Metronome ios快速击败视觉滞后

Question

我正试图通过实现苹果提供的示例代码来创建一个节拍器应用程序。一切都很好，但我看到了节拍视觉效果的延迟——它与玩家时间没有正确同步。这是苹果提供的示例代码

let secondsPerBeat = 60.0 / tempoBPM
let samplesPerBeat = Float(secondsPerBeat * Float(bufferSampleRate))
let beatSampleTime: AVAudioFramePosition = AVAudioFramePosition(nextBeatSampleTime)
let playerBeatTime: AVAudioTime = AVAudioTime(sampleTime: AVAudioFramePosition(beatSampleTime), atRate: bufferSampleRate)
// This time is relative to the player's start time.
player.scheduleBuffer(soundBuffer[bufferNumber]!, at: playerBeatTime, options: AVAudioPlayerNodeBufferOptions(rawValue: 0), completionHandler: {
self.syncQueue!.sync() {
self.beatsScheduled -= 1
self.bufferNumber ^= 1
self.scheduleBeats()
}
})
beatsScheduled += 1
if (!playerStarted) {
// We defer the starting of the player so that the first beat will play precisely
// at player time 0. Having scheduled the first beat, we need the player to be running
// in order for nodeTimeForPlayerTime to return a non-nil value.
player.play()
playerStarted = true
}
let callbackBeat = beatNumber
beatNumber += 1
// calculate the beattime for animating the UI based on the playerbeattime.
let nodeBeatTime: AVAudioTime = player.nodeTime(forPlayerTime: playerBeatTime)!
let output: AVAudioIONode = engine.outputNode
let latencyHostTicks: UInt64 = AVAudioTime.hostTime(forSeconds: output.presentationLatency)
//calcualte the final dispatch time which will update the UI in particualr intervals
let dispatchTime = DispatchTime(uptimeNanoseconds: nodeBeatTime.hostTime + latencyHostTicks)**
// Visuals.
DispatchQueue.global(qos: .userInitiated).asyncAfter(deadline: dispatchTime) {
if (self.isPlaying) {
// send current call back beat.
self.delegate!.metronomeTicking!(self, bar: (callbackBeat / 4) + 1, beat: (callbackBeat % 4) + 1)
}
}
}
// my view controller class where i'm showing the beat number
class ViewController: UIViewController ,UIGestureRecognizerDelegate,Metronomedelegate{
@IBOutlet var rhythmlabel: UILabel!
//view did load method
override func viewDidLoad() {

}
//delegate method for getting the beat value from metronome engine and showing in the UI label.
func metronomeTicking(_ metronome: Metronome, bar: Int, beat: Int) {
DispatchQueue.main.async {
print("Playing Beat (beat)")
//show beat in label
self.rhythmlabel.text = "(beat)"
}
}
}

Answer 1

我认为您处理这个问题有点过于复杂，这是没有原因的。您真正需要的只是在启动节拍器时设置DispatchTime，并在DispatchTime启动时激发函数调用，根据所需频率更新调度时间，并在节拍器启用时循环。

我为您准备了一个实现此方法的项目，以便您可以随意使用：https://github.com/ekscrypto/Swift-Tutorial-Metronome

祝你好运！

Metronomer.swift

import Foundation
import AVFoundation
class Metronome {
var bpm: Float = 60.0 { didSet {
bpm = min(300.0,max(30.0,bpm))
}}
var enabled: Bool = false { didSet {
if enabled {
start()
} else {
stop()
}
}}
var onTick: ((_ nextTick: DispatchTime) -> Void)?
var nextTick: DispatchTime = DispatchTime.distantFuture
let player: AVAudioPlayer = {
do {
let soundURL = Bundle.main.url(forResource: "metronome", withExtension: "wav")!
let soundFile = try AVAudioFile(forReading: soundURL)
let player = try AVAudioPlayer(contentsOf: soundURL)
return player
} catch {
print("Oops, unable to initialize metronome audio buffer: (error)")
return AVAudioPlayer()
}
}()
private func start() {
print("Starting metronome, BPM: (bpm)")
player.prepareToPlay()
nextTick = DispatchTime.now()
tick()
}
private func stop() {
player.stop()
print("Stoping metronome")
}
private func tick() {
guard
enabled,
nextTick <= DispatchTime.now()
else { return }
let interval: TimeInterval = 60.0 / TimeInterval(bpm)
nextTick = nextTick + interval
DispatchQueue.main.asyncAfter(deadline: nextTick) { [weak self] in
self?.tick()
}
player.play(atTime: interval)
onTick?(nextTick)
}
}

ViewController.swift

import UIKit
class ViewController: UIViewController {
@IBOutlet weak var bpmLabel: UILabel!
@IBOutlet weak var tickLabel: UILabel!
let myMetronome = Metronome()
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
myMetronome.onTick = { (nextTick) in
self.animateTick()
}
updateBpm()
}
private func animateTick() {
tickLabel.alpha = 1.0
UIView.animate(withDuration: 0.35) {
self.tickLabel.alpha = 0.0
}
}
@IBAction func startMetronome(_: Any?) {
myMetronome.enabled = true
}
@IBAction func stopMetronome(_: Any?) {
myMetronome.enabled = false
}
@IBAction func increaseBpm(_: Any?) {
myMetronome.bpm += 1.0
updateBpm()
}
@IBAction func decreaseBpm(_: Any?) {
myMetronome.bpm -= 1.0
updateBpm()
}
private func updateBpm() {
let metronomeBpm = Int(myMetronome.bpm)
bpmLabel.text = "(metronomeBpm)"
}
}

注意：似乎存在预加载问题，prepareToPlay((在播放之前没有完全加载音频文件，这会导致tick音频文件的首次播放出现一些时间问题。这个问题将留给读者来解决。最初的问题是同步，这应该在上面的代码中演示。