Enhancing your custom text engine with Writing Tools

At a glance

Project structure

Source bundle/
└── WritingToolsSampleApp/
    ├── WritingToolsSampleApp.swift
    ├── ViewModels/
    │   ├── DocumentViewModel.swift
    │   ├── DocumentViewModel+Formatting.swift
    │   ├── DocumentViewModel+Input.swift
    │   ├── DocumentViewModel+Pasteboard.swift
    │   └── DocumentViewModel+Selection.swift
    └── Views/
        ├── DocumentView/
        │   ├── DocumentView.swift
        │   └── DocumentView+WritingToolsCoordinator.swift
        ├── DocumentViewController.swift
        ├── Misc/
        │   ├── TextLayoutFragmentView.swift
        │   └── TextSelectionView.swift
        └── WindowController.swift

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: Swift.
• The verified tree contains 4 project/configuration file(s) and 8 source declaration(s).

Overall architecture

flowchart LR
    N1["WritingToolsSampleApp"]
    N2["DocumentViewController"]
    N3["DocumentViewModel"]
    N4["Cocoa APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

WritingToolsSampleApp/WritingToolsSampleApp.swift:10 — architecture anchor

@main
class AppDelegate: NSObject, NSApplicationDelegate {
    var windowController: NSWindowController?

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        let storyboard = NSStoryboard(name: "Main", bundle: nil)
        windowController = storyboard.instantiateController(withIdentifier: "MainWindowController") as? NSWindowController
        windowController?.showWindow(self)
        windowController?.window?.makeKeyAndOrderFront(self)
    }

    func applicationSupportsSecureRestorableState(_ app: NSApplication) -> Bool {
        return true
    }

}

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into AppKit.

Ownership and state

classDiagram
    AppDelegate o-- NSWindowController : windowController
    DocumentViewModel *-- NSTextContentStorage : textContentStorage
    DocumentViewModel *-- NSTextLayoutManager : textLayoutManager
    DocumentViewModel o-- ViewModelDelegate : viewModelDelegate

Ownership evidence

WritingToolsSampleApp/WritingToolsSampleApp.swift:12 — stored dependency or nearest verified ownership anchor

@main
class AppDelegate: NSObject, NSApplicationDelegate {
    var windowController: NSWindowController?
    // ...
}

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Concurrency, scheduling, and thread safety

Evidence limit: actor isolation, async/await, or Task creation does not by itself prove background-thread execution; Sendable conformance alone does not prove thread-safe mutation.

@MainActor/MainActor.run, DispatchQueue.main, and RunLoop.main are reported as distinct isolation, queue, and event-loop mechanisms. A plain Task is kept separate from Task.detached; neither is labeled as a background thread.

Reference code

WritingToolsSampleApp/Views/DocumentView/DocumentView.swift:269 — representative execution boundary

                Task { @MainActor in
                    self?.needsLayout = true
                }

State propagation, frameworks, and dependencies

Evidence limit: an import proves a source-level compilation dependency at the cited line; it does not prove runtime use, architectural adoption, or whether a Swift package is a direct application dependency.

receive(on:) describes downstream delivery scheduling, while subscribe(on:) describes upstream subscription/request/cancel scheduling. An import Combine alone establishes neither behavior nor a Store, reducer, Redux, or other application architecture.

Class and protocol design

WritingToolsSampleApp/ViewModels/DocumentViewModel.swift:70 — representative type boundary

protocol ViewModelDelegate: AnyObject {
    func textDidChange()
    func selectionDidChange()
}

No local protocol conformance is claimed as protocol-oriented design; external framework conformances are listed only as dependencies.

Access control

Reference code

WritingToolsSampleApp/Views/DocumentView/DocumentView+Editing.swift:113 — representative boundary

    private func performDelete(forward: Bool) {
        // ...
            return
        // ...
    }

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Naming conventions

• Types: Controller: DocumentViewController, WindowController; Delegate: AppDelegate, ViewModelDelegate; View: DocumentView, TextLayoutFragmentView, TextSelectionView; ViewModel: DocumentViewModel.
• Protocols: ViewModelDelegate.
• Methods: applicationDidFinishLaunching, applicationSupportsSecureRestorableState, textDidChange, selectionDidChange, replaceText, clearText, configureWritingTools, layout.
• Files: WritingToolsSampleApp/ViewModels/DocumentViewModel.swift, WritingToolsSampleApp/Views/DocumentView/DocumentView.swift, WritingToolsSampleApp/Views/DocumentViewController.swift, WritingToolsSampleApp/Views/Misc/TextLayoutFragmentView.swift, WritingToolsSampleApp/Views/Misc/TextSelectionView.swift, WritingToolsSampleApp/Views/WindowController.swift.

Architecture takeaways

• WritingToolsSampleApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches Cocoa through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• The source does not justify labeling the design protocol-oriented.

Source map

• Official Apple documentation