如何让AudioKit AKPluckedString优雅地淡出？

import AudioKit

// 初始化AKPluckedString
let pluckedString = AKPluckedString()

// 创建音量参数并设置初始音量
let volume = AUValue(0.5)

// 创建ADSR包络生成器
let adsr = AKOperationGenerator { _ in
    let trigger = AKOperation.trigger
    let attackDuration = AUValue(0.1)
    let decayDuration = AUValue(0.2)
    let sustainLevel = AUValue(0.3)
    let releaseDuration = AUValue(0.5)
    
    return AKOperation.pluckedString(trigger: trigger,
                                     frequency: pluckedString.frequency,
                                     amplitude: trigger.adsr(attackDuration: attackDuration,
                                                             decayDuration: decayDuration,
                                                             sustainLevel: sustainLevel,
                                                             releaseDuration: releaseDuration))
}

// 连接ADSR包络生成器到音量参数
let mixer = AKMixer(pluckedString, adsr)
mixer.volume = volume

// 启动AudioKit引擎
AudioKit.output = mixer
try? AudioKit.start()

// 播放AKPluckedString
pluckedString.trigger()

// 在需要淡出的时候，将ADSR包络生成器的释放时间设置为较短的值
adsr.parameters[3] = AUValue(0.2) // 设置释放时间为0.2秒，实现淡出效果