Q:

Swift Solid Metronome System

I am trying to build a reliable solid system to build a metronome in my app using SWIFT.
I Have built what seems to be a solid system using NSTimer so far.. The only issue I am having right now is when the timer starts the first 2 clicks are off time but then it catches into a solid timeframe. 
Now after all my research I have seen people mention you should use other Audio tools not relying on NSTimer.. Or if you choose use NSTimer then it should be on its own thread. Now I see many confused by this Including myself and I would love to get down to the bottom of this Metronome business and get this solved and share it with all those who are struggling. 
UPDATE
So I have implemented and cleaned up at this point after the feedback I had last recieved. At this point here is how my code is structured. Its playing back. But I am still getting 2 fast clicks in the beginning and then it settles in. 
I apologize on my ignorance for this one. I hope I am on the right path. 
I currently am prototyping another method as well. Where I have a very small audio file with one click and dead space at the end of it with the correct duration until for a loop point for specific tempos. I am looping this back and works very well. But the only thing Is I dont get to detect the loop points for visual updates so I have my basic NStimer just detecting the timing intervals underneath the audio being processed and it seems to matchup very well throughout and no delay. But I still would rather get it all with this NSTimer. If you can easily spot my error would be great for one more kick in the right direction and I am sure it can work soon! Thanks so much.  
    //VARIABLES 
    //AUDIO
    var clickPlayer:AVAudioPlayer = AVAudioPlayer()
    let soundFileClick = NSBundle.mainBundle().pathForResource("metronomeClick", ofType: ".mp3")

    //TIMERS
    var metroTimer = NSTimer()
    var nextTimer = NSTimer()

    var previousClick = CFAbsoluteTimeGetCurrent()    //When Metro Starts Last Click

    //Metro Features
    var isOn            = false
    var bpm             = 60.0     //Tempo Used for beeps, calculated into time value
    var barNoteValue    = 4        //How Many Notes Per Bar (Set To Amount Of Hits Per Pattern)
    var noteInBar       = 0        //What Note You Are On In Bar

    //********* FUNCTIONS ***********

func startMetro()
{
     MetronomeCount()

    barNoteValue    = 4         // How Many Notes Per Bar (Set To Amount Of Hits Per Pattern)
    noteInBar       = 0         // What Note You Are On In Bar
    isOn            = true      //

        }

        //Main Metro Pulse Timer
        func MetronomeCount()
        {
            previousClick = CFAbsoluteTimeGetCurrent()

        metroTimer = NSTimer.scheduledTimerWithTimeInterval(60.0 / bpm, target: self, selector: Selector ("MetroClick"), userInfo: nil, repeats: true)

        nextTimer = NSTimer(timeInterval: (60.0/Double(bpm)) * 0.01, target: self, selector: "tick:", userInfo: ["bpm":bpm], repeats: true)
    }

    func MetroClick()
    {
        tick(nextTimer)
    }

    func tick(timer:NSTimer)
    {
        let elapsedTime:CFAbsoluteTime = CFAbsoluteTimeGetCurrent() - previousClick
        let targetTime:Double = 60/timer.userInfo!.objectForKey("bpm")!.doubleValue!
        if (elapsedTime > targetTime) || (abs(elapsedTime - targetTime) < 0.003)
        {
            previousClick = CFAbsoluteTimeGetCurrent()

            //Play the click here
            if noteInBar == barNoteValue
            {
                clickPlayer.play()    //Play Sound
                noteInBar = 1
            }
            else//If We Are Still On Same Bar
            {
                clickPlayer.play()    //Play Sound
                noteInBar++             //Increase Note Value
            }

            countLabel.text = String(noteInBar)     //Update UI Display To Show Note We Are At
        }

    }

A:

A metronome built purely with NSTimer will not be very accurate, as Apple explains in their documentation.

Because of the various input sources a typical run loop manages, the effective resolution of the time interval for a timer is limited to on the order of 50-100 milliseconds. If a timer’s firing time occurs during a long callout or while the run loop is in a mode that is not monitoring the timer, the timer does not fire until the next time the run loop checks the timer.

