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A Flock of Swifts

We are a group of people excited by the Swift language. We meet each Saturday morning to share and discuss Swift-related topics.

All people and all skill levels are welcome to join.




Swift Ecosystem

Apple is now sponsoring the Swift Package Index.

Github Changed Host Key

Using the Camera

Jake working through some issues with purple errors and UIRepresentable.

What's Coming in Swift

A pitch about type-safe predicates:

Elide some:

Parameter Packs

TikTok Logo Effect: Metal Shaders in CoreImage

The finished project:



Swift Packages with C

Carlyn gave us a demo on wrapping a C package with Swift Package Manager:


Debugging Updates

TikTok Logo Effect: Metal Shaders in CoreImage

Josh began creating a sample App that lets you pick an image from your library and apply a color separating effect (think TikTok logo) on the image. This week he showed the main points:

  • Create a metal file
  • Add special flag -fcikernel to Other Metal Compiler Flags and Other Metal Linker Flags
  • Make the function extern "C" so metal can see it.
#include <metal_stdlib>
#include <CoreImage/CoreImage.h>
using namespace metal;

extern "C" {
    auto channelOffset(coreimage::sampler s, float2 redOffset, float2 greenOffset, float2 blueOffset) -> float4 {
        auto index = s.coord();
        return float4(
            s.sample(index - redOffset).r,
            s.sample(index - greenOffset).g,
            s.sample(index - blueOffset).b,

Using the - to subract the offsets is arbitrary. (It could have been + too.)

Learning Shader Graphics:


What's new in Swift

Swift 5.8 has been released to beta:

Also Swift 5.9 is on the way

New actor proposal

Frank's code for his talk about distributed actors:

Question from Allen: how to we sync state between SwiftUI and Scenekit

  • Answer: have a single source of truth:
    final class ViewModel: ObsevableObject {
        @Published var truth
  • share with SwiftUI with binding via the projected value of the StateObject:
  • share with SceneKit with a publisher projected fromt he @Published property

Demo: ImageIO

Josh went over the ImageIO library and using it to read files, and image meta data. The full project is below:



Escaping Closures

@escaping means they can be stored in a property and used as a callback later.

Also recommended by Carlyn:

Generative AI for Game Assets


Josh created an Apple cross-platform app to show how to get Metal going in SwiftUI and created a MetalView. (Watch does not support metal yet.)

Roughly based on:

Also see:

You can see some amazing WebGL based demos at:





Josh put together a new type of stack layout that lets each view communicate what weight it wants to the layout system. He wrote the logic for placing views in a functional style and utilized lazy sequences to avoid creation of temporary copies.


import PlaygroundSupport
import SwiftUI

struct V: View {
    var body: some View {
        WeightedStackLayout(axis: .vertical, spacing: 0) {
            Color.brown.stack(weight: 4)
            Color.cyan.stack(weight: 3)
            .frame(width: 400, height: 400)


struct WeightedStackLayoutKey: LayoutValueKey {
    static var defaultValue = 1.0

extension View {
    func stack(weight: Double) -> some View {
        layoutValue(key: WeightedStackLayoutKey.self, value: weight)

extension Layout.Subviews.Element {
    var stackWeight: Double {
        max(self[WeightedStackLayoutKey.self], 1e-6)

struct WeightedStackLayout: Layout {
    var axis: Axis = .vertical
    var spacing: CGFloat = 10

    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        proposal.replacingUnspecifiedDimensions(by: .zero)

