A blog about how Swift works and iOS tricks, by Bruno Rocha.
Articles about advanced Swift development in excruciating detail, for free!
I started SwiftRocks in 2018 as a tool to help me improve my iOS development skills. I've figured out that one of the best ways to properly learn something is to attempt to teach the same thing to someone else, and a blog turned out to be a great way to do so. I didn't expect anyone to read these posts, and I'm incredibly grateful for how many people I was able to reach and the opportunities I got as a result. Thank you so much!
Here's what I plan to do from now on!
Broken constraints are common, but there are good reasons you might wanted to get them sorted out. Even though iOS can sometimes provide a functional UI after breaking a constraint, the additional layout passes that this requires are usually a performance issue. If you care about your users' battery usage, you'll want to make sure your UI is always configured and rendered properly.
In this article, I'd like to show a way you can intercept UIKit's exceptions directly in code (not using breakpoints!) and what you can use an implementation like this for.
Closed: Cannot Reproduce
Ever had a crash in which you had absolutely no idea what was going on, and no amount of testing allowed you to reproduce the issue? If so, you've come to the right place!
Well, sort of. As you'll see in this article, the ability to debug complex crashes is not something immediate. Keep this out of your expectations: there's no magical instrument that you forgot to run that will give you the output you're expecting. When it comes to complex crashes, what we need to do instead is prepare our environment so that these issues are better understood when they arrive, making them more actionable. Let's see how to do that!
Module compiled with Swift X cannot be imported by the Swift Y compiler
Uh-oh! You've been visited by the module stability fairy. "Wait a second, didn't they fix this in Swift 5?" you ask, to no avail.
Code that relies on memory tricks to work can be complicated, but there's a solid way of unit testing these cases. I see people wonder about this every once in a while, so I think it's a good moment to share the trick I use to achieve this.
If you don't know what a CPU Usage termination is, consider yourself lucky. Not only this is one of the nastiest crashes you can get in iOS, but it's also not even considered as a crash by the system. These terminations are not reported to crash providers like Firebase, so if your app has a severe high CPU usage crash, you won't even know about it until your users start reporting it to you. Really nasty stuff!
As part of my series of articles about Swift's new Structured Concurrency features, let's take a look at how
AsyncSequence works behind the curtains!
Xcode has a feature called Organizer that shows you important information about builds you sent to the App Store, with the most relevant ones being crashes and energy reports (for CPU/memory usage). In my experience, this feature will work perfectly fine if you pushed the build from your own machine, but if that's not true for any reason (the most common being because you're using a CI pipeline to do) then you might have a frustrating experience.
Have you ever had issues with Xcode not symbolicating crashes? Me too, and I found the fix by reverse engineering the IDE.
Swift always had its own documentation format, but the introduction of DocC in Xcode 13 added some cool new features. Let's take a look at what we can do now!
I gave a talk about scalable iOS apps at SwiftHeroes 2021 in which I speak about an app's four "levels" of complexity, ending by stating that we still don't know what the "fifth level" would be.
I do have one guess though: I believe a "level five" iOS app will be an app that contains so many modules and architectural components that any small addition will require writing massive amounts of boilerplate code. In an app like this, the jump from level four to level five would be defined by the app's ability to generate most of its boilerplate and requirements. I've been playing with "intelligent code generation" for a couple of years now, and I'd like to show you how to achieve this with SourceKit.
StaticString is an interesting type in Swift. It's essentially nothing more than a
String that can't be modified for the purposes of referencing static content inside your binary. Let's see how it works!
I've been playing around with Combine for a while now and had some trouble navigating all the new protocols and types, especially when it came to extensions. In this article, I'll show you how to properly extend existing publishers and how to wrap them when creating your own custom publishers.
Hmmmm, how fast is this piece of code? Let's find out!
When it comes to benchmarking the speed of code, it's common for people to boot a playground, throw in a
Date object and calculate the time difference after a piece of code runs. That may give you a rough estimate, but it can also be very misleading. The difference between code running in debug builds versus release ones can be massive, and the size of the input (assuming we're benchmarking an algorithm) can also make a huge difference in the speed of a function. In this article, we'll see how to properly benchmark your Swift code. (Note that we're not talking about things like iOS startup time -- we're talking about the speed to run a specific piece of code.)