I would suggest using an NSTimer that fires on the order of 50 times per desired tick (for example, if you would like a 60 ticks per minute, you would have the NSTimeInterval to be about 1/50 of a second. 
You should then store a CFAbsoluteTime which stores the "last tick" time, and compare it to the current time. If the absolute value of the difference between the current time and the "last tick" time is less than some tolerance (I would make this about 4 times the number of ticks per interval, for example, if you chose 1/50 of a second per NSTimer fire, you should apply a tolerance of around 4/50 of a second), you can play the "tick."
You may need to calibrate the tolerances to get to your desired accuracy, but this general concept will make your metronome a lot more accurate.
Here is some more information on another SO post. It also includes some code that uses the theory I discussed. I hope this helps!
Update
The way you are calculating your tolerances is incorrect. In your calculations, notice that the tolerance is inversely proportional to the square of the bpm. The problem with this is that the tolerance will eventually be less than the number of times the timer fires per second. Take a look at this graph to see what I mean. This will generate problems at high BPMs. The other potential source of error is your top bounding condition. You really don't need to check an upper limit on your tolerance, because theoretically, the timer should have already fired by then. Therefore, if the elapsed time is greater than the theoretical time, you can fire it regardless. (For example if the elapsed time is 0.1s and and the actual time with the true BPM should be 0.05s, you should go ahead and fire the timer anyways, no matter what your tolerance is).
Here is my timer "tick" function, which seems to work fine. You need to tweak it to fit your needs (with the downbeats, etc.) but it works in concept.
func tick(timer:NSTimer) {
    let elapsedTime:CFAbsoluteTime = CFAbsoluteTimeGetCurrent() - lastTick
    let targetTime:Double = 60/timer.userInfo!.objectForKey("bpm")!.doubleValue!
    if (elapsedTime > targetTime) || (abs(elapsedTime - targetTime) < 0.003) {
        lastTick = CFAbsoluteTimeGetCurrent()  
        # Play the click here
    }
}

My timer is initialized like so: nextTimer = NSTimer(timeInterval: (60.0/Double(bpm)) * 0.01, target: self, selector: "tick:", userInfo: ["bpm":bpm], repeats: true)

A:

Ok! You can't get things right basing on time, because somehow we need to deal with DA converters and their frequency - samplerate. We need to tell them the exact sample to start play the sound. Add a single view iOS app with two buttons start and stop and insert this code into ViewController.swift. I keep things simple and it's just an Idea of how we can do this. Sorry for forcing try... This one is made with swift 3.  Also check out my project on GitHub https://github.com/AlexShubin/MetronomeIdea
Swift 3
   import UIKit
    import AVFoundation

    class Metronome {

        var audioPlayerNode:AVAudioPlayerNode
        var audioFile:AVAudioFile
        var audioEngine:AVAudioEngine

        init (fileURL: URL) {

            audioFile = try! AVAudioFile(forReading: fileURL)

            audioPlayerNode = AVAudioPlayerNode()

            audioEngine = AVAudioEngine()
            audioEngine.attach(self.audioPlayerNode)

            audioEngine.connect(audioPlayerNode, to: audioEngine.mainMixerNode, format: audioFile.processingFormat)
            try! audioEngine.start()

        }

        func generateBuffer(forBpm bpm: Int) -> AVAudioPCMBuffer {
            audioFile.framePosition = 0
            let periodLength = AVAudioFrameCount(audioFile.processingFormat.sampleRate * 60 / Double(bpm))
            let buffer = AVAudioPCMBuffer(pcmFormat: audioFile.processingFormat, frameCapacity: periodLength)
            try! audioFile.read(into: buffer)
            buffer.frameLength = periodLength
            return buffer
        }

        func play(bpm: Int) {

            let buffer = generateBuffer(forBpm: bpm)

   self.audioPlayerNode.play()

            self.audioPlayerNode.scheduleBuffer(buffer, at: nil, options: .loops, completionHandler: nil)

        }

        func stop() {

            audioPlayerNode.stop()

        }

    }

    class ViewController: UIViewController {

        var metronome:Metronome

        required init?(coder aDecoder: NSCoder) {

            let fileUrl = Bundle.main.url(forResource: "Click", withExtension: "wav")

            metronome = Metronome(fileURL: fileUrl!)

            super.init(coder: aDecoder)

        }

        @IBAction func StartPlayback(_ sender: Any) {

            metronome.play(bpm: 120)

        }

        @IBAction func StopPlayback(_ sender: Any) {

            metronome.stop()

        }

    }

A:

Thanks to the great work already done on this question by vigneshv & CakeGamesStudios, I was able to put together the following, which is an expanded version of the metronome timer discussed here.
Some highlights:

It's updated for Swift v5
It uses a Grand Central Dispatch timer to run on a separate queue, rather than just a regular NSTimer (see here for more details)
It uses more calculated properties for clarity
It uses delegation, to allow for any arbitrary 'tick' action to be handled by the delegate class (be that playing a sound from AVFoundation, updating the display, or whatever else - just remember to set the delegate property after creating the timer). This delegate would also be the one to distinguish beat 1 vs. others, but that'd be easy enough to add within this class itself if desired.
It has a % to Next Tick property, which could be used to update a UI progress bar, etc.

