ios - 如何从 iOS 中的音乐播放器(对于 Midi 文件)获取持续时间？

Question

我需要 MIDI 文件的持续时间和结束事件。我正在使用下面的代码来播放 midi 文件。我试过了，但没有发现任何东西。提前致谢

var s: MusicSequence?
NewMusicSequence(&s)

let midiFilePath = Bundle.main.path(forResource: "CCL-20180308-A-04", ofType: "mid")
let midiFileURL = URL(fileURLWithPath: midiFilePath ?? "")

MusicSequenceFileLoad(s!, midiFileURL as CFURL, MusicSequenceFileTypeID(rawValue: 0)!, [])

var p: MusicPlayer?
NewMusicPlayer(&p)

MusicPlayerSetSequence(p!, s)
MusicPlayerPreroll(p!)
MusicPlayerStart(p!)

usleep(3 * 100 * 100)
var now: MusicTimeStamp = 0
MusicPlayerGetTime(p!, &now)

Answer 1

这会起作用:

var s: MusicSequence!
NewMusicSequence(&s)

let midiFileURL = Bundle.main.url(forResource: "CCL-20180308-A-04", withExtension: "mid")!

MusicSequenceFileLoad(s!, midiFileURL as CFURL, .midiType, [])

var p: MusicPlayer!
NewMusicPlayer(&p)

MusicPlayerSetSequence(p, s)
MusicPlayerPreroll(p)
MusicPlayerStart(p)

var numTracks: UInt32 = 0
MusicSequenceGetTrackCount(s, &numTracks)
let length = (0..<numTracks).map { (index: UInt32) -> (MusicTimeStamp) in
    var track: MusicTrack?
    MusicSequenceGetIndTrack(s, index, &track)
    var size = UInt32(MemoryLayout<MusicTimeStamp>.size)
    var scratchLength = MusicTimeStamp(0)
    MusicTrackGetProperty(track!, kSequenceTrackProperty_TrackLength, &scratchLength, &size)
    return scratchLength
}.max() ?? 0
var lengthInSeconds = Float64(0)
MusicSequenceGetSecondsForBeats(s, length, &lengthInSeconds)

self.timer = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { (t) in
    var now: MusicTimeStamp = 0
    MusicPlayerGetTime(p, &now)
    var nowInSeconds = Float64(0)
    MusicSequenceGetSecondsForBeats(s, now, &nowInSeconds)
    print("\(nowInSeconds) / \(lengthInSeconds)")
})

您遗漏的重要部分是通过找到最长轨道的长度来获得总序列长度。您可以使用 MusicTrackGetProperty() 的 kSequenceTrackProperty_TrackLength 属性获取轨道的长度。

就其值(value)而言，CoreMIDI 已经够粗糙了，尤其是在 Swift 中，我认为值得使用更高级别的 API。查看AVMIDIPlayer ，它是 AVFoundation 的一部分。如果您需要更复杂的东西，您可以查看 MIKMIDI ，这是一个开源的 MIDI 库，它建立在 Core MIDI 的基础上，但增加了大量的附加功能，并且更易于使用。 (免责声明:我是 MIKMIDI 的原作者和维护者。)使用 MIKMIDI，您可以这样做:

let midiFileURL = Bundle.main.url(forResource: "CCL-20180308-A-04", withExtension: "mid")!
let sequence = try! MIKMIDISequence(fileAt: midiFileURL)
let sequencer = MIKMIDISequencer(sequence: sequence)
sequencer.startPlayback()

self.timer = Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true, block: { (t) in
    let now = sequencer.timeInSeconds(forMusicTimeStamp: sequencer.currentTimeStamp, options: [])
    let length = sequence.durationInSeconds
    print("\(now) / \(length)")
})

简单一点!如果您尝试进行录音、更复杂的合成、将 MIDI 路由到/从外部设备等，事情会变得更加有趣。