demo

출처

import Cocoa

extension NSColor {
    var swatch: NSImage {
        let size = NSSize(width: 25.0, height: 25.0)
        let image = NSImage(size: size)
        image.lockFocus()
        self.drawSwatch(in: NSRect(origin: .zero, size: size))
        image.unlockFocus()
        return image
    }
}

class ColorView: NSView {

    var color: NSColor = .white

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

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        color.setFill()
        NSBezierPath(rect: dirtyRect).fill()
    }

    override func mouseDragged(with event: NSEvent) {
        let draggingItem = NSDraggingItem(pasteboardWriter: color)
        let swatch = color.swatch
        draggingItem.setDraggingFrame(CGRect(origin: .zero, size: swatch.size), contents: swatch)
        beginDraggingSession(with: [draggingItem], event: event, source: self)
    }

    override func draggingEntered(_ sender: NSDraggingInfo) -> NSDragOperation {
        if let _ = NSColor(from: sender.draggingPasteboard) {
            return .copy
        }
        return []
    }

    override func draggingUpdated(_ sender: NSDraggingInfo) -> NSDragOperation {
        if let _ = NSColor(from: sender.draggingPasteboard) {
            return .copy
        }
        return []
    }

    override func performDragOperation(_ sender: NSDraggingInfo) -> Bool {
        guard let newColor = NSColor(from: sender.draggingPasteboard) else {
            return false
        }
        color = newColor
        self.needsDisplay = true
        return true
    }

}

extension ColorView: NSDraggingSource {
    func draggingSession(_ session: NSDraggingSession, sourceOperationMaskFor context: NSDraggingContext) -> NSDragOperation {
        return .copy
    }
}