表达式解析为未使用的函数

Question

首先让我说我对编程很新.我要做的是添加一个按钮,当按下该按钮播放音乐时,再次按下时按钮音乐停止.理想情况下,当第三次按下按钮时,音乐将重置.在尝试实现这一目标的同时,我收到错误消息"表达式解析为未使用的函数",因为我很新,我在网上找到的所有帮助对我都没有任何意义.

import UIKit
import AVFoundation

class ViewController: UIViewController {
    @IBOutlet weak var janitor: UIImageView!
    var pianoSound = NSURL(fileURLWithPath: NSBundle.mainBundle().pathForResource("C", ofType: "m4a")!)
    var audioPlayer = AVAudioPlayer()

    override func viewDidLoad() {
        super.viewDidLoad()

        audioPlayer = AVAudioPlayer(contentsOfURL: pianoSound, error: nil)
        audioPlayer.prepareToPlay()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }


        @IBAction func PianoC(sender: AnyObject) {

        audioPlayer.play()

            if audioPlayer.playing { audioPlayer.stop} else {  audioPlayer.play}
}
}

Answer 1

在Martin R的评论之后在这里俯冲......

if audioPlayer.playing { audioPlayer.stop} else {  audioPlayer.play}

在这一行中,您不是在调用stop和play函数,而只是访问它们.Resolving to an unused function是想告诉你,你有一个函数返回一个函数型的表达式,但你永远不会调用它(audioPlayer.stop和audioPlayer.play这里是有问题的表述).

要摆脱这个错误,并可能产生正确的行为,请尝试调用函数.

if audioPlayer.playing { 
    audioPlayer.stop()
} else {  
    audioPlayer.play()
}

Answer 2

Swift 4：只需在方法名称旁边添加大括号“()”

override func viewWillAppear(_ animated: Bool) {
   addView //error: Expression resolves to an unused function
}

func addView(){
}

解决方案：

override func viewWillAppear(_ animated: Bool) {
   addView()
}

Answer 3

这是Brian的答案的简化版本:

audioPlayer.playing
    ? audioPlayer.stop()
    : audioPlayer.play()