Actors is a feature that is part of Swift's Structured Concurrency, bringing a brand new format to write and deal with asynchronous code. Although what Swift brings is new to the language, it's not new to tech itself. Many languages supported actors and async/await before Swift, but one interesting thing is that they are all implemented similarly. By having these features now in Swift, there's much we can learn from what developers experienced by using them in other languages.
Just like other "How X works internally in Swift" SwiftRocks articles, the purpose of this article is to explore how actors work under the hood, using Swift's own source code as a guide to finding out how they work inside the compiler.
RealityKit Object Capture is a feature introduced in Xcode 13 that allows you to create 3D objects from photographs using a process called photogrammetry. Although intended for retailers to enhance their online shopping experience by creating models out of things like furniture and using them in augmented reality experiences, RealityKit is incredibly easy to use for anything you may want, like 3D printing random things from your house.
I've been meaning to write an article about computer science fundamentals and how it can improve a programmer's career for a long time, but I always had trouble finding a good way of introducing this topic.
In this article, we'll introduce the field of algorithms and data structures, show how this knowledge is applied in practice using an analogy, and finally use all of that to clarify why large companies like Google and Apple are so focused on it. I hope that this will give you the tools to make your own conclusions about this topic, allowing you to not only understand why they do what they do but also to help you determine if this is something you should be studying to achieve your goals.
Concurrency is the entry point for the most complicated and bizarre bugs a programmer will ever experience. In this article, I'll share my favorite methods of ensuring thread-safety, as well as analyzing the performance of the different mechanisms.
The easiest way is to create a paid macOS app is to simply put a price tag in the App Store, but it's a common practice nowadays to provide a free download that can later be upgraded to a pro version. In this article, we'll use our knowledge of serial numbers and asymmetric cryptography to create license files that cannot be reverse-engineered and use them to activate an app's premium features.
Bridging to/from Objective-C is an important part of Swift development due to the Obj-C history of Apple's platforms. Unfortunately, there are some hidden caveats that could lead to bizarre situations that would be impossible in a pure Swift environment. When dealing with Objective-C types, it's useful to check if they don't have a history of being too different from their Swift counterparts. Let's see some of these cases.
@inlinable attribute is one of Swift's lesser-known attributes. Like others of its kind, its purpose is to enable a specific set of micro-optimizations that you can use to increase the performance of your application. Let's take a look at how this one works.
DispatchSource family of types from GCD contains a series of objects that are capable of monitoring OS-related events. In this article, we'll see how to use
DispatchSource to monitor a log file and create a debug-only view that shows the app's latest logs.
We use API availability checks all the time to provide fallbacks for users running older iOS versions, but have you wondered how the Swift compiler handles this? In this article, we'll take a deep dive on how the
#availability condition works, how the Swift compiler is able to know if a specific symbol is available for usage and what the code you wrote looks like after being optimized.
Did you know it's possible to terminate your app in the middle of an XCUITest and launch it again from somewhere else? Let's see how to use this trick to test that deeplinks and universal links are properly launching our app when executed from Safari or iMessage.
Even if you don't know what the Observer pattern is, there's a good chance that you applied it somewhere in the past. This design pattern is used to create APIs that notify one or more subjects about changes in a certain object, with the
NotificationCenter being the most popular use of this pattern in iOS.
In this article, I'll show you a memory management trick with some old-school
Foundation types that can be used in any object that applies the Observer pattern to create better and safer APIs.
When jailbreaking of iOS devices first became popular, it was very common for iOS developers to try to defend their apps from users that altered their devices to enable piracy. As (possibly) a response to jailbreaking become popular again in recent times, Apple has released their own solution to this problem. In iOS 14, the new App Attest APIs provide you a secure way to sign server requests to prove to your server that they came from an uncompromised version of your app.
It's important to know that App Attest is not a "is this device jailbroken?" check, as that has been proven over and over to be impossible to pinpoint. Instead, it aims to protect server requests in order to make it harder for hackers to create compromised versions of your app that unlock premium features or inserts features like cheats. Note the word harder: as jailbreakers have physical access to their devices, nothing will completely safe-guard you from fraud in this case.
In this article, we'll see how Swift determines the entry point of an app, the different attributes used to alter this behavior in iOS, how they work internally, and what Swift 5.3's new
@main attribute brings to the table.
The compiler already does a good job translating Objective-C APIs to Swift's standards, but what if you want them to be translated to something completely different? Let's take a look at what this special compiler attribute can do.
