Get file path using drag and drop (Swift macOS)?

This implementation allows for multiple files.

Just set the view class to DragView in Interface Builder, implement DragViewDelegate in your controller, then hook up the delegate outlet in Interface Builder. This way you get a delegate callback with the file URLs.

func dragViewDidReceive(fileURLs: [URL])
{
    // Yay!
}

DragView.swift

//
//  DragView.swift
//
//  Copyright (c) 2020 Geri Borbás http://www.twitter.com/_eppz
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
//  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

import Cocoa


@objc protocol DragViewDelegate
{


    func dragViewDidReceive(fileURLs: [URL])
}


class DragView: NSView
{


    @IBOutlet weak var delegate: DragViewDelegate?
    let fileExtensions = ["kext"]


    required init?(coder: NSCoder)
    {
        super.init(coder: coder)
        color(to: .clear)
        registerForDraggedTypes([.fileURL])
    }

    override func draggingEntered(_ draggingInfo: NSDraggingInfo) -> NSDragOperation
    {
        var containsMatchingFiles = false
        draggingInfo.draggingPasteboard.readObjects(forClasses: [NSURL.self], options: nil)?.forEach
        {
            eachObject in
            if let eachURL = eachObject as? URL
            {
                containsMatchingFiles = containsMatchingFiles || fileExtensions.contains(eachURL.pathExtension.lowercased())
                if containsMatchingFiles { print(eachURL.path) }
            }
        }

        switch (containsMatchingFiles)
        {
            case true:
                color(to: .secondaryLabelColor)
                return .copy
            case false:
                color(to: .disabledControlTextColor)
                return .init()
        }
    }

    override func performDragOperation(_ draggingInfo: NSDraggingInfo) -> Bool
    {
        // Collect URLs.
        var matchingFileURLs: [URL] = []
        draggingInfo.draggingPasteboard.readObjects(forClasses: [NSURL.self], options: nil)?.forEach
        {
            eachObject in
            if
                let eachURL = eachObject as? URL,
                fileExtensions.contains(eachURL.pathExtension.lowercased())
            { matchingFileURLs.append(eachURL) }
        }

        // Only if any,
        guard matchingFileURLs.count > 0
        else { return false }

        // Pass to delegate.
        delegate?.dragViewDidReceive(fileURLs: matchingFileURLs)
        return true
    }

    override func draggingExited(_ sender: NSDraggingInfo?)
    { color(to: .clear) }

    override func draggingEnded(_ sender: NSDraggingInfo)
    { color(to: .clear) }

}


extension DragView
{


    func color(to color: NSColor)
    {
        self.wantsLayer = true
        self.layer?.backgroundColor = color.cgColor
    }
}

[Update to Swift 4.0 and Xcode 9]

Inspired by Implementing a drag-and-drop zone in Swift

Add a NSView to your main view and subclass. This code works perfect in Swift 4.0 and macOS 10.13 High Sierra!

import Cocoa

class DropView: NSView {

    var filePath: String?
    let expectedExt = ["kext"]  //file extensions allowed for Drag&Drop (example: "jpg","png","docx", etc..)

    required init?(coder: NSCoder) {
        super.init(coder: coder)

        self.wantsLayer = true
        self.layer?.backgroundColor = NSColor.gray.cgColor

        registerForDraggedTypes([NSPasteboard.PasteboardType.URL, NSPasteboard.PasteboardType.fileURL])
    }

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        // Drawing code here.
    }

    override func draggingEntered(_ sender: NSDraggingInfo) -> NSDragOperation {
        if checkExtension(sender) == true {
            self.layer?.backgroundColor = NSColor.blue.cgColor
            return .copy
        } else {
            return NSDragOperation()
        }
    }

    fileprivate func checkExtension(_ drag: NSDraggingInfo) -> Bool {
        guard let board = drag.draggingPasteboard().propertyList(forType: NSPasteboard.PasteboardType(rawValue: "NSFilenamesPboardType")) as? NSArray,
              let path = board[0] as? String
        else { return false }

        let suffix = URL(fileURLWithPath: path).pathExtension
        for ext in self.expectedExt {
            if ext.lowercased() == suffix {
                return true
            }
        }
        return false
    }

    override func draggingExited(_ sender: NSDraggingInfo?) {
        self.layer?.backgroundColor = NSColor.gray.cgColor
    }

    override func draggingEnded(_ sender: NSDraggingInfo) {
        self.layer?.backgroundColor = NSColor.gray.cgColor
    }

    override func performDragOperation(_ sender: NSDraggingInfo) -> Bool {
        guard let pasteboard = sender.draggingPasteboard().propertyList(forType: NSPasteboard.PasteboardType(rawValue: "NSFilenamesPboardType")) as? NSArray,
              let path = pasteboard[0] as? String
        else { return false }

        //GET YOUR FILE PATH !!!
        self.filePath = path
        Swift.print("FilePath: \(path)")

        return true
    }
}

To use this code you have to set "macOS Deployment Target" to 10.13