Supporting Collection View Drag and Drop Through File Promises

At a glance

Project structure

Source bundle/
├── CollectionViewDragDrop/
│   ├── LoadPhotosOperation.swift
│   ├── LoadThumbnailOperation.swift
│   ├── AppDelegate.swift
│   ├── PromiseFileProvider.swift
│   ├── ViewController.swift
│   ├── PhotoItem.swift
│   ├── CollectionViewItem.swift
│   ├── ViewController+DragAndDrop.swift
│   └── Base.lproj/
│       ├── CollectionViewItem.xib
│       └── Main.storyboard
└── CollectionViewDragDrop.xcodeproj/
    ├── .xcodesamplecode.plist
    └── project.pbxproj

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 7 project/configuration file(s) and 10 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["ViewController"]
    N3["FilePromiseProvider"]
    N4["Cocoa / UniformTypeIdentifiers APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

CollectionViewDragDrop/AppDelegate.swift:10 — architecture anchor

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        // Insert code here to initialize your application.
    }

    func applicationWillTerminate(_ aNotification: Notification) {
        // Insert code here to tear down your application.
    }

}

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
    LoadPhotosOperation *-- URL : directoryURL
    LoadThumbnailOperation *-- CGFloat : thumbHeight
    LoadThumbnailOperation *-- CGFloat : thumbWidth
    LoadThumbnailOperation *-- URL : fileURL

Ownership evidence

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

class LoadPhotosOperation: Operation {
    var directoryURL: URL!
    // ...
}

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.

No source-visible execution, scheduling, or synchronization boundary was found in the indexed source.

@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.

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

CollectionViewDragDrop/PhotoItem.swift:16 — representative type boundary

protocol ThumbnailDelegate: AnyObject {
    func thumbnailDidFinish(_ photoItem: PhotoItem)
}

The source explicitly defines local protocol relationships: ViewController → ThumbnailDelegate.

Access control

Reference code

CollectionViewDragDrop/CollectionViewItem.swift:26 — representative boundary

    private func updateSelectionHighlighting() {
        // ...
            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: ViewController; Delegate: AppDelegate, ThumbnailDelegate; Operation: LoadPhotosOperation, LoadThumbnailOperation; Provider: FilePromiseProvider.
• Protocols: ThumbnailDelegate.
• Methods: main, applicationDidFinishLaunching, applicationWillTerminate, writableTypes, pasteboardPropertyList, writingOptions, viewDidLoad, setupProgressIndicator.
• Files: CollectionViewDragDrop/LoadPhotosOperation.swift, CollectionViewDragDrop/LoadThumbnailOperation.swift, CollectionViewDragDrop/AppDelegate.swift, CollectionViewDragDrop/ViewController.swift, CollectionViewDragDrop/PhotoItem.swift, CollectionViewDragDrop/CollectionViewItem.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches Cocoa, UniformTypeIdentifiers 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.
• Local protocol relationships provide an explicit substitution boundary.

Source map

• Official Apple documentation