Debug menus in iOS are a very effective way to make your day more productive as a developer. If you find yourself doing the same debugging tasks over and over, such as printing backend responses, skipping to specific view controllers or copying user identifiers, it's nice to have special developer menus in your app that automatically handles these tasks for you. Let's see how we can use
UIContextMenuInteraction to create debug-only special menus.
Widgets existed in iOS for a long time, but iOS 14 completely revamped them. With the new Widget APIs, not you're able to add widgets directly to your home screen, but they are also completely written in SwiftUI. iOS 14's Widgets come in a variety of shapes (literally), ranging from simple information squares to widgets that can retrieve and display information from its parent app's Siri Intents.
One trick that has drawn my attention the most however is that having widgets directly in your home means that technically you're now able to make visual "mini apps". If you find yourself doing the same tasks over and over in your job such as checking the number of crashes in a release or the latest release supported by one of your apps, you can create a Widget in iOS 14 that essentially behaves like a super-charged, UI-enabled Siri Shortcut.
I've always found throwing functions (
do/catch) to be a very underrated feature in the Swift community. While many native APIs from iOS rely on it, very few people seem to actually use them in their daily projects. I've recently refactored one of my tools to completely rely on it, and the results exceeded my expectations.
Before Swift 4.2, generating random numbers involved using raw C APIs. With the addition of the RandomNumberGenerator protocol in Swift 4.2, developers were graced with an easy way to generate random numbers. Let's see how to use the new APIs, but most importantly, how they are implemented inside the Swift compiler.
Preprocessor macros are used to bring context to a build, allowing you to transform how your app is compiled depending on why it's being built. Let's see how we can unit test their presence / absence while still keeping your app's unreachable code uncompiled.
The DispatchQueue class has lots of interesting and undocumented (unfortunately) hidden gems. Let's see how we can use one of these gems to get a method's current DispatchQueue -- a trick that can also be used to send and retrieve information from a DispatchQueue.
When dealing with dictionaries and arrays in Swift, it's very important to know that any reference types used as a key or value will be retained by default. Let's see some interesting cases where this behavior is problematic, and how we can use types like NSMapTable, NSHashTable and NSPointerArray to create weak collections that perform better than their Swift counterparts.
ObjectIdentifier is one of these obscure Swift types that you only see when looking at the answers for very specific problems in StackOverflow, and chances are that you have never really needed to use it. However,
ObjectIdentifier plays an important part in Swift and is actually pretty useful as a solution to problems that involve metatypes. Let's take a look at why this type exists and what you can use it for.
Global functions, or functions that can be accessed from anywhere without the scope of a specific type is an old concept that was popular in languages like C and Objective-C, but unrecommended in Swift as we would rather have things that are nicely typed and scoped ("swifty").
For historical reasons, the Swift Standard Library still has quite a few public global functions, and some of them are still useful to this day. Let's take a look at functions like
Ordered Collection Diffing is a feature added in Swift 5.1 that allows you to calculate and apply the difference between two collections. Using diffing libraries is common in iOS for a few reasons, the most popular one being to handle the addition and removal of elements in
UITableViews. With the addition of this feature, developers can now diff
Collections without having to bother with external libraries.
Have you wondered what efficiently diffing something looks like? In this article, we'll see and analyze the diffing APIs as well as the Myers's Diffing Algorithm used by the Swift Standard Library -- the same used in git.
The Codable protocols are one of the coolest recent additions to Swift. Even though it works similarly to its community counterparts like Unbox, Codable has the advantage of being powered by the compiler.
One of my favorite features in Unbox was to give a context to the decoding operation. Let's take a look at how we can achieve the same with Codable and how I use to power a type-erased Decodable type.
While dividing an app into several modules can improve the build time of an iOS app, the result heavily depends on what is being changed. If you're not careful with how your dependency graph is laid out, you can often have results that are worse than non-modularized apps. Let's take a look at a technique used at iFood to have big and consistent improvements to the build times of our app.
Hashing algorithms are functions that map an arbitrary string to a seemingly "random" output with a fixed length. Commonly associated as a component of
Dictionaries, hashing algorithms are a major component of several branches of computer science, including cryptography.
The internal algorithm used to calculate a type's
Hasher in Swift 4.2) and the related compiler features changed several times throughout Swift's releases, and it was only after Swift 4.2 that a real universal hashing algorithm was added to Swift. To see how this is implemented today, let's take a look at how this looked like throughout the history of Swift.
