Accept deep-links to Mail.app messages through macOS drag and drop

MailURLDropView.swift

// Accept deep-links to Mail.app messages through macOS drag and drop
class MailURLDropView: NSView {
    
    var filePath: String?
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        
        self.wantsLayer = true
        self.layer?.backgroundColor = NSColor.gray.cgColor
        
        let draggedType = NSPasteboard.PasteboardType(kUTTypeURL as String)
        registerForDraggedTypes([draggedType])
    }
    
    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
    }
    
    override func draggingEntered(_ sender: NSDraggingInfo) -> NSDragOperation {
        if extractURL(sender.draggingPasteboard()) == nil {
            return NSDragOperation()
        }
        
        self.layer?.backgroundColor = NSColor.blue.cgColor
        return .copy
    }
    
    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 url = extractURL(sender.draggingPasteboard()) else {
            return false
        }
        
        print(url)
        return true
    }
    
    func extractURL(_ pasteboard: NSPasteboard) -> URL? {
        guard let data = pasteboard.data(forType: NSPasteboard.PasteboardType(rawValue: "public.url")) else {
            return nil
        }
        
        guard let string = String(data: data, encoding: String.Encoding.utf8) else {
            return nil
        }
        
        guard let url = URL(string: string) else {
            return nil
        }
        
        if url.scheme != "message" {
            return nil
        }
        
        return url
    }
}