Swift错误“ UIButton.currentTitle必须仅在主线程中使用”

Question

我刚刚通过以下链接关注了您的教程：

URL：https://www.youtube.com/watch?v=aiXvvL1wNUc

我遇到一个错误，提示“ UIButton.currentTitle必须仅在主线程中使用”

由于我是制作应用程序的新手，所以不太确定如何解决此问题，有人可以帮助我吗？

非常感谢，并提前致谢！

import UIKit
import MediaPlayer

class GenreButtonScreen: UIViewController {
    var musicPlayer = MPMusicPlayerController.applicationMusicPlayer

    @IBAction func genreButtonTapped(_ sender: UIButton) {
        MPMediaLibrary.requestAuthorization { (status) in
            if status == .authorized {
                self.playGenre(genre: sender.currentTitle!)
            }
        }
    }

    @IBAction func stopButtonTapped(_ sender: UIButton) {
        musicPlayer.stop()
    }

    @IBAction func nextButtonTapped(_ sender: UIButton) {
        musicPlayer.skipToNextItem()
    }

    func playGenre(genre:String) {
        musicPlayer.stop()

        let query = MPMediaQuery()
        let predicate = MPMediaPropertyPredicate(value: genre, forProperty: MPMediaItemPropertyGenre)

        query .addFilterPredicate(predicate)

        musicPlayer.setQueue(with: query)
        musicPlayer.shuffleMode = .songs 
        musicPlayer.play()
    }
}

Answer 1

MPMediaLibrary.requestAuthorization是一个异步调用，并且只能在主线程中修改布局组件。您应该使用这种方式：

 MPMediaLibrary.requestAuthorization { (status) in
    if status == .authorized {
        DispatchQueue.main.async { 
           self.playGenre(genre: sender.currentTitle ?? String()) 
        }
    }
}

Answer 2

由于requestAuthorization的完成是在后台线程中，因此您需要像这样

一样在主队列中嵌入任何 UIKit 代码

DispatchQueue.main.async { 
  self.playGenre(genre: sender.currentTitle!) 
}

---

@IBAction func genreButtonTapped(_ sender: UIButton) {
    MPMediaLibrary.requestAuthorization { (status) in
        if status == .authorized {
            DispatchQueue.main.async {
              self.playGenre(genre: sender.currentTitle!) 
            }
        }
    }
}