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        let totalSpace = CGFloat(subviews.count - 1) * spacing
        let dimension = (axis == .vertical ? bounds.height : bounds.width) - totalSpace
        let pointsPerWeight = dimension /\.stackWeight).reduce(0, +)
        let weights =\.stackWeight).accumulated(0, +)
        let points = zip(
        ).enumerated() { offset, element in
            let (startWeight, endWeight) = element
            let spaceBefore = spacing * CGFloat(offset)
            let offset = axis == .vertical ? bounds.origin.y : bounds.origin.x
            return (
                start: startWeight * pointsPerWeight + spaceBefore + offset,
                end: endWeight * pointsPerWeight + spaceBefore + offset
        zip(subviews, points).forEach { view, point in
            let distance = point.end - point.start
            switch axis {
            case .vertical:
                    at: .init(x: bounds.midX, y: point.start + distance / 2),
                    anchor: .center,
                    proposal: .init(width: bounds.width, height: distance)
            case .horizontal:
                    at: .init(x: point.start + distance / 2, y: bounds.midY),
                    anchor: .center,
                    proposal: .init(width: distance, height: bounds.height)


struct LazyAccumulatedSequence<Accumulated>: LazySequenceProtocol {
    private let _makeIterator: () -> Iterator
        underlying: some Sequence<Element>,
        initial: Accumulated,
        accumulate: @escaping (Accumulated, Element) -> Accumulated
    ) {
        _makeIterator = {
            var iterator = underlying.makeIterator()
            var nextAccumulated: Accumulated? = initial
            return Iterator {
       { accumulated in
                    nextAccumulated = { next in
                        accumulate(accumulated, next)
                    return accumulated
    func makeIterator() -> Iterator { _makeIterator() }
    struct Iterator: IteratorProtocol {
        let _next: () -> Accumulated?
        mutating func next() -> Accumulated? { _next() }

extension LazySequenceProtocol {
    func accumulated<Accumulated>(
        _ initial: Accumulated,
        _ accumulate: @escaping (Accumulated, Element) -> Accumulated
    ) -> LazyAccumulatedSequence<Accumulated> {
        LazyAccumulatedSequence(underlying: self, initial: initial, accumulate: accumulate)

extension Sequence {
    func accumulated<Accumulated>(
        _ initial: Accumulated,
        _ accumulate: (Accumulated, Element) throws -> Accumulated
    ) rethrows -> [Accumulated] {
        var current = initial
        var accumulated = [Accumulated]()
        accumulated.reserveCapacity(underestimatedCount + 1)
        for element in self {
            current = try accumulate(current, element)
        return accumulated

print([1,2,3].accumulated(0, +))

Deep linking in Apps

Josh had an example:

Learning AI

General Neural Networks

Running on you own computer

Facebook's Large Language Model

ChatGPT Client for iOS


Upcoming Conferences

Edit Kit Pro

From iOSDev weekly, Ed told us about


Carlyn is doing some work on the


You can do a lot of work to cleanup models by declaring custom initializers. (You don't have to expose ViewModels but can construct those internally.) You can also use custom Codable to cleanup your code.


Ray revisited the Identifier<Tag> type that we looked at in November. The folks over at point free have a library that makes it easy to make unique little types.

import Tagged

struct User {
    var id: Tagged<Self, UUID>

The point free guys have a whole series on modern SwiftUI development that you can check out on their blog.

Swift Evolution Proposals


Swift Strings are unique in the way that they handle Unicode. Several examples from Josh about how mutation is handled with both String and bridged NSString and NSMutableString. Swift makes a defensive copy for you when assigning.

var a: String = "a"
var b: UnsafeMutablePointer<String> = .init(&a)
a = "b"

var c: NSMutableString = "c"
var d: NSString = c
c = "d"

struct Reference<Value> {
    final class Wrapper<Value> {
        var value: Value
        init(value: Value) {
            self.value = value
    private var wrapper: Wrapper<Value>
    var wrappedValue: Value {
        get { wrapper.value }
        set { wrapper.value = newValue }
    init(wrappedValue: Value) {
        wrapper = .init(value: wrappedValue)

var e = Reference(wrappedValue: "e")
var f = e
e.wrappedValue = "f"
print(e.wrappedValue, f.wrappedValue)

["âa", "áb", "àc", "ad", "äe"] { string in
}.forEach { print($0) }

["âa", "z", "áb", "àc", "ad", "äe"].sorted { lhs, rhs in
}.forEach { print($0) }

["âa", "áb", "àc", "ad", "äe"] { string in
}.forEach { print($0) }

["âa", "z", "áb", "àc", "ad", "äe"].sorted { lhs, rhs in
    lhs.localizedCaseInsensitiveCompare(rhs) == .orderedAscending
}.forEach { print($0) }


Virtual Buddy

A useful tool for creating clean installs. Frank will soon be making a pull request for linux VMs.

