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Below are the blog posts I have published since 2015. You can also subscribe to new posts with your favourite news reader.

Smuggling Values Across Actors


I recently started updating an app to use the Swift 6 language mode and ran in to an issue using AVCaptureMetadataOutputObjectsDelegate. The issue is that the compiler cannot reason about which actor the delegate function is called on and it must be treated as nonisolated, however we know that it's being called on a specific actor (in this case the MainActor), so how can we tell the compiler?

In Swift 5 language mode we can use MainActor.assumeIsolated and call it a day. But in Swift 6 this will produce an error:

import AVFoundation
import UIKit

@MainActor
final class ViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
    private var output: AVCaptureMetadataOutput?

    override func viewDidLoad() {
        super.viewDidLoad()

        let output = AVCaptureMetadataOutput()
        output.setMetadataObjectsDelegate(self, queue: .main)
        self.output = output
    }

    private func doSomethingWithMetadataObjects(_ metadataObjects: [AVMetadataObject]) {}

    // Must be marked nonisolated because the AVCaptureMetadataOutputObjectsDelegate protocol cannot declare the actor on which the function will be called.
    nonisolated func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
        // We know it's on the main actor so we should be able to assume
        // isolated and pass metadata objects to another function.
        MainActor.assumeIsolated {
            // This does not compile though, presumably because AVMetadataObject
            // is not Sendable.
            doSomethingWithMetadataObjects(metadataObjects) // ❌ Error: Sending 'metadataObjects' risks causing data races
        }
    }
}

This reminded me of a macro I created recently, or more specifically the type used to workaround a similar issue with stored properties in Swift 5: UnsafeSendable. This type works by using @unchecked Sendable to send store values we know are sendable but have no native way of telling the compiler.

In this case, however, it can also be used to smuggle values across an actor boundary that we know is safe.

public struct Smuggler<Smuggled>: @unchecked Sendable {
    public var smuggled: Smuggled

    public init(unsafeSmuggled smuggled: Smuggled) {
        self.smuggled = smuggled
    }

    @available(*, deprecated, message: "Smuggler is not needed when `Smuggled` is Sendable")
    public init(unsafeSmuggled smuggled: Smuggled) where Smuggled: Sendable {
        self.smuggled = smuggled
    }
}

With this we can smuggle our value through:

    nonisolated func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) {
        // We know it's on the main actor so we should be able to assume
        // isolated and pass metadata objects to another function.
        let smuggler = Smuggler(unsafeSmuggled: metadataObjects)
        MainActor.assumeIsolated {
            let metadataObjects = smuggler.smuggled
            doSomethingWithMetadataObjects(metadataObjects) // ✅ Works
        }
    }

We can add an extension to MainActor that makes this a little more convenient.

extension MainActor {
    public static func assumeIsolated<Smuggled>(
        smuggling smuggled: Smuggled,
        operation: @MainActor (_ smuggled: Smuggled) -> Void,
        file: StaticString = #fileID,
        line: UInt = #line
    ) {
        let smuggler = Smuggler(unsafeSmuggled: smuggled)
        withoutActuallyEscaping(operation) { escapingOperation in
            let smuggledOperation = Smuggler(unsafeSmuggled: escapingOperation)
            assumeIsolated({
                smuggledOperation.smuggled(smuggler.smuggled)
            }, file: file, line: line)
        }
    }
}

I have been playing around with trying to add a function to GlobalActor that would make the API a bit nicer but I think it's going to require a macro, lest we resort to reimplementing MainActor.assumeIsolated 😊

Although this isn't really "smuggling" because the value is not actually crossing an actor boundary I like the term and I'm sticking to it! Especially because I assume this workaround will be temporary.

P.S. I created a demo project of the issue, which I also submitted to Apple as feedback FB13950073 ✌️

The Bug That Bit Me Twice


I've been working on a fix for a bug in Overamped, which causes the popover UI shown when tapping on an image in Google Images to be blank, if the link goes to an AMP page. This was a silly bug that never should've happened; knowing that Google can change their page structure at any time I should've been more cautious with my checks.

As a temporary quick fix I removed all custom handling of Google results, tested my changes in the simulator, and uploaded a new build to TestFlight.

After installing the TestFlight update on my phone I checked a search result that I knew recreated the problem, but it was still happening! I have other extensions installed so I disabled some, refreshed, and the bug was fixed!

