ContentView.swift

//
//  ContentView.swift
//  RigelianTagger
//
//  Created by Berrie Kremers on 05/03/2022.
//
//  This shows how a directory structure can be shown in a sidebar, with dynamic loading
//  of additional content as the user drills down in the hierarchy.
//

import SwiftUI

class FileItem: ObservableObject, Identifiable {
    private(set) var name: String
    private(set) var url: URL
    @Published private(set) var children: [FileItem]?
    private var loaded = false
    var isDirectory: Bool {
        children != nil
    }
    var isFile: Bool {
        children == nil
    }
    
    static func root(_ root: URL? = nil) -> FileItem {
        let url = root ?? URL(fileURLWithPath: "/Users/berrie/Music", isDirectory: true)
        let item = FileItem(name: url.lastPathComponent, url: url, children: [])
        
        // Immediately load the children of this item, so that this data is available when the item is opened
        Task {
            await item.loadChildren()
        }
        
        return item
    }
    
    internal init(name: String, url: URL, children: [FileItem]? = nil) {
        self.name = name
        self.url = url
        self.children = children
    }
    
    func reset(_ root: URL) {
        loaded = false
        url = root
        name = root.lastPathComponent
        children = []
        
        Task {
            await loadChildren()
        }
    }

    func loadChildren() async {
        guard loaded == false, isDirectory else { return }
        
        loaded = true
        do {
            let sortedChildren: [FileItem] = try FileManager.default.contentsOfDirectory(at: url, includingPropertiesForKeys: nil, options: [.skipsHiddenFiles]).map {
                let child = FileItem(name: $0.lastPathComponent, url: $0)
                
                if $0.hasDirectoryPath {
                    child.children = []
                }
                return child
            }
            .sorted(by: { $0.name < $1.name })
            
            DispatchQueue.main.async {
                self.children = sortedChildren
            }
        }
        catch {
            print("\(error)")
        }
    }
    
    var id: String { url.absoluteString }
}

private func toggleSidebar() {
    NSApp.keyWindow?.contentViewController?.tryToPerform(#selector(NSSplitViewController.toggleSidebar(_:)), with: nil)
}

struct Sidebar: View {
    @ObservedObject var root = FileItem.root()
    
    var body: some View {
        List([root], children: \.children) { item in
            NavigationLink(destination: Text("Selected " +  item.name)) {
                HStack {
                    Image(systemName: item.isDirectory ? "folder" : "doc")
                    Text(item.name)
                }
                .onAppear {
                    // Whenever an item appears, load its children.
                    Task {
                        await item.loadChildren()
                    }
                }
            }
        }
        .listStyle(.sidebar)
        .frame(minWidth: 150)
        .toolbar {
            Button(action: toggleSidebar, label: {
                Image(systemName: "sidebar.left").help("Toggle Sidebar")
            })
            Button("Select Root") {
                let openPanel = NSOpenPanel()
                openPanel.prompt = "Select Directory"
                openPanel.allowsMultipleSelection = false
                openPanel.canChooseDirectories = true
                openPanel.canCreateDirectories = false
                openPanel.canChooseFiles = false
                openPanel.begin { (result) -> Void in
                    if result.rawValue == NSApplication.ModalResponse.OK.rawValue {
                        guard let url = openPanel.url else { return }
                        root.reset(url)
                    }
                }
            }
        }
    }
}

struct ContentView: View {
    var body: some View {
        NavigationView {
            Sidebar()
            Text("Main Window")
                .frame(minWidth: 200)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

• very nice - thanks!