iOS音乐播放器应用

Question

以下代码给我一个错误。它给出错误消息“类型'值'UIToolbar'没有成员值”

此行发生错误：musicPlayer.volume = volumeSlider.value

musicPlayer.delegate = self
musicSlider.minimumValue = 0
musicSlider.maximumValue = Float(musicPlayer.duration)

musicSlider.value = Float(musicPlayer.currentTime)

// error line below:       
musicPlayer.volume = volumeSlider.value

musicPlayer.play()
songNameLabel.text = musicFiles[currentIndex]
animateSongNameLabel()

为什么我收到此错误？