I thought it would be very strange for the same – very specific – bug to appear in multiple extensions, so I did a little digging.

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Partial in Swift


Partial is now available in its own Swift package on GitHub. This post is still valid, but somewhat out of date.

Structs are incredibly useful in Swift, especially when representing static read-only data. However, the values of a struct often come from multiple sources, such as view controllers, network requests, and files on disk, which can make the creation of these structs cumbersome.

There are numerous methods to work around this, but each have their downsides. One of these methods is to change the struct to a class and update the properties to vars, but this removes the advantages of read-only structs. Another is to make a "builder" object, but the API of this object must be kept in-sync with the object is wraps.

Partial eliminates these problems by providing a type-safe API for building structs by utilising generics and KeyPaths. Although I learned of the concept of Partial through TypeScript – which [provides Partial as a built-in type][1] – the Swift implementation supports many more use cases.

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Don't Use Scope Modifiers with Extensions


Extending types in Swift support setting the scope for the extension, i.e. public, internal, or private, with internal being implicit if nothing is specified.

This may seem useful, but given the following snippet it's impossible to know what the scope of a function is:

func doSomething() {
    // Do the thing
}
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Overamped 1.1.0 and the Year of Small


So far my Year of Small is going really well; as I write this I'm making some final changes and preparing to push out v1.1.0-RC.1, which I hope to submit the App Store in the next couple of days.

Overamped 1.1.0 is an update I first started working on almost 3 months ago, and it should've been released earlier.

My initial plan for 1.1.0 was to:

  • Add a screen showing recently logged events
  • Add an option to send a notification when the Web Extension redirects a link
  • Simplify the permissions model
  • Add widgets

Easy, right? Well, adding widgets is why this update has been so delayed.

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pastelghouls Available Again


pastelghouls is once again available for download on the App Store. It's a free sticker pack containing 6 ghoulish sticker. Originally released 19th October 2016, just in time for halloween, it was removed from the App Store December 7th 2019 due to not having an update for a substantial period of time.

As part of my year of small I wanted to make this available again, and at the same time setup fastlane to automate the screenshots and store the app metadata to make future updates easier.

pastelghouls was created by my friend Joshua Robins and published by my company Yetii Ltd.

The Year of Small


I've become accustomed to using yearly themes – rather than New Years resolution – thanks to CGP Grey, who has a very good summary of yearly themes.

It took me a while to home in on it but this year the title of my theme will be "Year of Small."

I'll be focusing on all things small:

  • Small people: spending time with and giving the best life to my now 9-month-old son.
  • Small home improvements: finally doing those small tasks and improvements, such as buying a bedside table and getting rid of things I no longer use.
  • Small work: I want to focus on releasing smaller updates for existing project, alongside a higher quantity of smaller projects that serve as proof of concepts and methods of learning new technology.
  • Small activities: small hobbies I can with my wife, such as new board games and outside activities.

At the end of the year I'll post a "year in review," although I hope to revisit this every 3 months to make sure I'm still focusing on the right things.

Anything related to this theme will be posted under the year-of-small tag.

Swift Package Collection Signing Using the Terminal


Swift Packages are JSON files that describe a collection of packages. This post will explain how to sign these packages with a trusted certificate entirely from the terminal. These methods should work on Linux and macOS alike. At the end I describe how to have Swift on Linux implicitly trust these packages.

Using this technique I have published my own package collection.

If you're targeting macOS only and find GUIs more intuitive I recommend following the “Swift Package Collection” blog post from Shaps, which is the post that finally made this “click” for me.

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Hosting DocC Archives


At WWDC21 Apple introduced DocC, a tool for creating archives of Swift documentation that includes the static files required to host a version of the documentation on a website.

In this post I will summarise various methods of serving a DocC archive:

  • Netlify
  • Vapor middleware
  • nginx
  • Apache

All the examples provided here are hosting the DocC archive for VaporDocC, the Vapor middleware I wrote for hosting DocC archives.

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Handling iCal Files in iOS


The iCal format, first defined as a standard as RFC 2445 in 1998, is the universally accepted format for distributing calendar files, mainly used for distributing events.

As part of my QR code scanning app Scanula I added support for detecting events in scanned objects. Thanks to the fantastic libical and the Swift wrapper swift-ical it's fairly easy to parse an iCal feed, but adding it to iOS is a bit trickier.

