Sample CodemacOSReviewed 2026-07-21View on Apple Developer

Supporting Table View Drag and Drop Through File Promises

At a glance

Item	Summary
Purpose	Load photos into a table, reorder rows, receive external images/promises, and promise files to other apps.
Architecture	A view controller owns the bound content array and operation queues; model objects lazily load thumbnails; a capability extension implements table and promise protocols.
Table model	`@objc dynamic contentArray` is the Cocoa-binding and row-order source of truth.

Project structure

TableViewDragDrop/
├── ViewController.swift
├── ViewController+DragAndDrop.swift
├── PhotoItem.swift
├── TableCellView.swift
├── LoadPhotosOperation.swift
├── LoadThumbnailOperation.swift
└── PromiseFileProvider.swift

This is the table-view counterpart to the collection sample, with binding-friendly model properties and row numbers replacing snapshots and index paths.

Overall architecture

flowchart LR
    Loader["LoadPhotosOperation"] --> Array["contentArray"]
    Array --> Table["NSTableView + Cocoa bindings"]
    Table --> Adapter["ViewController data source"]
    Adapter --> Provider["FilePromiseProvider"]
    Adapter --> Receiver["NSFilePromiseReceiver"]
    Provider --> External["Finder / other apps"]
    Receiver --> Array

Reference code

TableViewDragDrop/ViewController.swift:18 — the controller’s array is deliberately exposed to Objective-C KVC for storyboard bindings.

/** This contentArray need @objc to make it key value compliant with this view controller,
    and so they are accessible and usable with Cocoa bindings.
*/
@objc dynamic var contentArray = [PhotoItem]()

Ownership and state

classDiagram
    ViewController *-- Array : contentArray
    ViewController *-- OperationQueue : loaderQueue
    ViewController *-- OperationQueue : filePromiseQueue
    Array o-- PhotoItem
    PhotoItem *-- LoadThumbnailOperation : per load
    PhotoItem --> ThumbnailDelegate : weak
    FilePromiseProvider o-- Dictionary : row and URL userInfo

Ownership evidence

TableViewDragDrop/PhotoItem.swift:20 — each model owns its file/display state and guards its own asynchronous load.

class PhotoItem: NSObject {
    // ...
}

The view controller owns ordering, UI, import destination, and I/O queues. PhotoItem owns per-file and thumbnail state. The provider carries a row plus URL for drag representations; the controller resolves that row when the destination requests the file.

Class and protocol design

Type	Responsibility
`ViewController`	Bound table content, queue ownership, initial loading, and thumbnail refresh.
`ViewController` drag/drop extension	`NSTableViewDataSource` and `NSFilePromiseProviderDelegate` behavior.
`PhotoItem`	KVC-visible row model and lazy thumbnail state.
`TableCellView`	Row presentation and image loading trigger.
Operation subclasses	Discover photos and derive thumbnails.
`FilePromiseProvider`	Add row and file representations to the pasteboard.

Access control

Boundary	Effect and rationale
`private imageLoading`	Makes one `PhotoItem` the authority preventing duplicate loads.
`private` cell presentation details	Keeps row-view internals out of controller logic.
`weak thumbnailDelegate`	Avoids model-to-controller ownership.
`@objc dynamic` properties	Enable Cocoa bindings/KVO but remain implicit `internal`, not public API.
`public override writingOptions`	Matches the superclass override surface; the internal subclass caps external use.

TableViewDragDrop/PromiseFileProvider.swift:63 is the explicit public override; TableViewDragDrop/PhotoItem.swift:30 is the private load guard.

Logic ownership and placement

Logic	Owner
Bound row ordering and UI refresh	`ViewController`
Drop validation, row moves, imports, and promised writes	`ViewController+DragAndDrop`
Pasteboard type/property-list translation	`FilePromiseProvider`
Lazy thumbnail lifecycle	`PhotoItem` plus `LoadThumbnailOperation`
Row rendering	`TableCellView`

Design patterns

Pattern	Evidence	Purpose
MVC	`TableViewDragDrop/PhotoItem.swift:20`, `TableViewDragDrop/ViewController.swift:14`	Separates row state from table coordination.
Cocoa binding	`TableViewDragDrop/ViewController.swift:21`	Lets the storyboard observe the content array.
Operation	`TableViewDragDrop/LoadPhotosOperation.swift:12`	Moves file discovery off the main thread.
Delegate	`TableViewDragDrop/PhotoItem.swift:16`	Reports thumbnail completion to the table owner.
File promise	`TableViewDragDrop/ViewController+DragAndDrop.swift:382`	Defers file output until drop time.

Naming conventions

Table-specific identity is explicit: rowDragType, rowNumberKey, and TableCellView.
Background workers use Load...Operation; the displayed domain object is PhotoItem.
ViewController+DragAndDrop signals where protocol-heavy integration lives.

Architecture takeaways

Make one array authoritative for table order and promise row lookup.
Distinguish KVC visibility from public module access.
Keep slow work on operation queues and marshal UI mutations to the main queue.
Put table policy in its data-source extension and representation policy in the provider.

Source map

Source	Role
`TableViewDragDrop/ViewController.swift:14`	Table state owner
`TableViewDragDrop/ViewController+DragAndDrop.swift:13`	Drag/drop data source
`TableViewDragDrop/PhotoItem.swift:20`	Bound photo model
`TableViewDragDrop/PromiseFileProvider.swift:10`	Pasteboard provider
`TableViewDragDrop/TableCellView.swift:10`	Row presentation

Official Apple documentation