With Swift being a very type-safe and namespaced language, you'll find that certain tasks are really hard to complete if at some point you can't determine the types that are being handled -- mostly when generics are involved. Using an automatic dependency injector as an example, let's see how generic arguments and closures can be "erased" to trick the compiler into compiling code that it would otherwise claim to be impossible. (when it's clearly not!)
While this isn't your usual SwiftRocks-compiler-feature-deconstruction article, we'll take an exciting look at how the treatment of methods/closures as properties can be used in this context to bypass one of the Swift Compiler's most annoying compilation errors.
CollectionOfOne is a type inside the Swift Standard Library that defines a collection of a single element. While this sounds a bit weird, there's a good reason for this to exist. Let's check out why.
OptionSet protocol is the bridged version of Objective-C's
NS_OPTIONS enum -- a handy tool that allows you to very efficiently define a combination of "options" that can be used for a specific operation. Let's see what this protocol is, why
OptionSets are preferred over a regular
Set in some cases and finally what compiler magics it possesses.
Even in apps written in full Swift, interfacing with Objective-C is still a very big part of iOS development. Many types and semantics from Objective-C are hidden underneath the code we write today, and understanding where they come from can help you make better code decisions and just better understand the platform in general. This time, we'll take a look at what
NSCopying and its companion class
NSZone are and what they can do for Swift apps.
One of my favorite things about Swift is that almost every compiler feature is built on top of actual classes and protocols of the language. This means that if you see a native type that has some special magical property (like SwiftUI's declarative syntax, which was covered in another post here in SwiftRocks), it's likely that you can reproduce that behavior in your custom types.
In this article, I'll cover one of my favorite syntax sugars in the language: the protocols and the internal compiler behavior that allow you to write
for loops. You might have heard already of the
Sequence family of protocols, and here, we'll see how the compiler uses them as the building blocks for
iOS 13 marks the release of long-waited features like Dark Mode, but it also brought some needed changes on less popular aspects. Prior to iOS 13, creating socket connections required coding very low-level network interactions which made libraries like Starscream and Socket.io the go-to solution for sockets in iOS. Now with iOS 13, a new native
URLSessionWebSocketTask class is available to finally make creating and managing socket connections easier for developers.
Some time ago I created a little side project that involved an Arduino-powered servo motor that menacingly pointed at people's faces with the help of CoreML, mimicking the Team Fortress 2 Engineer's Sentry Gun. With iOS 13, I decided to re-write that using the new Socket APIs and SwiftUI.
SIMD Vector Types is a feature added in Swift 5 that exposes Apple's
<simd/simd.h> module to Swift, allowing you to calculate multiple results with a single instruction. Let's see how that works.
Have you ever asked yourself which algorithm is used by Swift's sorting method? There are many sorting algorithms out there, and chances are that you'll rarely have to use something other than the language's builtin
sort() method. However, knowing the properties of the sorting algorithm built into your language is important if you want to prevent unwanted behaviors and nasty edge cases.
While Xcode provides several visual abstractions for lldb commands like adding breakpoints by clicking the lines of code and running by clicking the play button, lldb provides several useful commands that are not present in Xcode's UI. This can range from creating methods on the fly to even changing the CPU's registers to force a specific flow on the app without having to recompile it, and knowing them can greatly improve your debugging experience.
ExpressibleBy represents a series of protocols in the Swift Standard library that allows you to instantiate objects directly from token literals, like a string, a number, a floating-point and so on, if the object can be "expressed" like that. Let's see how the compiler applies this magic.
Foundation provides you a lot of the bread-and-butter needed for your daily iOS development, ranging from structures like
Data all the way to complete APIs like
URLSession. But as it turns out, we only use a fraction of what
There's a bunch of Foundation types that are so situational that people doubt they even existed in the first place! In fact, they are so rarely mentioned as solutions to daily problems that developers may end up coding things that may already exist in these SDKs. But they do exist -- and although some of them are really old, most of these types are still very useful. Let's take a look at some of them!
Opaque Return Types is a feature added in Swift 5.1 that is a big part of the new SwiftUI framework's functionality. It comes to finally fix a fundamental problem of the usage of protocols and the design of Swift APIs, opening new possibilities for the creation and usage of public APIs. Let's see the compiler magic behind it and why SwiftUI uses it to return the
some View type.
Announced in WWDC 2019, SwiftUI is an incredible UI building framework that might forever change how iOS apps are made. For years we've engaged in the war of writing views via Storyboard or View Code, and SwiftUI seems to finally end this. With its release, not only Storyboards are now pretty much irrelevant, but the old fashioned View Code is also very threatened as SwiftUI mixes the best of both worlds.
