An autocomplete popup for text entry. Ported to Swift from this example: https://github.com/danjonweb/NCRAutocompleteTextView

AutocompleteView.swift

//
//  AutocompleteView.swift
//
//  Originally created by Daniel Weber on 9/28/14.
//  Copyright (c) 2014 Null Creature. All rights reserved.
//  Ported to Swift by Martin Pichlmair 2018
//

import Cocoa

let MAX_RESULTS = 10
let HIGHLIGHT_STROKE_COLOR = NSColor.selectedMenuItemColor
let HIGHLIGHT_FILL_COLOR = NSColor.selectedMenuItemColor
let HIGHLIGHT_RADIUS = 0.0
let INTERCELL_SPACING = NSMakeSize(20.0, 3.0)
let WORD_BOUNDARY_STRING="().#abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
let POPOVER_WIDTH = 250.0
let POPOVER_PADDING = 0.0
let POPOVER_APPEARANCE = NSAppearance.Name.vibrantLight
let POPOVER_FONT = NSFont.init(name: "Menlo", size: 12)

// The font for the characters that have already been typed
let POPOVER_BOLDFONT = NSFont.init(name: "Menlo-Bold", size: 12)
let POPOVER_TEXTCOLOR = NSColor.black


protocol AutocompleteTableViewDelegate {
	func textView(_ textView: NSTextView, completions words: [String], forPartialWordRange: NSRange, indexOfSelectedItem:  UnsafeMutablePointer<Int>?) -> [String]
	func textView(_ textView: NSTextView, imageForCompletion word: String) -> NSImage?
}

class AutocompleteTableRowView : NSTableRowView {
	
	override func drawSelection(in dirtyRect: NSRect) {
		if selectionHighlightStyle != .none {
			let selectionRect: NSRect = NSInsetRect(bounds, 0.5, 0.5)
			HIGHLIGHT_STROKE_COLOR.setStroke()
			HIGHLIGHT_FILL_COLOR.setFill()
			let selectionPath: NSBezierPath = NSBezierPath(roundedRect: selectionRect, xRadius: CGFloat(HIGHLIGHT_RADIUS), yRadius: CGFloat(HIGHLIGHT_RADIUS))
			selectionPath.fill()
			selectionPath.stroke()
		}
	}

	override var interiorBackgroundStyle: NSView.BackgroundStyle { get {
			if (isSelected) {
				return NSView.BackgroundStyle.dark
			} else {
				return NSView.BackgroundStyle.light
			}
		}
	}

	
}

// MARK: -

class AutocompleteTextView : NSTextView, NSTableViewDelegate, NSTableViewDataSource {
	
	var autocompletePopover: NSPopover?
	weak var autocompleteTableView: NSTableView?
	var matches: [String]? = [String]()
	var substring: String?
	var lastPos: Int?
	
	override func awakeFromNib() {
		// Make a table view with 1 column and enclosing scroll view. It doesn't
		// matter what the frames are here because they are set when the popover
		// is displayed
		let column1: NSTableColumn = NSTableColumn(identifier: NSUserInterfaceItemIdentifier(rawValue: "text"))
		column1.isEditable = false
		column1.width = CGFloat(POPOVER_WIDTH-2*POPOVER_PADDING)
		let tableView: NSTableView = NSTableView(frame: NSZeroRect)
		tableView.selectionHighlightStyle = .regular
		tableView.backgroundColor = NSColor.clear
		tableView.rowSizeStyle = .small
		tableView.intercellSpacing = INTERCELL_SPACING
		tableView.headerView = nil
		tableView.refusesFirstResponder = true
		tableView.target = self
		tableView.doubleAction = #selector(NSArrayController.insert(_:))
		tableView.addTableColumn(column1)
		tableView.delegate = self
		tableView.dataSource = self
		autocompleteTableView = tableView
		let tableScrollView: NSScrollView = NSScrollView(frame: NSZeroRect)
		tableScrollView.drawsBackground = false
		tableScrollView.documentView = tableView
		tableScrollView.hasVerticalScroller = true
		let contentView: NSView = NSView(frame: NSZeroRect)
		contentView.addSubview(tableScrollView)
		let contentViewController: NSViewController = NSViewController()
		contentViewController.view = contentView
		autocompletePopover = NSPopover()
		autocompletePopover!.appearance = NSAppearance(named: POPOVER_APPEARANCE)
		autocompletePopover!.animates = false
		autocompletePopover!.contentViewController = contentViewController
		matches = [String]()
		lastPos = -1
		NotificationCenter.default.addObserver(self, selector: #selector(didChangeSelection(_:)), name: NSNotification.Name(rawValue: "NSTextViewDidChangeSelectionNotification"), object: nil)
	}
	
	override func keyDown(with theEvent: NSEvent) {
		let row: Int = autocompleteTableView!.selectedRow
		var shouldComplete: Bool = true
		switch theEvent.keyCode {
		case 51:
			
