快速询问初始视图控制器的音乐权限

Question

我有一个分割视图控制器，顶部视图控制器设置为表视图控制器，即显示供选择的播放列表列表。第一次加载该应用程序时，它要求获得音乐访问权限。回答“是”确实给了它权限，但“表”视图没有显示播放列表。最后，我不得不杀死这个应用程序，并再次运行它。我是不是在错误的地方要求音乐图书馆的许可？它是在顶级视图控制器的viewWillAppear中，并将我使用的播放列表(因为有些是筛选出来的)存储在一类播放列表中。

        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

代码的主要问题是

• 您完全无法处理在后台线程上调用requestAuthorization完成函数这一事实。您需要跳出主线程来完成接口的工作。
• 您忽略了最重要的.authorized案例。当您有工作要做，这取决于您的授权状态，您必须现在做，如果您是授权的，但授权后，如果您还没有确定。

因此，这是进行一致授权检查的正确方案(如果可以的话，f()是您一直想要做的事情)：

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

如果您将这些代码抽象为实用程序方法(其中f可以是任何东西)，那么您可以在应用程序中的任何地方都这样做--在应用程序中需要授权--而不仅仅是在启动时。

Answer 2

谢谢你的评论，它为我提供了正在进行的多线程的线索，如果课堂上没有播放列表来继续检查和重新加载表数据，我就可以通过计时器调用来修复它。

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)
    }
}