Smooth Data

Common methods include moving averages, exponential smoothing, kernel smoothing, and splines.

We discussed a moving average which is probably the simplest. You can do this with a queue.

Passing Data

Although not discussed on video, this was a side discussion in the chat.

Facebook Engineering

Discussion of the technology Facebook uses (C++, Obj-C++)

If you want to do hardcore Swift, Facebook might not be your first choice.

Cross Platform SwiftUI

Although they might only have a small subset of things done.

Swift on the Command Line (Scripting)

Carlyn talked about work she is doing in VS Code and Swift

Link mentioned in chat:

A side topic to this was Swift on the Server:

Frank mentioned that the biggest gotcha is differences between implementations on Linux and macOS. Several (Josh and Franklin) noting that this will eventually be solved when Foundation becomes pure Swift:

Wordle Animation!

Josh shows how animation can be brought into the wordle app with a little restructuring. Shows the difference between implicit and explicit animation.


Parsing RSS Feeds

RSS is simple XML, so the the old XML Parser API should work.

Frank did some work on this previously. This project might help.

Josh implemented a partial SVG reader where he didn't implement all of the callbacks but got the information that he was interested in. See:

Using Custom Fonts

Ed wants his custom fonts to play nicely with the OS (and respect dynamic type).

Franklin mentioned:

@ScaledMetric(relativeTo: .largeTitle) var dynamicHeader1Size: CGFloat = 24

iOS Podcast GPT

Writing Apps with ChatGPT

Emil reports that ChatGPT is helping him to get going with SwiftUI. He has reproduced a major portion of his previous app in SwiftUI in a few days where the original took him months (as he was learning).

Noting that some of the code is old (because ChatGPT was trained before 2021).

The licensing of the code is ambiguous.

Space, the final Frontier

Carlyn mentioned that many of her space friends are excited about this new release:

The Nature of Code

Carlyn has been working through these (knows the author) in the last couple of months:

Wordle Clone!

Josh implemented all of the game logic for his wordle clone. He used a reducer that took inputs through a tap gesture recognizer and reduced the state, modifying two properties that drive the UI updates.


AsyncImage Fix

Jake had an update to how he was able to fix the animation problem with AsyncImage. Here is the code:

AsyncImage(url: user.profileURL,
           transaction: Transaction(animation: .default)) { phase in
  switch phase {
    case .success(let image):
                  .aspectRatio(contentMode: .fit)
            ZStack {
                      .frame(width: 90, height: 90)
              Image(systemName: "person")
}.frame(width: 90, height: 90)

Non-uniform shuffling

Carlyn asked about the most Swifty way to enable non-uniform probability picking.

Some suggestions from the group:

  • Look around in GameKit? (Perhaps look at this:
  • Create an array with duplicates of the number of elements in the probablilty you want.
  • Create a special collection that vends the duplicates without actually hosting them in memory.

A related topic:

Speeding up Conformance Checking

We reviewed this blog post about how you can re-order conformance records to get a 20% performance boost.

Wurdle (Wordle Clone)

Josh continued his epic presentation on a Wordle clone. Attempted to make it work pretty. Namely, GeometryReader is not greedy and anchors things to the upper-left. You can work around it by adding a ZStack that contains a greedy view like Color.clear or Rectangle().hidden() to the ZStack.

We used the Layout protocol which is a type of view that can explicitly control the layout of a view and its subviews. Josh created a AnchorInParentLayout that lets you align an arbitrary position (UnitPoint) with an arbitrary position of the subview(s). A view modifier makes it easy to use.


