Drag and Drop View on macOS

DragView.swift

protocol DragViewDelegate {
    var acceptedFileExtensions: [String] { get }
    func dragView(dragView: DragView, didDragFileWith URL: NSURL)
}

class DragView: NSView {
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        registerForDraggedTypes([NSFilenamesPboardType, NSURLPboardType])
    }
    
    private var fileTypeIsOk = false
    private var acceptedFileExtensions: [String] {
        return delegate?.acceptedFileExtensions ?? []
    }
    
    var delegate: DragViewDelegate?
    
    override func draggingEntered(sender: NSDraggingInfo) -> NSDragOperation {
        if checkExtension(sender) {
            fileTypeIsOk = true
            return .Copy
        } else {
            fileTypeIsOk = false
            return .None
        }
    }
    
    override func draggingUpdated(sender: NSDraggingInfo) -> NSDragOperation {
        if fileTypeIsOk {
            return .Copy
        } else {
            return .None
        }
    }
    
    override func performDragOperation(sender: NSDraggingInfo) -> Bool {
        guard let draggedFileURL = sender.draggedFileURL else {
            return false
        }
        
        delegate?.dragView(self, didDragFileWith: draggedFileURL)
        
        return true
    }
    
    func checkExtension(drag: NSDraggingInfo) -> Bool {
        guard let fileExtension = drag.draggedFileURL?.pathExtension?.lowercaseString else {
            return false
        }
        
        return acceptedFileExtensions.contains(fileExtension)
    }
}

NSDraggingInfo+DraggedFileURL.swift

extension NSDraggingInfo {
    var draggedFileURL: NSURL? {
        let filenames = draggingPasteboard().propertyListForType(NSFilenamesPboardType) as? [String]
        let path = filenames?.first
        
        return path.map(NSURL.init)
    }
}