Simple table data source for NSTableView

example.swift

struct Person: CustomStringConvertible {
	var name: String
	var birthday: Date
	var friends: [Person]
	
	var description: String { return name + "(\(friends.count))" }
}

enum TableRow: String, NSUserInterfaceItemIdentifierConvertible {
	case name, birthday, age
}

let michel = Person(name: "Michel", birthday: Date(), friends: [])
let dorothea = Person(name: "Dorothea", birthday: Date(), friends: [])

let dataSource = TableDataSource<[Person]>(
	data: [michel, dorothea],
	columns: [
		TableRow.name: .init(\.name),
		.birthday: .init(\.birthday),
		.age: .init(\.birthday) { -$0.timeIntervalSinceNow }
	]
)

NSTableViewDataSource.swift

//  Created by Damiaan on 17/01/2019.
//  Copyright © 2019 Devian. All rights reserved.

import AppKit

class TableDataSource<C: RandomAccessCollection>: NSObject, NSTableViewDataSource {
	public struct ColumnSpecifier {
		let keyPath: PartialKeyPath<C.Element>
		fileprivate let transformer: Any?
		
		func getValue(from element: C.Element) -> Any {
			if let transformer = transformer {
				return (keyPath as! FullKeyPath).value(for: element, transformedBy: transformer)
			} else {
				return element[keyPath: keyPath]
			}
		}
		
		public init<T>(_ key: KeyPath<C.Element, T>, transform: ((T)->Any)? = nil) {
			keyPath = key
			transformer = transform
		}
	}
	
	var data: [C.Element]
	let columnSpecifiers: [NSUserInterfaceItemIdentifier: ColumnSpecifier]
	let columnIdentifiers: [PartialKeyPath<C.Element>: [NSUserInterfaceItemIdentifier]]
	
	init(data initialData: [C.Element], columns: [NSUserInterfaceItemIdentifier: ColumnSpecifier]) {
		data = initialData
		columnSpecifiers = columns
		columnIdentifiers = Dictionary(columns.map{($0.value.keyPath, [$0.key])}, uniquingKeysWith: +)
	}
	
	convenience init<Identifier: NSUserInterfaceItemIdentifierConvertible>(data initialData: [C.Element], columns: [Identifier: ColumnSpecifier]) {
		self.init(
			data: initialData,
			columns: Dictionary(uniqueKeysWithValues: columns.map{ ($0.key.id, $0.value) })
		)
	}
	
	func numberOfRows(in tableView: NSTableView) -> Int {
		return data.count
	}
	
	func tableView(_ tableView: NSTableView, objectValueFor tableColumn: NSTableColumn?, row: Int) -> Any? {
		if let column = tableColumn {
			return value(for: column.identifier, of: data[row])
		}
		return nil
	}
	
	func value(for uiIdentifier: NSUserInterfaceItemIdentifier, of row: C.Element) -> Any {
		guard let specifier = columnSpecifiers[uiIdentifier] else {
			fatalError("There is no ColumnSpecifier set up for column \(uiIdentifier).")
		}
		return specifier.getValue(from: row)
	}
}

protocol NSUserInterfaceItemIdentifierConvertible: RawRepresentable where RawValue == String {}
extension NSUserInterfaceItemIdentifierConvertible {
	var id: NSUserInterfaceItemIdentifier { return .init(rawValue) }
}

fileprivate protocol FullKeyPath {
	func value(for object: Any, transformedBy transform: Any) -> Any
}

extension KeyPath: FullKeyPath {
	func value(for object: Any, transformedBy transform: Any) -> Any {
		let element = (object as! Root)[keyPath: self]
		let function = transform as! ((Value)->Any)
		return function(element)
	}
}