Layout with AsyncImage

Jake was seeing a problem with AsyncImage where the transition animation would be cancelled when the async image loaded. The image would appear at the destination without loading. Josh theorized that the problem was happening because its identity was changing. However, we could not seem to fix the problem by explicitly setting id on the views.

One recommendation is to use a much more capable third party library like Nuke.


Using Core Data to sort by date. Ed says, "CalendarComponents is your friend." Trevor recommended this resource:

Inspiration for UI

Jacey noted that SnapKit is also good (easy) way to layout views.


Sendable conformance will be one of the important areas to be aware of as come into Swift 6. You can enable strict concurrency warnings in your build settings. The default is minimal but you can use "targetted" to check your own code.

When you enabled this checking, you will see warnings where an instance is passed across a concurrency domain and is not Sendable. For example:

func findInBackground(quadTree: QuadTree,
                      region: CGRect) async -> Task<[CGPoint], Never> {
  Task.detached {
    quadTree.find(in: region) // WARNING: QuadTree is not Sendable

You can mark QuadTree as Sendable to fix this warning. This will, in turn, lead to a warning that Node, the reference type, is not sendable. If you mark Node sendable you get more warnings. This is because the class contains multiple immutable stored properties that could get modified from another concurrency domain. In this case you can mark Node with @unchecked Sendable since you know that all mutation is protected by isKnownUniquelyReferenced and makes a deep copy if it is not unique. (Aka COW.)


The collection-benchmark project allows you to create benchmarks where the time might be dependent on the size of the input. We created a command-line target and included the benchmark package.

We wrote the following benchmarks:

import CollectionsBenchmark
import CoreGraphics.CGBase

struct TestPoints {
  let region: CGRect
  let points: [CGPoint]
  init(size: Int) {
    region = CGRect(origin: .zero, size: CGSize(width: size, height: size))
    points = zip((0..<size).shuffled(), (0..<size).shuffled())
      .map { CGPoint(x: $0.0, y: $0.1) }

var benchmark = Benchmark(title: "QuadTree Benchmarks")

benchmark.registerInputGenerator(for: TestPoints.self) { size in
  TestPoints(size: size)

benchmark.add(title: "QuadTree find",
              input: TestPoints.self) { testPoints in
  let tree = QuadTree(region: testPoints.region, points: testPoints.points)
  return { timer in
    testPoints.points.forEach { point in
          let searchRegion = CGRect(origin: point, size: .zero).insetBy(dx: -1, dy: -1)
          blackHole(tree.find(in: searchRegion))

benchmark.addSimple(title: "Array<CGPoint> filter",
                    input: TestPoints.self) { testPoints in
  testPoints.points.forEach { point in
    let searchRegion =  CGRect(origin: point, size: .zero).insetBy(dx: -1, dy: -1)
    blackHole(testPoints.points.filter { candidate in


Then we ran the following commands arguments:

run QuadFindResult.json --cycles 1
render QuadFindResults.json QuadFindResults.png

This produced the following results:

Benchmark Results for Find

This is a log-log chart and you can see that the growth of the array implementation is linear.

You can see the jump at 4 items which is where the QuadTree logic is kicking in. I found that on my machine, I can boost this constant to 512 to make it always perform better than array.

You can also use a special group file to automatically produce sets of benchmarks and multiple graphs.


Josh continued working on the Wordle game example getting through a lot of the layout issues of the words and keyboard (adding return and backspace).

