AudioKit Apple Sequencer初始化报错：Invalid beat range or track is empty

已将AudioKit SDK接入简易项目，参照AudioKit Cookbook中的鼓序列器示例创建基础序列。已复制示例中的MIDI和音频文件到Xcode项目并确认路径正确，但初始化AppleSequencer时始终报错：

2023-06-11 20:13:22.124246-0600 NoteFlow[4788:2349120] [midi] MIDI.swift:init():52:Initializing MIDI (MIDI.swift:init():52)
2023-06-11 20:13:22.694782-0600 NoteFlow[4788:2349120] [seq] SequenceTrack.cpp:977 Clear: Invalid beat range or track is empty

已注释所有sequencer相关代码、开启后台音频模式，问题仍存在。代码如下：

import AudioKit
import AudioKitEX
import AVFoundation

class DrumSequencerConductor: ObservableObject, HasAudioEngine {
    let engine = AudioEngine()
    let drums = MIDISampler(name: nil)
    var sequencer: AppleSequencer = AppleSequencer(fromURL: Bundle.main.url(forResource: "4tracks", withExtension: "mid")!) 
    init() {
        engine.output = drums
        do {
            let clapURL: URL = Bundle.main.url(forResource: "clap_D#1", withExtension: "wav", subdirectory: "Samples")!
            let clapFile = try AVAudioFile(forReading: clapURL)
            let snareURL: URL = Bundle.main.url(forResource: "snare_D1", withExtension: "wav", subdirectory: "Samples")!
            let snareFile = try AVAudioFile(forReading: snareURL)
            let hihatURL: URL = Bundle.main.url(forResource: "closed_hi_hat_F#1", withExtension: "wav", subdirectory: "Samples")!
            let hihatFile = try AVAudioFile(forReading: hihatURL)
            try drums.loadAudioFiles([
                clapFile,
                snareFile,
                hihatFile
            ])
        } catch {
            print("error while finding audio files \(error)")
        }
//        sequencer.clearRange(start: Duration(beats: 0), duration: Duration(beats: 100))
//        sequencer.debug()
//        sequencer.setGlobalMIDIOutput(drums.midiIn)
//        sequencer.enableLooping(Duration(beats: 4))
//        sequencer.setTempo(150)
//        sequencer.tracks[0].add(noteNumber: 60, velocity: 100, position: Duration(beats: 0), duration: Duration(beats: 1))
//        sequencer.tracks[0].add(noteNumber: 60, velocity: 100, position: Duration(beats: 1), duration: Duration(beats: 1))
//        sequencer.tracks[3].add(noteNumber: 30, velocity: 127, position: Duration(beats: 2.5), duration: Duration(beats: 0.5))
//        for i in 0...40 {
//            sequencer.tracks[0].add(noteNumber: MIDINoteNumber(i), velocity: 100, position: Duration(beats: 0), duration: Duration(beats: 1))
//        }
    }
}

1. 检查MIDI文件有效性 #

报错核心提示“Invalid beat range or track is empty”，大概率是4tracks.mid文件本身存在问题：

• 用MIDI编辑工具（如GarageBand、MidiEditor）打开文件，确认至少有一条包含有效节拍范围的轨道，文件本身不是空的。
• 若原文件无效，重新导出示例中的MIDI文件，或创建一个包含4小节空白轨道的测试MIDI文件替换。

2. 安全初始化AppleSequencer #

当前代码在属性定义阶段强制解包文件URL，且初始化逻辑早于init()执行，容易引发隐性问题。修改为在init()中安全初始化：

var sequencer: AppleSequencer?

init() {
    engine.output = drums
    do {
        // 音频文件加载逻辑保留...
        
        // 安全加载MIDI文件
        guard let midiURL = Bundle.main.url(forResource: "4tracks", withExtension: "mid") else {
            print("MIDI文件未找到")
            return
        }
        sequencer = try AppleSequencer(fromURL: midiURL)
        
        // 后续操作需添加可选绑定
        if let seq = sequencer {
            seq.setGlobalMIDIOutput(drums.midiIn)
            seq.enableLooping(Duration(beats: 4))
            seq.setTempo(150)
            // 其他sequencer配置代码
        }
    } catch {
        print("初始化错误: \(error)")
    }
}

3. 确认文件的Bundle配置 #

即使路径正确，Xcode可能漏加文件到目标Bundle：

• 选中4tracks.mid和Samples文件夹下的音频文件，在右侧“File Inspector”中，确认“Target Membership”已勾选项目目标。
• 添加Samples文件夹时，选择“Create folder references”而非“Create groups”，否则subdirectory: "Samples"无法正确定位文件。

4. 用空序列器验证逻辑 #

若MIDI文件问题暂时无法解决，可先创建空序列器验证基础逻辑：

sequencer = AppleSequencer()
// 添加对应数量的空轨道
sequencer?.newTrack()
sequencer?.newTrack()
sequencer?.newTrack()
sequencer?.newTrack()
// 再执行音符添加等操作

若此方式无报错，即可确认问题出在导入的MIDI文件上。

内容的提问来源于stack exchange，提问作者iamluisg