无法使用MPMusicPlayerMediaItemQueueDescriptor设置MPMusicPlayerController队列 [英] Can't set MPMusicPlayerController queue with MPMusicPlayerMediaItemQueueDescriptor

问题描述

此代码导致静音:

let query = MPMediaQuery.songs()
let result = query.items
guard let items = result, items.count > 0 else {return}
let song = items[0]

let player = MPMusicPlayerController.applicationQueuePlayer

let coll = MPMediaItemCollection(items: [song])
let q = MPMusicPlayerMediaItemQueueDescriptor(itemCollection: coll)
player.setQueue(with: q)
player.play()

我已经遍历了代码，我们到达了player.play().我有一个MPMediaItem，并从中正确地形成了MPMediaItemCollection，并从那个中形成了MPMusicPlayerMediaItemQueueDescriptor.那为什么我的玩家不玩呢?

I've stepped through the code, and we reach player.play(). I have an MPMediaItem and I've correctly formed an MPMediaItemCollection from that, and an MPMusicPlayerMediaItemQueueDescriptor from that. So why isn't my player playing?

推荐答案

您发现了一个错误.似乎初始化程序MPMusicPlayerMediaItemQueueDescriptor(itemCollection:)已完全损坏:它导致队列描述符不可用.

You've found a bug. It appears that the initializer MPMusicPlayerMediaItemQueueDescriptor(itemCollection:) is completely broken: it results in an unusable queue descriptor.

可能的解决方法是改为使用另一个初始化程序，即MPMusicPlayerMediaItemQueueDescriptor(query:).

The workaround, where possible, is to use the other initializer instead, namely MPMusicPlayerMediaItemQueueDescriptor(query:).

例如，在这种情况下，您可以编写(在原始代码的最后四行中选择):

For example, in this case, you could write (picking up in the last four lines of the original code):

let coll = MPMediaItemCollection(items: [song])
let predicate = MPMediaPropertyPredicate(
                value: song.persistentID,
                forProperty: MPMediaItemPropertyPersistentID)
let query = MPMediaQuery(filterPredicates: [predicate])
let q = MPMusicPlayerMediaItemQueueDescriptor(query: query)
player.setQueue(with: q)
player.play()

不幸的是，在许多情况下，您无法形成单个查询来获取您真正想要的MPMediaItemCollection.

Unfortunately, there are many circumstances where you can't form a single query that gets the MPMediaItemCollection you really wanted.

在此特定示例中，您也可以通过直接使用MPMediaItemCollection而不是由MPMediaItemCollection制作的MPMusicPlayerMediaItemQueueDescriptor设置播放器的队列来解决此问题.

In this particular example you could work around that, too, by setting the player's queue directly with an MPMediaItemCollection instead of a MPMusicPlayerMediaItemQueueDescriptor made from an MPMediaItemCollection.

但是，有些命令(例如append(_:))需要 一个MPMusicPlayerMediaItemQueueDescriptor，对于这样的事情，整个API基本上都已完成.自iOS 10.1起已对其进行清理，并且在iOS 11.1中仍进行清理.

But alas, there are some commands (such as append(_:)) that require an MPMusicPlayerMediaItemQueueDescriptor, and for things like that, this entire API is basically hosed. It has been hosed since iOS 10.1 and it remains hosed in iOS 11.1.

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