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The iPod touch Is My Favourite Device for iOS Development


It's important to test across various screen sizes, which the iOS simulator is good for, but it's also important to test on real devices where possible. I currently have an iPhone 11 Pro, an iPhone 6, and 2 iPod touches. Out of all these I find the iPod touch to be the best device for a lot of iOS development.

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My WWDC 2021 Wishlist


With WWDC 2021 just around the corner I've been thinking about what I'd like to see there.

A lot of the popular discourse around this time of year is focussed on features of the operating systems but I want to look at what I'd like to see as a developer for Apple platforms.

I love to develop for Apple platforms but it can often be a painful process. May is like a christmas for Apple developers.

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Mapping Optional Binding to Bool


When displaying an alert in SwiftUI, if the value used to calculate whether the alert is presented is both Optional and does not conform to Identifiable1 it is often recommended to use a separate flag, similar to:

struct ContentView: View {
    @State private var alertText: String?
    @State private var isPresentingAlert = false

    var body: some View {
        Button("Show Alert") {
            self.alertText = "Alert Text"
            self.isPresentingAlert = true
        }
        .alert(isPresented: $isPresentingAlert) {
            Alert(title: Text(alertText!))
        }
    }
}

There are 2 main downsides to this:

  1. alertText is not set back to nil, which may cause bugs and will increase memory usage (even if only a little in this case)
  2. The isPresentingAlert flag needs to be managed

To work around these issues I create a small extension to Binding the allows this same code to be updated to:

struct ContentView: View {
    @State private var alertText: String?

    var body: some View {
        Button("Show Alert") {
            self.alertText = "Alert Text"
        }
        .alert(isPresented: $alertText.mappedToBool()) {
            Alert(title: Text(alertText!))
        }
    }
}

The extension is fairly small and simple:

import os.log
import SwiftUI

extension Binding where Value == Bool {
    /// Creates a binding by mapping an optional value to a `Bool` that is
    /// `true` when the value is non-`nil` and `false` when the value is `nil`.
    ///
    /// When the value of the produced binding is set to `false` the value
    /// of `bindingToOptional`'s `wrappedValue` is set to `nil`.
    ///
    /// Setting the value of the produce binding to `true` does nothing and
    /// will log an error.
    ///
    /// - parameter bindingToOptional: A `Binding` to an optional value, used to calculate the `wrappedValue`.
    public init<Wrapped>(mappedTo bindingToOptional: Binding<Wrapped?>) {
        self.init(
            get: { bindingToOptional.wrappedValue != nil },
            set: { newValue in
                if !newValue {
                    bindingToOptional.wrappedValue = nil
                } else {
                    os_log(
                        .error,
                        "Optional binding mapped to optional has been set to `true`, which will have no effect. Current value: %@",
                        String(describing: bindingToOptional.wrappedValue)
                    )
                }
            }
        )
    }
}

extension Binding {
    /// Returns a binding by mapping this binding's value to a `Bool` that is
    /// `true` when the value is non-`nil` and `false` when the value is `nil`.
    ///
    /// When the value of the produced binding is set to `false` this binding's value
    /// is set to `nil`.
    public func mappedToBool<Wrapped>() -> Binding<Bool> where Value == Wrapped? {
        return Binding<Bool>(mappedTo: self)
    }
}

The extension isn't tied directly to showing an alert or a sheet and can be used in any context, but this is one of the better examples of its usage.

This extension is available on GitHub under the MIT license.

1 If it does conform to Identifiable use alert(item:content:)

Supporting Multiple Swift Package Versions Without Breaking Compatibility


The Xcode 12 beta includes Swift 5.3 but drops support for iOS 8.x. This means that Swift packages that support iOS 8 will cause a warning:

The iOS deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 14.0.99.

It's not possible to remove this warning within a project that depends on a Swift package with a deployment target of iOS 8, but it is possible to fix this in the dependency without removing support for iOS 8 for older versions of Swift. There are multiple way this can be accomplished.

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Running UI Tests on Mac Catalyst


While working on the 2.0 update for Gathered I have been trying to develop the app multiple platforms simultaneously. SwiftUI will solve this problem in the future, but I wish to support some OS versions that SwiftUI does not support.