import SwiftUI

extension Color {
    static let darkGray = Color(#colorLiteral(red: 0.4784313725, green: 0.4823529412, blue: 0.4980392157, alpha: 1))
    static let lightGray = Color(#colorLiteral(red: 0.8274509804, green: 0.8431372549, blue: 0.8549019608, alpha: 1))
    static let darkGreen = Color(#colorLiteral(red: 0.4117647059, green: 0.6705882353, blue: 0.3803921569, alpha: 1))
    static let darkYellow = Color(#colorLiteral(red: 0.7960784314, green: 0.7098039216, blue: 0.3137254902, alpha: 1))

struct Row: Hashable, Identifiable {
    var id: Int
    var letters: [Letter]

struct Letter: Hashable, Identifiable {
    var id: Int
    var character: Character
    var status: Status
    enum Status: Int, Comparable {
        static func < (lhs: Letter.Status, rhs: Letter.Status) -> Bool { lhs.rawValue < rhs.rawValue }
        case unguessed, wrong, wrongPosition, correct

final class GameViewModel: ObservableObject {
    @Published private(set) var words: [Row] = []
    @Published private(set) var keys: [Row] = []
    init() {
        words = ["SWIFT", "CODER", "PLAYA", "     ", "     ", "     "]
            .map { word in
                    id: word.offset,
                    letters: word.element.enumerated().map { character in
                            id: word.offset * 10 + character.offset,
                            character: character.element,
                            status: {
                                switch character.element {
                                case " ": return Letter.Status.unguessed
                                case "O", "A": return .correct
                                case "L": return .wrongPosition
                                default: return .wrong
            .map { rows in
                let keys = rows
                    .reduce(into: [Character: Letter.Status]()) { accumulated, next in
                        accumulated[next.character] = accumulated[next.character].map { max($0, next.status) } ?? next.status
                return ["QWERTYUIOP", "ASDFGHJKL", "ZXCVBNM"]
                    .map { string in
                            id: string.offset,
                            letters: string.element.enumerated().map { character in
                                    id: string.offset * 100 + character.offset,
                                    character: character.element,
                                    status: keys[character.element] ?? .unguessed
            .assign(to: &$keys)

struct ContentView: View {
    @StateObject var viewModel = GameViewModel()
    var body: some View {
        GeometryReader { reader in
            VStack(spacing: 36) {
                Grid(alignment: .topLeading, horizontalSpacing: 12, verticalSpacing: 12) {
                    ForEach(viewModel.words) { row in
                        GridRow {
                            ForEach(row.letters) { letter in
                                LetterView(style: .word(letter))
                .frame(maxWidth: 600)
                VStack (spacing: 12) {
                    ForEach(viewModel.keys) { row in
                        HStack(spacing: 8) {
                            if viewModel.keys.last == row {
                                LetterView(style: .keyImage("return"))

                                keys(for: row)
                                LetterView(style: .keyImage("delete.backward"))

                            } else {
                                keys(for: row)

    private func keys(for row: Row) -> some View {
        ForEach(row.letters) { letter in
            LetterView(style: .key(letter))

struct LetterView: View {
    var style: Style
    enum Style {
        case key(Letter), word(Letter), keyImage(String)
    private let fontSize: CGFloat
    private let aspectRatio: CGFloat
    private let textColor: Color
    private let backgroundColor: Color
    private let outlineColor: Color

    init(style: Style) { = style
        switch style {
        case .keyImage:
            aspectRatio = 0.66 * 1.5
            fontSize = 48
            textColor = .black
            backgroundColor = .lightGray
            outlineColor = .clear
        case let .key(letter):
            aspectRatio = 0.66
            fontSize = 48
            (textColor, backgroundColor, outlineColor) = colors(for: letter, isKey: true)
        case let .word(letter):
            aspectRatio = 1
            fontSize = 100
            (textColor, backgroundColor, outlineColor) = colors(for: letter, isKey: false)
        func colors(for letter: Letter, isKey: Bool) -> (Color, Color, Color) {
            switch (letter.status, isKey) {
            case (.unguessed, true): return (.black, .lightGray, .clear)
            case (.unguessed, false): return (.black, .white, .black)
            case (.correct, _): return (.white, .darkGreen, .clear)
            case (.wrongPosition, _): return (.white, .darkYellow, .clear)
            case (.wrong, _): return (.white, .darkGray, .clear)

    var body: some View {
        GeometryReader { proxy in
            RoundedRectangle(cornerRadius: 8)
                .strokeBorder(outlineColor, lineWidth: 4)
                .background(RoundedRectangle(cornerRadius: 8).fill(backgroundColor))
                .overlay {
                    switch style {
                    case let .keyImage(name):
                        Image(systemName: name)
                    case let .key(letter), let .word(letter):
                        Text(String(describing: letter.character))
                .font(.system(size: fontSize, weight: .bold))
        .aspectRatio(aspectRatio, contentMode: .fit)