The thing is, it looks very different from the Swift we're used to, and the reason is because it empowers a few new "unreleased" compiler features. Let's see how this works.
NSAutoreleasePool type, later abstracted to the
@autoreleasepool block, is a very old concept in iOS development. During the Obj-C era of iOS, usage of this type was important to prevent your app's memory from blowing up in specific cases. As ARC and Swift came around and evolved, very few people still have to manually play around with memory, making seeing it become a rare occurrence.
Having developed tools that need to allocate enourmous amounts of memory, I asked myself if there's still a case where
autoreleasepool is useful in Swift. Here are the answers.
When teams start to grow, it becomes to difficult to track what everyone is doing. If the team has different ideas on what a specific piece of code should look like, you can very quickly end up with a project where nobody wanders outside the things they coded themselves. Style guides can be seen as an anti-pattern when the subject is speed and there is often resistence from developers that haven't been part of large projects, and indeed having a traditional bureaucratic process in this modern tech world seems very odd, but a strict style guide is critical to have a project scale to the point where you can have hundreds of developers like Uber.
When deadlines are tight and the product faces considerable changes, it's common for developers to make concessions in the project's quality to make sure it gets shipped in time. This leads to release anxiety - that stressful feeling where you're unsure if you're shipping something that actually works. As a result, teams resort to heavy amounts of manual testing, staying overtime to make sure nothing fell apart and an unhappy environment in general. Avoiding Release Anxiety is a series of posts where I show the things I do to develop durable Swift apps that allow me and my team to sleep worry-free at night.
In this edition, I show why I like UI Tests and how I build them to make my projects reliable.
In iOS, the Responder Chain is the name given to an UIKit-generated linked list of
UIResponder objects, and is the foundation for everything regarding events (like touch and motion) in iOS.
The Responder Chain is something that you constantly deal with in the world of iOS development, and although you rarely have to directly deal with it outside of
UITextField keyboard shenanigans, knowledge of how it works allows you to solve event-related problems in very easy/creative ways - you can even build architectures that rely on Responder Chains.
iOS already allows you to add "shortcuts" to an app in the shape of 3D Touch Quick Actions, but they are very limited, hidden from the user, and let's be honest - it's one of these features that nobody remembers. You can also use Siri Extensions, but then you have to deal with Siri, and the regular Siri Shortcuts relies on the Shortcuts app, so it's not something that you can provide to your users. Wouldn't it be great if you could give your users the ability to add shortcuts to things like ordering their favorite coffee in the shape of a home icon - just like the app itself?
Well, after reverse engineering Apple's Shortcuts app, it turns out that you can! That's exactly what it does when you add a system shortcut to the home screen, and we're going to replicate it in order to give users the ability to add home icons that directly open a specific part of an app.
Being able to work in projects of the most diverse varieties gave me the chance to be in contact with several types of developers and code bases. Besides their core differences, what stood out to me during this process is that projects with a lower level of maturity will always face similar problems.
Perhaps they choose the wrong architecture, or the lack of unit tests caused a nasty bug to sneak into production, but there's a specific problem that always draws my attention - callback hell. If not treated from the very beginning, these awful pyramids of braces you get when chaining callbacks inside other callbacks or conditions plague code bases with an eternity of impossible code-reviews and distant screams of "what the hell is this method supposed to be doing?". I've shared a few tips on how to avoid them.
@dynamicMemberLookup attribute was introduced in Swift 4.2 to add a certain degree of dynamism into the language similar to what is seen in languages like Python. I was interested in learning how the attributes worked inside the compiler to know more about how this attribute is able to transform fake expressions into legit ones, so I've once again reverse-engineered the Swift compiler to find these answers - and used this knowledge to create a brand new
Added in Swift 3, the
Never type allows you to define a method that is guaranteed to crash your app. Although we rarely write methods that use this type directly, we do interact with it constantly given that it is the return type of all forced-crash methods like
fatalError(). The advantage this type brings is that a method that calls another
Never-returning method does not need to provide a return value, after all, a crash is certain to happen. I investigated how that works.
Metatypes are pretty useful in Swift, and you have certainly used it in multiple occasions. Unfortunately they look pretty weird in code, which can cause some confusion when trying to understand what they really are.
I for one know how these weird suffixes can confuse you, but don't worry, they are actually pretty straightforward once you get to know the differences between each of them.
