Extension to add a volume property to MIKMIDISynthesizer on iOS

MIKMIDISynthesizer+Volume.swift

//
//  MIKMIDISynthesizer+Volume.swift
//
//  Created by Andrew Madsen on 3/22/16.
//  Copyright © 2016 Mixed In Key. All rights reserved.
//

import Foundation
import CoreAudio
import MIKMIDI

extension MIKMIDISynthesizer {
    private func linearGainToDB(gain: Float) -> Float { return 20.0 * log10(gain) }
    private func DBToLinearGain(db: Float) -> Float { return pow(10.0, db / 20.0) }

    var volume: Float {
        get {
            var result: Float = 0.0
            let err = AudioUnitGetParameter(instrumentUnit, kAUSamplerParam_Gain, kAudioUnitScope_Global, 0, &result)
            if err != noErr {
                NSLog("Error getting volume of MIKMIDISynthesizer \(self): \(err)")
                return 0.0
            }
            return min(max(DBToLinearGain(result), 0.0), 1.0)
        }
        set {
            let valueInDB = linearGainToDB(min(max(newValue, 0.0), 1.0))
            let err = AudioUnitSetParameter(instrumentUnit, kAUSamplerParam_Gain, kAudioUnitScope_Global, 0, valueInDB, 0); 
            if err != noErr { NSLog("Error setting volume of MIKMIDISynthesizer \(self): \(err)") }
        }
    }
}