A simple synthesizer class in Swift

Synth.swift

class Synthesizer {

    // MARK: - Variables
    var audioEngine: AVAudioEngine!
    var sourceNode: AVAudioSourceNode!

    var time = Float.zero
    var frequencyRamp = Float.zero
    var currentFrequency: Float = 20 {
        didSet {
            frequencyRamp = currentFrequency - oldValue
        }
    }

    // MARK: - Setup
    public func setup() {
        audioEngine = AVAudioEngine()
        sourceNode = AVAudioSourceNode(renderBlock: { (_, _, frameCount, bufferList) -> OSStatus in
            let listPointer = UnsafeMutableAudioBufferListPointer(bufferList)
            let rampValue = self.frequencyRamp
            let frequency = self.currentFrequency
            let period = 1 / frequency

            for frame in 0..<Int(frameCount) {
                let completion = self.time
                let sample = sin(2.0 * .pi * (frequency + rampValue * completion) * self.time)
                self.time += 1 / Float(self.audioEngine.outputNode.inputFormat(forBus: 0).sampleRate)
                self.time = fmod(self.time, period)
                for buffer in listPointer {
                    let bufferPointer = UnsafeMutableBufferPointer<Float>(buffer)
                    bufferPointer[frame] = sample
                }
            }
            return noErr
        })

        let format = audioEngine.outputNode.inputFormat(forBus: 0)
        let inputFormat = AVAudioFormat(commonFormat: format.commonFormat,
                                        sampleRate: format.sampleRate,
                                        channels: 1,
                                        interleaved: format.isInterleaved)

        audioEngine.attach(sourceNode)
        audioEngine.connect(sourceNode, to: audioEngine.outputNode, format: inputFormat)

        do {
            try audioEngine.start()
        } catch {
            print("Error: " + error.localizedDescription)
        }
    }
}