Siri Shortcuts are definitely my favorite iOS 12 feature. Ever since SiriKit first came out I was very disappointed that you couldn't use it for your own custom commands, but the new Shortcuts app solves this problem. Not only it allows you to create your custom commands, but it also provides a very intuitive UI to allow even non-devs to automate tons of kinds of workflows in their iPhones.
We've all seen how to create a workflow and make your app expose custom actions, but one thing that I haven't seen people mention is that the Shortcuts app exposes several deeplinks for opening and running shortcuts - meaning that you can make an app that runs other apps' Siri Shortcuts. I used this to make an app run my "Take Picture" shortcut every time I tapped my AirPods.
CaseIterable is one of my favorite features in Swift 4.2. Despite being a simple protocol, it solves the common problem (that I personally faced many times) of needing access to an array containing all the cases of a certain enum. But this protocol is special: The requirements are automatically filled by the compiler. I investigated how that works - and as a bonus, modified it to also provide a
I was browsing the Swift forums when I stumbled across a discussion regarding an undocumented feature of Swift (as of this post's date): the ability to give associated types a default value. Here's how they can improve your objects.
Usage of high-order functions like
filter are very common in Swift projects, as they are simple algorithms that allow you to convert extensive ideas into simple one-liners. Unfortunately, they don't solve every issue - at least not in their default implementations. High-order functions are eager: they use the closure immediately and return a new array, regardless if you need an early return or only going to use specific elements.
Pattern matching is available everywhere in Swift, and you have likely used it tons of times to deconstruct and bind values in things like
switch cases. While regular
switch cases are the most common use for patterns, Swift has several types of patterns which can be mixed and even used outside of
switches to result in really cool and short lines of code. One thing that interests me in particular is that pattern matching can be used for a wide variety of things.
The choice between using Storyboards and writing views programatically is very subjetive. Having dealt with both in the past, I personally vouch for writing projects entirely through view code as it allows multiple people to work in the same class without nasty conflicts, and easier code reviews.
When starting with the practice of writing views programatically, a common problem people face is where to put the code in the first place. If you follow the regular storyboard approach of putting everything view related in your view controller, it's very easy to end up with gigantic classes. However, there's also a very easy way to overcome this problem.
@inline attribute is one of those obscure things in Swift - it's nowhere to be found in Apple's docs, doesn't help you write cleaner code and has no purpose but to help the compiler make optimization decisions, but it's related to a pretty important aspect of your app's performance.
Competitive programming is a great way to master a specific programming language. Even if you're not interested in competing in world events like the Facebook Hacker Cup, tackling difficult algorithm problems using nothing but the language's bread and butter will expose you to aspects/shortcuts of the language you would otherwise never see, such as how efficient certain methods/operations are and how to code better alternatives.
It's very common to use the
weak keyword in order to prevent reference cycles in properties like delegates.
Unfortunately, since it protects you from references being lost, it forces the usage of
var and an optional type, which can be quite jarring if you're building something like an
UIView that is not supposed to be used without a delegate - much less change delegates suddenly. The
unowned keyword allows you to overcome these issues.
It's very likely that you asked yourself at least once in your iOS career what's the difference between a
struct and a
class. While realistically the choice between using one or another always boils down to value semantics versus reference semantics, the performance differences between the two are expressive and can heavily favor one or another depending on the contents of your object, specially when dealing with value types.
Some people might say that knowledge of memory architecture is irrelevant for application level developers, and I agree partially. Knowing how to save a few bits here and there will make no visible difference on newer iPhones, and premature optimization is a highly shunned practice.
However, both reference and value types can severely slow down your app when misused, and such knowledge will define whether or not you can fix the problem efficiently.
Understanding how threads operate on the CPU level is the key to writing good concurrent code. This article explains how the CPU executes multithreaded processes, what exactly are iOS's DispatchQueues, what's the point of their "Quality of Services", and why do you have to process UI code in the so called Main Thread.
If you're using TouchID/FaceID as a security measure, chances are you want to protect your user's data in case someone tries to break into his phone. By default, any fingerprint/face is able to validate an user - even if they were added after your app was installed. Learn how to detect system changes in order to stop thieves from accessing your user's data.
In this article, I show how simple is the process of reverse-engineering an App Store's app by using dumped class information and a debugger hooked to a jailbroken device to make Facebook's Messenger allow me to select custom chat bubble colors.