The Blog

Understanding how DispatchQueue.sync can cause deadlocks

As a developer, you’ll come across the term "deadlock" sooner or later. When you do, it’s usually pretty clear that a deadlock is bad (the name alone implies this) and if you’ve experienced one in your code, you’ll know that your application crashes with EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) when a deadlock occurs. A few weeks ago, I wrote about dispatching code synchronously and asyncronously. Several people pointed out that that post does not mention deadlocks. Instead of making that post longer Read more…

Configuring error types when using flatMap in Combine

When you’re using Combine for an app that has iOS 13 as its minimum deployment target, you have likely run into problems when you tried to apply a flatMap to certain publishers in Xcode 12. To be specific, you have probably seen the following error message: flatMap(maxPublishers:_:) is only available in iOS 14.0 or newer. When I first encountered this error I was somewhat stumped. Surely we had flatMap in iOS 13 too! If you’ve encountered this error and thought Read more…

3 tips to work through a technical coding interview

If you’re a programmer looking for a job it’s very likely that you’ll have to do a coding interview at some point. Every company conducts these interviews differently. Some may have you work through several heavy computer science problems, others may present you with a task that’s related to the job you’re interviewing for and others might do both. No matter what the exact form is, you’ll want to nail these interviews as they are a big part of whether Read more…

Dispatching async or sync? The differences explained

When writing iOS apps, we regularly run into code that is asynchronous. Sometimes you know you’re writing something that will run asynchronously and other times you’re passing a completion handler to code that may or may not run asynchronously on a different dispatch queue. If you’re familiar with using DispatchQueue.main, you have probably written code like this: DispatchQueue.main.async { // do something } And while writing this, you may have encountered a second method on DispatchQueue.main called sync. In this Read more…

Implementing a one-way sync strategy with Core Data, URLSession and Combine

A common use of a Core Data store is to cache data from a remote resource locally to support offline functionality of an app. There are multiple ways to implement a caching mechanism and many of them don’t involve Core Data. For example, you could simply fetch a JSON file from a server and store the contents of that JSON file on disk. A downside of fetching a full data set every time is that you risk using a lot Read more…

Understanding Swift’s OptionSet

Every once in a while I look at a feature in Swift and I fall down a rabbit hole to explore it so I can eventually write about it. The OptionSet protocol is one of these Swift features. If you’ve ever written an animation and passed it a list of options like this, you have already used OptionSet: UIView.animate( withDuration: 0.6, delay: 0, options: [.allowUserInteraction, .curveEaseIn], animations: { myView.layer.opacity = 0 }, completion: { _ in }) You may not Read more…

Fetching objects from Core Data in a SwiftUI project

When you’ve added Core Data to your SwiftUI project and you have some data stored in your database, the next hurdle is to somehow fetch that data from your Core Data store and present it to the user. In this week’s post, I will present two different ways that you can use to retrieve data from Core Data and present it in your SwiftUI application. By the end of this post you will be able to: Fetch data using the Read more…

Using Codable with Core Data and NSManagedObject

If you’ve ever wanted to decode a bunch of JSON data into NSManagedObject instances you’ve probably noticed that this isn’t a straightforward exercise. With plain structs, you can conform your struct to Codable and you convert the struct from and to JSON data automatically. For an NSManagedObject subclass it’s not that easy. If your Core Data data model is configured to automatically generate your entity class definitions for you (which is the default), you may have tried to write the Read more…

Setting up a Core Data store for unit tests

Unit testing is an important skill in the toolbox of any engineer. Knowing how to write a reliable, robust test suite helps you write reliable and robust code. If you’ve followed my introduction to unit testing part one and part two, or if you’re experienced with unit testing you know that your tests should run isolated from any other tests. You also know that you should make sure that your test relies on as few external dependencies as possible. When Read more…

Using Core Data with SwiftUI 2.0 and Xcode 12

This article covers beta technology and it’s up to date for Xcode 12 beta 5 In Xcode 12 you can create projects that no longer use an AppDelegate and SceneDelegate to manage the application lifecycle. Instead, we can use Swift’s new @main annotation to turn a struct that conforms to the App protocol into the main entry point for our applications. When you create a new project in Xcode 12, you have the option to use the SwiftUI App application Read more…