As part of this I have been creating UI tests to test performance, but ran in to an issue when running the UI tests on macOS using Mac Catalyst:

Running tests...
The bundle “PerformanceXCTests” couldn’t be loaded because it is damaged or missing necessary resources. Try reinstalling the bundle.
(dlopen_preflight(...): no suitable image found.  Did find:
	...: code signature in (...) not valid for use in process using Library Validation: mapped file has no Team ID and is not a platform binary (signed with custom identity or adhoc?))
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My Swift Package Manager Release Workflow


I am currently maintaining numerous Swift Packages that don't receive a constant flow of updates, but do receive updates when new Swift updates come out, or as I think of useful additions.

To ensure that I can make some of these less frequent updates without too much friction and with confidence in their correctness I rely heavily on GitHub Actions, which I'll go over in this blog post.

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Capturing More Than `self`


A common pattern when using closures in Swift is to add [weak self] in the captures list to hold a weak reference to self and avoid a retain cycle. This is then often followed by the following:

guard let self = self else { return }

But I often forget that capture lists can capture other variables in the current scope, so I thought I'd highlight some other use cases.

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HashableByKeyPath framework release 1.0.0


Today I have released the 1.0.0 version of a Swift package that aids with adding Equatable and Hashable conformance by using KeyPaths.

The package is available on GitHub.

I created the Swift Playground that sparked this concept in December 2018, so this concept has been rattling around in my brain for a couple of years. The API has changed a lot since the original concept, but the core has stayed the same: a protocol that requires a single function to be implemented that uses KeyPaths to synthesise Equatable and/or Hashable conformance.

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Partial framework release 1.0.0


Today marks 1 year since I released a blog post demonstrating an implementation of Partial in Swift, and it also marks the release of the 1.0.0 version of a Swift package for Partial.

The package is available on GitHub and supports SwiftPM, Carthage, and CocoaPods.

This blog post will go over some of the changes that have been made since the original blog post, my rationale when making certain decisions, and how I have thought about maintenance. If you want to try out Partial and see how it can be used head over to the GitHub page.

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Gathered 1.3 Release Notes


Gathered 1.3 has been released and is now available on the App Store. Version 1.3 brings 2 new data sources, app-wide speed and UX improvements, and support for various features added in recent versions of iOS.

This update also has lots of behind-the-scenes changes that will make future updates easier to create and deploy, which – along with my features roadmap – should mean more frequent updates.

I wasn't very happy removing the Heart Rate data source but Apple weren't very happy with the use of HealthKit.

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iOS Share Sheets the Proper Way - Locations


Sharing a location on iOS is something that not a lot of apps need, but after requiring it for my latest app, Scanula, I found that there isn't a good resource explaining how to do it properly. This is the first post in a series of planned posts going over a few of the tips, tricks, and common pitfalls I have found while working with iOS Share Sheets.

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Exploiting University Security for My Own Convenience


This blog post covers an open-source timetable parsing project I released a couple of months ago. It is available at https://timetable.josephduffy.co.uk and the source is available on GitHub. The post won't go too in-depth on the technical side of the project, but rather the story of how I discovered it was possible.

Since starting my studies at the University of Huddersfield I've always wanted an easy way to see my timetable on my phone. The timetable available on the website isn't responsive and relies on POST data to display future weeks timetables, 2 things that don't work great on mobile, especially when the page is kept open in the background.

To get around this I would manually add each of my lectures and practicals to my calendar. These events could be set as recurring, however they would often need removing on specific days (such as during holidays) or have different information on another date, such as a room change. All of this eventually led me think about the famous XKCD Automation comic, so I started work on a method of automating adding it to my calendar.

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It's a Duffy Thing


I recently released a major overhaul for this website. The old website used an old version Node.js and used Ghost to power the blog. I didn't find it very easy to maintain and wanted more flexibility. While the new website may not have the best design, I'm a lot happier with it overall. Along with the rewrite of the website itself, I also gave it a new name: It's a Duffy Thing. This was inspired by a shirt that my Dad bought me.

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Touch ID on the Lock Screen


Touch ID is a wonderful piece of technology, to the point where wouldn't buy an iOS device without it. It had many great uses, such as:

  • Unlock the device
  • Authorise Apple Pay payments
  • Add biometric restrictions within apps

However, I wish to discuss the first of these: unlocking the device.

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