Any feedback on how this can be improved further is welcome!
protocol BPMTimerDelegate: class {
    func bpmTimerTicked()
}

class BPMTimer {

    // MARK: - Properties

    weak var delegate: BPMTimerDelegate? // The class's delegate, to handle the results of ticks
    var bpm: Double { // The speed of the metronome ticks in BPM (Beats Per Minute)
        didSet {
            changeBPM() // Respond to any changes in BPM, so that the timer intervals change accordingly
        }
    }
    var tickDuration: Double { // The amount of time that will elapse between ticks
        return 60/bpm
    }
    var timeToNextTick: Double { // The amount of time until the next tick takes place
        if paused {
            return tickDuration
        } else {
            return abs(elapsedTime - tickDuration)
        }
    }
    var percentageToNextTick: Double { // Percentage progress from the previous tick to the next
        if paused {
            return 0
        } else {
            return min(100, (timeToNextTick / tickDuration) * 100) // Return a percentage, and never more than 100%
        }
    }

    // MARK: - Private Properties

    private var timer: DispatchSourceTimer!
    private lazy var timerQueue = DispatchQueue.global(qos: .utility) // The Grand Central Dispatch queue to be used for running the timer. Leverages a global queue with the Quality of Service 'Utility', which is for long-running tasks, typically with user-visible progress. See here for more info: https://www.raywenderlich.com/5370-grand-central-dispatch-tutorial-for-swift-4-part-1-2
    private var paused: Bool
    private var lastTickTimestamp: CFAbsoluteTime
    private var tickCheckInterval: Double {
        return tickDuration / 50 // Run checks many times within each tick duration, to ensure accuracy
    }
    private var timerTolerance: DispatchTimeInterval {
        return DispatchTimeInterval.milliseconds(Int(tickCheckInterval / 10 * 1000)) // For a repeating timer, Apple recommends a tolerance of at least 10% of the interval. It must be multiplied by 1,000, so it can be expressed in milliseconds, as required by DispatchTimeInterval.
    }
    private var elapsedTime: Double {
        return CFAbsoluteTimeGetCurrent() - lastTickTimestamp // Determine how long has passed since the last tick
    }

    // MARK: - Initialization

    init(bpm: Double) {

        self.bpm = bpm
        self.paused = true
        self.lastTickTimestamp = CFAbsoluteTimeGetCurrent()
        self.timer = createNewTimer()
    }

    // MARK: - Methods

    func start() {

        if paused {
            paused = false
            lastTickTimestamp = CFAbsoluteTimeGetCurrent()
            timer.resume() // A crash will occur if calling resume on an already resumed timer. The paused property is used to guard against this. See here for more info: https://medium.com/over-engineering/a-background-repeating-timer-in-swift-412cecfd2ef9
        } else {
            // Already running, so do nothing
        }
    }

    func stop() {

        if !paused {
            paused = true
            timer.suspend()
        } else {
            // Already paused, so do nothing
        }
    }

    // MARK: - Private Methods

    // Implements timer functionality using the DispatchSourceTimer in Grand Central Dispatch. See here for more info: http://danielemargutti.com/2018/02/22/the-secret-world-of-nstimer/
    private func createNewTimer() -> DispatchSourceTimer {

        let timer = DispatchSource.makeTimerSource(queue: timerQueue) // Create the timer on the correct queue
        let deadline: DispatchTime = DispatchTime.now() + tickCheckInterval // Establish the next time to trigger
        timer.schedule(deadline: deadline, repeating: tickCheckInterval, leeway: timerTolerance) // Set it on a repeating schedule, with the established tolerance
        timer.setEventHandler { [weak self] in // Set the code to be executed when the timer fires, using a weak reference to 'self' to avoid retain cycles (memory leaks). See here for more info: https://learnappmaking.com/escaping-closures-swift/
            self?.tickCheck()
        }
        timer.activate() // Dispatch Sources are returned initially in the inactive state, to begin processing, use the activate() method

        // Determine whether to pause the timer
        if paused {
            timer.suspend()
        }

        return timer
    }

    private func cancelTimer() {

        timer.setEventHandler(handler: nil)
        timer.cancel()
        if paused {
            timer.resume() // If the timer is suspended, calling cancel without resuming triggers a crash. See here for more info: https://forums.developer.apple.com/thread/15902
        }
    }

    private func replaceTimer() {

        cancelTimer()
        timer = createNewTimer()
    }

    private func changeBPM() {

        replaceTimer() // Create a new timer, which will be configured for the new BPM
    }

    @objc private func tickCheck() {

        if (elapsedTime > tickDuration) || (timeToNextTick < 0.003) { // If past or extremely close to correct duration, tick
            tick()
        }
    }

    private func tick() {

        lastTickTimestamp = CFAbsoluteTimeGetCurrent()
        DispatchQueue.main.sync { // Calls the delegate from the application's main thread, because it keeps the separate threading within this class, and otherwise, it can cause errors (e.g. 'Main Thread Checker: UI API called on a background thread', if the delegate tries to update the UI). See here for more info: https://stackoverflow.com/questions/45081731/uiapplication-delegate-must-be-called-from-main-thread-only
            delegate?.bpmTimerTicked() // Have the delegate respond accordingly
        }
    }

    // MARK: - Deinitialization

    deinit {

        cancelTimer() // Ensure that the timer's cancelled if this object is deallocated
    }
}