			// Delete
			autocompletePopover!.close()
			shouldComplete = false
			break
		case 53:
			
			// Esc
			if autocompletePopover!.isShown {
				autocompletePopover!.close()
			}
			return
		// Skip default behavior
		case 125:
			
			// Down
			if autocompletePopover!.isShown {
				autocompleteTableView!.selectRowIndexes(NSIndexSet(index: row+1) as IndexSet, byExtendingSelection: false)
				autocompleteTableView!.scrollRowToVisible(autocompleteTableView!.selectedRow)
				return
				// Skip default behavior
			}
			break
		case 126:
			
			// Up
			if autocompletePopover!.isShown {
				autocompleteTableView!.selectRowIndexes(NSIndexSet(index: row-1) as IndexSet, byExtendingSelection: false)
				autocompleteTableView!.scrollRowToVisible(autocompleteTableView!.selectedRow)
				return
				// Skip default behavior
			}
			break
		case 49:
			
			// Space
			if autocompletePopover!.isShown {
				autocompletePopover!.close()
			}
			break
		default:
			// Return or tab
			if autocompletePopover!.isShown && (
				theEvent.keyCode == 0x24 || theEvent.keyCode == 0x4C || theEvent.keyCode == 0x30) {
				insert(self)
				return
				// Skip default behavior
			}
		}
		super.keyDown(with: theEvent)
		if shouldComplete && theEvent.characters != nil && WORD_BOUNDARY_STRING.contains(theEvent.characters![(theEvent.characters?.startIndex)!]) {
			complete(self)
		}
	}
	
	func insert(_ sender: Any) {
		if autocompleteTableView!.selectedRow >= 0 && autocompleteTableView!.selectedRow < matches!.count {
			let string: String = (matches![autocompleteTableView!.selectedRow])
			let beginningOfWord: Int = selectedRange.location-substring!.count
			let range: NSRange = NSMakeRange(beginningOfWord, substring!.count)
			if shouldChangeText(in: range, replacementString: string) {
				replaceCharacters(in: range, with: string)
				didChangeText()
			}
		}
		autocompletePopover!.close()
	}
	
	@objc func didChangeSelection(_ notification: NSNotification) {
		if labs(selectedRange.location-lastPos!) > 1 {
			// If selection moves by more than just one character, hide autocomplete
			autocompletePopover!.close()
		}
	}
	
	func complete(_ sender: Any) {
		
		let str = string as NSString
		var wordStart = selectedRange.location - 1
		
		for i in stride(from: selectedRange.location - 1, to: 0, by: -1) {
			let char = Character(UnicodeScalar(str.character(at: i)) ?? " ")
			
			if !WORD_BOUNDARY_STRING.contains(char) {
				wordStart = i + 1
				break
			}
		}
		var wordLength = 0
		for i in wordStart ... str.length {
			let char = Character(UnicodeScalar(str.character(at: i)) ?? " ")
			if !WORD_BOUNDARY_STRING.contains(char) {
				wordLength = i - wordStart
				break
			}
		}

		let substringRange = NSMakeRange(wordStart, wordLength)
		substring = str.substring(with: substringRange)
		
