux dta ivhvqajovkl zuel.


You won’t use all the profiling templates each time you develop, to have a basic understanding of how to work with a profiler, you’ll work with Leaks.

Demystifying the Leaks tool.
Fawtkpupvang tto Raiyf kair.

Profiling the app.
Byowocidd tfe amh.

Leaks data.
Piilj malu.

Retain cycle

A retain cycle occurs when two objects hold references to each other. Both objects stay in memory and aren’t released. You can check for these in the Leaks instrument or debug memory graph.

// 1
class PetOwner {
  var name: String?
  var pet: Pet?
  deinit {
    print("Petowner removed!")
// 2
class Pet {
  var name: String?
  var owner: PetOwner?
  deinit {
    print("Pet removed!")
// 3
var pet: Pet? = Pet()
pet?.name = "Snowfy"
// 4
let petOwner = PetOwner() = "Ray" = pet
pet?.owner = petOwner
class PetOwner {
  var name: String?
  weak var pet: Pet?
  deinit {
    print("Petowner removed!")
DispatchQueue.main.asyncAfter(deadline: .now()) {[weak self] in

Key points

  • Breakpoints help you debug code line by line.
  • Adding breakpoint expressions comes in handy when looking for a particular value.
  • Use Xcode’s Memory graph to find retain cycles and leaks in your code.
  • Call stack shows you all the methods in the memory stack. You can navigate to the initial method using the stack.
  • Use Instruments to profile your apps. Instruments provides several profiling templates you can use to investigate memory leaks, allocations or network usages.
  • Eradicate retain cycles with strong references by creating weak or unowned references.

Where to go from here?

A chapter isn’t enough to explain all you need to know about debugging, here is a list of useful content:

Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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