PointFree Dependency Injection

A new library for dependency injection was announced this week by the folks at Josh gave us a quick tour of the library and an additions library:

Peter posted this example of using the additions library:

Ed Launches Testflight

Ed launched a private testfligt build for his new app. During the coarse of the meeting was able to find and fix an out of bounds crasher when there is no data. The power of testing in action.

Learning Swift

Some of the tried and true:

Async Result

You can create an async init for result types to get clean monadic chaining instead of nested do {} catch {} blocks. Daniel and Josh showed us how!


Core Data is a deep subject. A good place to start:

Noise Generation

GameKit although old and written in ObjectiveC, has API for creating "natural" noise often used for procedural terrain generation in games.


As part of another epic, multipart demo, Josh is implementing a version of the popular game in SwiftUI. Today he created the basic model for Rows and Letters and Keys as well as the Status for each.


Using contraMap Example

You can write a contramap function on CurrentValueSubject<Int> to make functions that can send other types into the current value.

import Foundation
import Combine

extension CurrentValueSubject {
    func contraMap<Value>(transform: @escaping (Value) -> Output) -> (Value) -> Void {
      { [weak self] input in

let subject = CurrentValueSubject<Int, Never>(0)
let sendBool = subject.contraMap { (bool: Bool) in bool ? 1 : 0 }

Learning SwiftUI

Core Data Debugging

Suggestions for Dan's app that has a crashing problem:

  • Log non-fatal errors
  • Think about all of the validations done by the Core Data model
  • It reliably crashes on start
  • Take note of the OS / devices it is happening on in the crash logs
  • Debugging concurrency issues with 1

Returning Swift Conferences

Swift Charts Performance

Ed wrote his own charts with GeometryReader instead of SwiftUI Charts because of performance problems on rotation and scrolling.

Photo Picker iOS16

New Swift Proposals

  • SE-0383 Deprecate @UIApplicationMain and @NSApplicationMain
  • SE-0384 Importing Forward Declared Objective-C Interfaces and Protocols
  • SE-0382 Expression Macros


The new multicore APFS aware Apple Archive framework:

Swift Collections

Josh took us on a guided tour of the Swift Collections package.

Exploration of CHAMP:

Aside: A Benchmarking Tool

JSON with decode indirect enum

You can represent JSON with swift with this:

enum JSON {
	indirect case array([JSON])
	indirect case dictionary([String: JSON])
	case boolean(Bool)
	case number(Double)
	case string(String)
	case null

You can implement Decodable using a single value container. Naively it is a bunch of nested do {} catch {} blocks but it can be done quite succinctly by using a Result type and flatMapError to implement successive retries.

enum JSON {
    indirect case array([JSON])
    indirect case dictionary([String: JSON])
    case boolean(Bool)
    case number(Double)
    case string(String)
    case null

extension JSON: Decodable {
    init(from decoder: Decoder) throws {
        self = try Result { try decoder.singleValueContainer() }
            .flatMap { container in
                    ? .success(JSON.null)
                    : Result { JSON.boolean(try container.decode(Bool.self)) }
                        .flatMapError { _ in Result { JSON.number(try container.decode(Double.self)) } }
                        .flatMapError { _ in Result { JSON.string(try container.decode(String.self)) } }
                        .flatMapError { _ in Result { JSON.array(try container.decode([JSON].self)) } }
                        .flatMapError { _ in Result { JSON.dictionary(try container.decode([String: JSON].self)) } }


Meeting minutes and learnings from the physical space meeting.






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