快速询问音乐权限以获取初始视图控制器

Question

我有一个拆分视图控制器，其顶视图控制器设置为表视图控制器，该控制器用于显示播放列表列表供选择。 首次加载应用程序时，它会请求音乐访问权限。 回答“是”确实会授予它权限，但表视图不显示任何播放列表。 我最终不得不杀死该应用程序并再次运行。 我是在错误的位置申请音乐库许可吗？ 正是在该顶部视图控制器的viewWillAppear中，并将我正在使用的播放列表（因为某些播放列表被筛选掉）存储在一个播放列表中。

        override func viewWillAppear(_ animated: Bool) {
    self.clearsSelectionOnViewWillAppear = self.splitViewController!.isCollapsed
    super.viewWillAppear(animated)

    checkMediaAccessAndSetup()
}

func checkMediaAccessAndSetup() {
    let authorizationStatus = MPMediaLibrary.authorizationStatus()
    switch authorizationStatus {
    case .notDetermined:
        // Show the permission prompt.
        MPMediaLibrary.requestAuthorization({[weak self] (newAuthorizationStatus: MPMediaLibraryAuthorizationStatus) in
            // Try again after the prompt is dismissed.
            self?.checkMediaAccessAndSetup()
        })
    case .denied, .restricted:
        // Do not use MPMediaQuery.
        return
    default:
        // Proceed as usual.
        break
    }
    // Do stuff with MPMediaQuery
    self.setupPlaylistStore()
    tableView.reloadData()
}

Answer 1

您的代码的主要问题是

• 您完全无法解决在后台线程上调用requestAuthorization完成函数这一事实。 您需要跳出主线程才能在接口上工作。

• 您已省略了所有重要的.authorized案例。 当你有工作要做，这取决于你的授权状态，你必须现在就做，如果你被授权，但授权后，如果您没有确定。

因此，这是进行一致性授权检查的正确方案（其中f()是您可能总是想做的事情）：

let status = MPMediaLibrary.authorizationStatus()
switch status {
case .authorized:
    f()
case .notDetermined:
    MPMediaLibrary.requestAuthorization() { status in
        if status == .authorized {
            DispatchQueue.main.async {
                f()
            }
        }
    }
// ...
}

如果将代码抽象为实用工具方法，其中f可以是任意值，则可以在应用程序中可能需要授权的任何地方进行此操作-不仅仅是在启动时。

Answer 2

感谢您的评论，它为我提供了正在进行的多线程的线索，并且如果类中没有播放列表可以继续检查并重新加载表数据，我可以通过计时器调用对其进行修复。

override func viewWillAppear(_ animated: Bool) {
    self.clearsSelectionOnViewWillAppear = self.splitViewController!.isCollapsed
    super.viewWillAppear(animated)

    checkMediaAccess()
    self.setupPlaylistStore()
    tableView.reloadData()
    if store.allPlaylists.count < 1 {
        playlistTimer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector:     #selector(self.playlistTimerCall), userInfo: nil, repeats: true)
    }
}

@objc func playlistTimerCall() {
    self.setupPlaylistStore()
    if store.allPlaylists.count > 1 {
        tableView.reloadData()
        playlistTimer?.invalidate()
    }
}
func checkMediaAccess() {
    let authorizationStatus = MPMediaLibrary.authorizationStatus()
    switch authorizationStatus {
    case .notDetermined:
        // Show the permission prompt.
        MPMediaLibrary.requestAuthorization({[weak self] (newAuthorizationStatus: MPMediaLibraryAuthorizationStatus) in
            // Try again after the prompt is dismissed.
            self?.checkMediaAccess()
        })
    case .denied, .restricted:
        // Do not use MPMediaQuery.
        return
    default:
        // Proceed as usual.
        break
    }
}


func setupPlaylistStore() {

    // purge store
    store.clearAllPlaylists()

    // create a query of media items in playlist
    let myPlayListsQuery = MPMediaQuery.playlists()
    if myPlayListsQuery.collections != nil {
        playlists = myPlayListsQuery.collections!
    }

    // add playlists to MyPlaylist(s)
    if playlists.count > 0 {
        for index in 0...playlists.count - 1 {
            let playlist = playlists[index]
            store.addPlaylist(playlist: playlist as! MPMediaPlaylist)
        }
    }
    var toBeRemoved = [Int]()
    let defaults = UserDefaults.standard
    if defaults.bool(forKey: "exclude_smart_playlists") {
        //smart
        for index in 0...(playlists.count - 1) {
            let playlist = playlists[index]
            let theAttributes = playlist.value(forProperty: MPMediaPlaylistPropertyPlaylistAttributes) as! Int
            if theAttributes == 2 {
                toBeRemoved.append(index)
            }
        }
    }
    if defaults.bool(forKey: "exclude_folders") {
        //folders
        for index in 0...(playlists.count - 1) {
            let playlist = playlists[index]
            let isFolder = playlist.value(forProperty: "isFolder")
            let stringIsFolder = String("\(String(describing: isFolder))")
            if ((stringIsFolder.range(of: "1")) != nil) {
                toBeRemoved.append(index)
            }
        }
    }
    //sort from the last to the first so i don't reindex
    let reverseSortedPlaylists = toBeRemoved.sorted(by: >)

    // remove the unwanted playlists
    for list in reverseSortedPlaylists {
        store.removePlaylist(index: list)
    }
}