import Cocoa

class JuxtaposeSlider: NSView {

    // The slider's position is determined by a value between 0 and 1
    var value: CGFloat = 0.5

    var rightImage = NSImage(named: "rightImage")
    var leftImage = NSImage(named: "leftImage")

    @IBInspectable var handleColor: NSColor = .white

    var mouseLocation: CGPoint? {
        didSet {
            if let location = self.mouseLocation {
                let local = self.convert(location, from: nil)
                self.value = min(1, max(0, local.x / self.bounds.width))
            }
            self.needsDisplay = true
        }
    }



    // MARK: - Drawing

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)

        // Draw the right image completely, as it is usually displayed somehow
        self.rightImage?.draw(at: .zero, from: self.bounds, operation: .copy, fraction: 1)

        // Then draw the left image on top (partially)
        if self.value > 0 {
            let leftRect = NSRect(x: 0,
                                  y: 0,
                              width: self.bounds.width * self.value,
                             height: self.bounds.height)
            self.leftImage?.draw(at: .zero, from: leftRect, operation: .copy, fraction: 1)
        }


        // Draw a handle while dragging
        if self.mouseLocation == nil {return}

        // You could also use an NSBezierPath for a more high-level API of drawing lines
        guard let context = NSGraphicsContext.current?.cgContext else {return}

        self.handleColor.set()


        context.beginPath()

        context.move(to: NSPoint(x: self.value * self.bounds.width,
                                 y: 0))
        context.addLine(to: NSPoint(x: self.value * self.bounds.width,
                                    y: self.bounds.size.height))

        context.setLineWidth(4)
        context.strokePath()

    }


    // MARK: - Event Handling
    override func mouseDown(with event: NSEvent) {
        self.mouseLocation = event.locationInWindow
    }

    override func mouseDragged(with event: NSEvent) {
        self.mouseLocation = event.locationInWindow
    }

    override func mouseUp(with event: NSEvent) {
        self.mouseLocation = nil
    }
}