Reading Fastfiles with document based SwiftUI app on macOS

WWDC’20 brought an addition to SwiftUI apps which enables to create document based applications in SwiftUI. Along with DocumentGroup API two new protocols were added: FileDocument and ReferenceFileDocument. The first one is meant for value types and the latter one for class type document types. For seeing how much effort it takes to create a document based SwiftUI app, let’s create a small macOS app which reads Fastlane’s Fastfiles.

Mac app window showing a list of lanes from a Fastfile
The final applications which displays a list of lanes.

Xcode comes with a pretty nice template for document based application. For this specific case we will go with macOS’ document app template.

A template picker in Xcode showing macOS document app template being selected.
Xcode template picker.

The default template is set up in a way that it can be used for reading and writing text documents. Let’s go and modify the created app. Fastfiles in Fastlane do not have any file extensions and therefore we’ll need to use Uniform Type Identifiers (UTI) type which enables the app to open it. This has a side-effect as well, now the app can open lots of other files as well, so we should probably add a validation step which makes sure we are trying to read a Fastfile. As the app is going to deal with UTI types and the app itself does not define any custom UTI types then Document Types and Imported Type Identifiers in the Info.plist can be removed.

Like mentioned before, SwiftUI brought a new way of creating document based applications. Document types are presented either with value or reference type. FileDocument is a protocol which adds an init method with read support, a write method and supported UTI type declarations for reading and writing. It is a pretty compact protocol when thinking about the interface UIDcoument has. Something to keep in mind is that every implemented method in the document must be thread-safe because reading and writing always happens on background threads. But let’s take a look at the implementation of a document which represents a Fastfile document:

struct FastfileDocument: FileDocument {
let fastfileContents: String
init(contents: String) {
self.fastfileContents = contents
init(configuration: ReadConfiguration) throws {
// TODO: validate the file name
guard let data = configuration.file.regularFileContents else { throw CocoaError(.fileReadCorruptFile) }
guard let string = String(data: data, encoding: .utf8) else { throw CocoaError(.fileReadCorruptFile) }
fastfileContents = string
static var readableContentTypes: [UTType] {
return [UTType("")!]
func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
let data = .utf8)!
return FileWrapper(regularFileWithContents: data)
// MARK: Accessing Lanes
func lanes() -> [Lane] {
return FastfileParser.lanes(in: fastfileContents)
struct Lane: Equatable, Identifiable {
let name: String
let documentation: String
var id: String {
return name

The FastfileParser was covered by the previous blog post if you would like to take a look: Adding prefixMap for expensive operations in Swift. In summary, the document just reads the whole Fastfile into the memory and provides a method for parsing lanes. Note that the protocol requires to have a write method defined as well although, at least for now, we are not going to use it. Having the document created, the next step is building a small UI which shows a list of lanes.

DocumentGroup is a new scene type which manages everything around creating, viewing, and saving documents. Therefore, for viewing a document we’ll need to create a DocumentGroup for viewing and provide a SwiftUI view which can display the document. DocumentGroup takes care of showing the open panel and coordinating the view creation. The example SwiftUI app looks like this:

struct LaneControlApp: App {
var body: some Scene {
DocumentGroup(viewing: FastfileDocument.self) { file in
LaneListView(viewModel: LaneListView.ViewModel(document: file.document))
view raw App.swift hosted with ❤ by GitHub

A list view for showing the list of lanes is pretty straight-forward: LaneListView uses the List view component and displays individual rows with the LaneRowView. The row view shows the name of the lane and the description found in the Fastfile document.

struct LaneListView: View {
@StateObject var viewModel: ViewModel
var body: some View {
List {
ForEach(viewModel.lanes) { lane in
LaneRowView(lane: lane)
.frame(minWidth: 200, minHeight: 200)
final class ViewModel: ObservableObject {
let lanes: [Lane]
init(document: FastfileDocument) {
lanes = document.lanes()
view raw LaneListView.swift hosted with ❤ by GitHub
struct LaneRowView: View {
let lane: Lane
var body: some View {
VStack(spacing: 8) {
if !lane.documentation.isEmpty {
.frame(maxWidth: .greatestFiniteMagnitude)
view raw LaneRowView.swift hosted with ❤ by GitHub


DocumentGroup, FileDocument, and ReferenceFileDocument APIs are the building blocks for building document based apps with SwiftUI. Getting a simple document based app up and running does not require much code at all.

If this was helpful, please let me know on Twitter @toomasvahter. Feel free to subscribe to RSS feed. Thank you for reading.

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