		if selectedRange.location == wordStart || wordLength == 0 {
			// This happens when we just started a new word or if we have already typed the entire word
			autocompletePopover!.close()
			return
		}
		let index: Int = 0
		matches = completions(range:substringRange)
		if matches!.count > 0 {
			lastPos = selectedRange.location
			autocompleteTableView!.reloadData()
			autocompleteTableView!.selectRowIndexes(NSIndexSet(index: index) as IndexSet, byExtendingSelection: false)
			autocompleteTableView!.scrollRowToVisible(index)
			// Make the frame for the popover. We want it to shrink with a small number
			// of items to autocomplete but never grow above a certain limit when there
			// are a lot of items. The limit is set by MAX_RESULTS.
			let numberOfRows: Int = min(autocompleteTableView!.numberOfRows, MAX_RESULTS)
			let height: CGFloat = (autocompleteTableView!.rowHeight+autocompleteTableView!.intercellSpacing.height)*CGFloat(numberOfRows)+2.0*CGFloat(POPOVER_PADDING)
			let frame: NSRect = NSMakeRect(0, 0, CGFloat(POPOVER_WIDTH), height)
			autocompleteTableView!.enclosingScrollView!.frame = NSInsetRect(frame, CGFloat(POPOVER_PADDING), CGFloat(POPOVER_PADDING))
			autocompletePopover!.contentSize = NSMakeSize(NSWidth(frame), NSHeight(frame))
			// We want to find the middle of the first character to show the popover.
			// firstRectForCharacterRange: will give us the rect at the begeinning of
			// the word, and then we need to find the half-width of the first character
			// to add to it.
			var rect: NSRect = firstRect(forCharacterRange: substringRange, actualRange: nil)
			rect = window!.convertFromScreen(rect)
			rect = convert(rect, from: nil)
			let firstChar: String = String(substring![(substring?.startIndex)!..<(substring?.index((substring?.startIndex)!, offsetBy: 1))!])
			let firstCharSize: NSSize = firstChar.size(withAttributes: [NSAttributedString.Key.font: font!])
			rect.size.width = firstCharSize.width
			autocompletePopover!.show(relativeTo: rect, of: self, preferredEdge: .maxY)
		} else {
			autocompletePopover!.close()
			
		}
	}

	func completions(range:NSRange) -> [String] {
		if ((delegate as? AutocompleteTableViewDelegate) != nil) {
			var index: Int = 0
			return delegate!.textView!(self, completions: [], forPartialWordRange: range, indexOfSelectedItem: &index)
		}
		return []
	}

	func numberOfRows(in tableView: NSTableView) -> Int {
		return matches!.count
	}
	
	func tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int) -> NSView? {
		
		var cellView = tableView.makeView(withIdentifier: NSUserInterfaceItemIdentifier(rawValue: "MyView"), owner: self)
		
		if cellView == nil {
			cellView = NSTableCellView(frame: NSZeroRect)
			let textField: NSTextField = NSTextField(frame: NSZeroRect)
			textField.isBezeled = false
			textField.drawsBackground = false
			textField.isEditable = false
			textField.isSelectable = false
			cellView!.addSubview(textField)
			(cellView as! NSTableCellView).textField = textField
			if delegate!.responds(to: Selector(("textView:imageForCompletion:"))) {
				let imageView: NSImageView = NSImageView(frame: NSZeroRect)
				imageView.imageFrameStyle = .none
				imageView.imageScaling = NSImageScaling.scaleNone
				cellView!.addSubview(imageView)
				(cellView as! NSTableCellView).imageView = imageView
			}
			cellView!.identifier = NSUserInterfaceItemIdentifier(rawValue: "MyView")
		}
		let astring: NSMutableAttributedString = NSMutableAttributedString(string: matches![row] , attributes: [NSAttributedString.Key.font:POPOVER_FONT!, NSAttributedString.Key.foregroundColor:POPOVER_TEXTCOLOR])
		if (substring != nil) {
			let range = astring.mutableString.range(of: substring!, options: NSString.CompareOptions(rawValue: NSString.CompareOptions.anchored.rawValue | NSString.CompareOptions.caseInsensitive.rawValue))
			astring.addAttribute(NSAttributedString.Key.font, value: POPOVER_BOLDFONT!, range: range)
		}
		(cellView as! NSTableCellView).textField!.attributedStringValue = astring
		if delegate!.responds(to: Selector(("textView:imageForCompletion:"))) {
			let image: NSImage = (delegate! as! AutocompleteTableViewDelegate).textView(self, imageForCompletion: matches![row] )!
			(cellView as! NSTableCellView).imageView?.image = image
		}
		return cellView
	}
	
	func tableView(_ tableView: NSTableView, rowViewForRow row: Int) -> NSTableRowView? {
		return AutocompleteTableRowView()
	}
	
}