DisplayLink for OSX

DisplayLink.swift

//
//  DisplayLink.swift
//  MetalMac
//
//  Created by Jose Canepa on 8/18/16.
//  Copyright © 2016 Jose Canepa. All rights reserved.
//

import AppKit

/**
 Analog to the CADisplayLink in iOS.
 */
class DisplayLink
{
    let timer  : CVDisplayLink
    let source : DispatchSourceUserDataAdd
    
    var callback : Optional<() -> ()> = nil
    
    var running : Bool { return CVDisplayLinkIsRunning(timer) }
    
    /**
     Creates a new DisplayLink that gets executed on the given queue
     
     - Parameters:
        - queue: Queue which will receive the callback calls
     */
    init?(onQueue queue: DispatchQueue = DispatchQueue.main)
    {
        // Source
        source = DispatchSource.makeUserDataAddSource(queue: queue)
        
        // Timer
        var timerRef : CVDisplayLink? = nil
        
        // Create timer
        var successLink = CVDisplayLinkCreateWithActiveCGDisplays(&timerRef)
        
        if let timer = timerRef
        {
            // Set Output
            successLink = CVDisplayLinkSetOutputCallback(timer,
            {
                (timer : CVDisplayLink, currentTime : UnsafePointer<CVTimeStamp>, outputTime : UnsafePointer<CVTimeStamp>, _ : CVOptionFlags, _ : UnsafeMutablePointer<CVOptionFlags>, sourceUnsafeRaw : UnsafeMutableRawPointer?) -> CVReturn in
            
                // Un-opaque the source
                if let sourceUnsafeRaw = sourceUnsafeRaw
                {
                    // Update the value of the source, thus, triggering a handle call on the timer
                    let sourceUnmanaged = Unmanaged<DispatchSourceUserDataAdd>.fromOpaque(sourceUnsafeRaw)
                    sourceUnmanaged.takeUnretainedValue().add(data: 1)
                }
                
                return kCVReturnSuccess
            
            }, Unmanaged.passUnretained(source).toOpaque())
            
            guard successLink == kCVReturnSuccess else
            {
                NSLog("Failed to create timer with active display")
                return nil
            }
            
            // Connect to display
            successLink = CVDisplayLinkSetCurrentCGDisplay(timer, CGMainDisplayID())
            
            guard successLink == kCVReturnSuccess else
            {
                NSLog("Failed to connect to display")
                return nil
            }
            
            self.timer = timer
        }
        else
        {
            NSLog("Failed to create timer with active display")
            return nil
        }
        
        // Timer setup
        source.setEventHandler(handler:
        {
            [weak self] in self?.callback?()
        })
    }
    
    /// Starts the timer
    func start()
    {
        guard !running else { return }
        
        CVDisplayLinkStart(timer)
        source.resume()
    }
    
    /// Cancels the timer, can be restarted aftewards
    func cancel()
    {
        guard running else { return }
        
        CVDisplayLinkStop(timer)
        source.cancel()
    }
    
    deinit
    {
        if running
        {
            cancel()
        }
    }
}

Note that since mac 10.11 there's CVDisplayLinkSetOutputHandler which makes setting up of the callback simpler.

And they say swift is easy and requires fewer lines of code than objc... I setup a DisplayLink in about 5 lines of code in ObJC.
Swift is for people who love writing complicated code

This is for the Mac though. CADisplayLink for iOS is easy in either Swift or ObjC.