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Introduction – please watch!

The Inside Swift series is designed to explore Swift's own source code, so you can better understand how it works and also pick up techniques you can apply to your own code.

Watch the video here, or read the article below

Quick links

How to read the code

Large parts of Swift is written in C++, but almost as much is written in Swift itself – almost all the Swift standard library, for example, is written in Swift code, and therefore makes for excellent learning material if you know where to look.

In this series I'll be picking out various aspects of Swift and the standard library from Apple's own code. As a result, it's important you have easy access to that code, and here there are two options:

  • You can clone the Swift repository from, giving you access to the code on your desktop.
  • You can also browse all the source code online through your web browser at that same URL.

Both work, but the former is significantly faster and easier to use – you can move through directories in Finder, open files in Xcode, search through file contents using your preferred tools, and more.

In this series I'll be using the local clone option. So, you'll see me browse through the directories locally on my Mac and open code right inside Xcode. If you want the easiest approach to follow along, I highly recommend you do the same.

If you're using the command-line, running git clone should be enough to get you a full copy of the files. Inside there the most important directory for us is stdlib > public > core, where all the base standard library files are.

A brief explanation of GYB

The standard library is made up of a whole bunch of Swift files, but there is one important exception: files that end in "gyb".

GYB stands for "generate your boilerplate", and it's used by the Swift team to generate Swift code for times when there is lots of duplication. For example, FloatingPointTypes.swift.gyb generates the types Float, Double, and more – types that are the same apart from the size of their storage.

So, rather than the Swift team having lots of code duplicated in places, they instead run a little Python code create all those types from one template. These files contain Python code such as conditions and loops to customize each type as needed, but the result is it generates a bunch of Swift files that can then be built for real to make the standard library.

A reminder to update

Before I'm done with this intro, I want to mention one important thing: Swift is an actively developed project by Apple, so the code we're examining might change in the future. That's okay, though: we're not trying to actually memorize Apple's code, we're just looking for ways they've used Swift and how we can use similar techniques ourselves.

That being said, if you find me referencing some code that doesn't exist in your version of the code, your first port of call should be to update your local clone of the repository just in case your version is out of date.

If you liked this, you'd love Hacking with Swift+…

Here's just a sample of the other tutorials, with each one coming as an article to read and as a 4K Ultra HD video.

Find out more and subscribe here

The pitfalls of string bridging



FREE: The pitfalls of string bridging

Swift's strings are designed to work flawlessly with languages around the world, but sometimes – just sometimes – you need to be careful using them. Let's explore why…

Creating a WaveView to draw smooth waveforms



FREE: Creating a WaveView to draw smooth waveforms

In this article I’m going to walk you through building a WaveView with SwiftUI, allowing us to create beautiful waveform-like effects to bring your user interface to life.

Creating a custom property wrapper using DynamicProperty



FREE: Creating a custom property wrapper using DynamicProperty

It’s not hard to make a basic property wrapper, but if you want one that automatically updates the body property like @State you need to do some extra work. In this article I’ll show you exactly how it’s done, as we build a property wrapper capable of reading and writing documents from our app’s container.

Understanding generics – part 1



FREE: Understanding generics – part 1

Generics are one of the most powerful features of Swift, allowing us to write code once and reuse it in many ways. In this article we’ll explore how they work, why adding constraints actually helps us write more code, and how generics help solve one of the biggest problems in Swift.

Controlling views using the accelerometer



FREE: Controlling views using the accelerometer

Reading device motion and orientation is a fast and slightly magical way to incorporate the real world into your apps, and can do a huge amount to add a little spark of delight to your UI. In this article I’m going to show you how easy it is to control SwiftUI layouts using the accelerometer, and give you a few ideas for special effects.

User-friendly network access



FREE: User-friendly network access

Anyone can write Swift code to fetch network data, but much harder is knowing how to write code to do it respectfully. In this article we’ll look at building a considerate network stack, taking into account the user’s connection, preferences, and more.

Shadows and glows



FREE: Shadows and glows

SwiftUI gives us a modifier to make simple shadows, but if you want something more advanced such as inner shadows or glows, you need to do extra work. In this article I’ll show you how to get both those effects and more in a customizable, flexible way.

Understanding assertions



FREE: Understanding assertions

Assertions allow us to have Swift silently check the state of our program at runtime, but if you want to get them right you need to understand some intricacies. In this article I’ll walk you through the five ways we can make assertions in Swift, and provide clear advice on which to use and when.

Functional programming in Swift: Introduction



FREE: Functional programming in Swift: Introduction

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Interview questions: Introduction



FREE: Interview questions: Introduction

Getting ready for a job interview is tough work, so I’ve prepared a whole bunch of common questions and answers to help give you a jump start. But before you get into them, let me explain the plan in more detail…

Using memoization to speed up slow functions



FREE: Using memoization to speed up slow functions

In this article you’ll learn how memoization can dramatically boost the performance of slow functions, and how easy Swift makes it thanks to its generics and closures.




FREE: Trees

Trees are an extraordinarily simple, extraordinarily useful data type, and in this article we’ll make a complete tree data type using Swift in just a few minutes. But rather than just stop there, we’re going to do something quite beautiful that I hope will blow your mind while teaching you something useful.

Making the most of optionals



FREE: Making the most of optionals

Swift’s optionals are implemented as simple enums, with just a little compiler magic sprinkled around as syntactic sugar. However, they do much more than people realize, and in this article I’m going to demonstrate some of their power features that can really help you write better code – and blow your mind along the way.

Ultimate Portfolio App: Introduction



FREE: Ultimate Portfolio App: Introduction

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Transforming data with map()



FREE: Transforming data with map()

In this article we’re going to look at the map() function, which transforms one thing into another thing. Along the way we’ll also be exploring some core concepts of functional programming, so if you read no other articles in this course at least read this one!

How to use phantom types in Swift



FREE: How to use phantom types in Swift

Phantom types are a powerful way to give the Swift compiler extra information about our code so that it can stop us from making mistakes. In this article I’m going to explain how they work and why you’d want them, as well as providing lots of hands-on examples you can try.

Actors and actor reentrancy



Actors and actor reentrancy

Swift's actors are an incredibly powerful way to schedule work safely, but they have a catch: actors allow reentrant code, which can cause surprising problems. Let's explore how actors work, and how reentrancy can catch you out…

Last but not least: widgets



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For our last topic, we’re going to explore widgets. iOS has had widget-like behavior for some time through its Today extensions, but in iOS 14 they gained a lot more functionality.

Layout and Geometry



Layout and Geometry

This challenge asks you make views fade out, scale down, and change their color, all synchronized with the movement in our ScrollView. Let’s tackle it now…

Can you give some examples of where singletons might be a good idea?



Can you give some examples of where singletons might be a good idea?

This sounds like a trick question because so many people rail against singletons, but the real challenge is actually providing a good answer – places where singletons actually work.

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