以下是一个AVAudioEngine MIDI文件播放器的示例代码，包括当前进度和MIDI结束回调的实现：

import AVFoundation

class MidiPlayer {

    var engine: AVAudioEngine!
    var player: AVAudioPlayerNode!
    var sequencer: AVAudioSequencer!
    var currentPosition: TimeInterval = 0
    var duration: TimeInterval = 0
    var completionHandler: (() -> Void)?

    init() {
        engine = AVAudioEngine()
        player = AVAudioPlayerNode()
        engine.attach(player)
        engine.connect(player, to: engine.mainMixerNode, format: nil)
    }

    func loadMidiFile(url: URL) {
        do {
            sequencer = try AVAudioSequencer(url: url)
            sequencer.prepareToPlay()
            duration = sequencer.duration.seconds
        } catch {
            print("Failed to load MIDI file: \(error.localizedDescription)")
        }
    }

    func play() {
        do {
            try engine.start()
            player.play()
            sequencer.currentPosition = currentPosition
            sequencer.addPlayer(player)
        } catch {
            print("Failed to start audio engine: \(error.localizedDescription)")
        }
    }

    func pause() {
        player.pause()
        currentPosition = sequencer.currentPosition.seconds
        engine.stop()
    }

    func stop() {
        player.stop()
        currentPosition = 0
        engine.stop()
    }

    func addCompletionHandler(completion: @escaping () -> Void) {
        completionHandler = completion
        NotificationCenter.default.addObserver(self, selector: #selector(handleMidiCompletion), name: .AVAudioEngineConfigurationChange, object: nil)
    }

    @objc func handleMidiCompletion(notification: NSNotification) {
        if sequencer.isPlaying {
            return
        }

        completionHandler?()
        completionHandler = nil
        NotificationCenter.default.removeObserver(self, name: .AVAudioEngineConfigurationChange, object: nil)
    }
}

使用示例：

let midiPlayer = MidiPlayer()
let midiFileURL = Bundle.main.url(forResource: "example", withExtension: "mid")!
midiPlayer.loadMidiFile(url: midiFileURL)
midiPlayer.addCompletionHandler {
    print("MIDI playback completed")
}
midiPlayer.play()

这个示例代码创建了一个MidiPlayer类，封装了AVAudioEngine和AVAudioPlayerNode的功能。它提供了加载MIDI文件、播放、暂停和停止的方法，并且可以在MIDI播放完成时添加一个回调处理程序。要使用它，首先创建一个MidiPlayer对象，然后调用loadMidiFile方法加载MIDI文件，使用addCompletionHandler方法添加一个回调处理程序，最后调用play方法开始播放MIDI文件。

请将"example.mid"替换为您自己的MIDI文件的名称，并确保将其添加到项目的资源中。