iOS 13 上的 AVAssetReferenceRestrictions

Question

工作区：iOS 13.0、Xcode 11.0

TL;DR：iOS 13 中的 AVAssetReferenceRestrictions 有问题吗？

第 1 部分：

在AVAsset.h 中，AVAssetReferenceRestrictions 定义为：

  @enum         AVAssetReferenceRestrictions
  @abstract     These constants can be passed in to AVURLAssetReferenceRestrictionsKey to control the resolution of references to external media data.

  @constant     AVAssetReferenceRestrictionForbidNone
    Indicates that all types of references should be followed.
  @constant     AVAssetReferenceRestrictionForbidRemoteReferenceToLocal
    Indicates that references from a remote asset (e.g. referenced via http URL) to local media data (e.g. stored in a local file) should not be followed.
  @constant     AVAssetReferenceRestrictionForbidLocalReferenceToRemote
    Indicates that references from a local asset to remote media data should not be followed.
  @constant     AVAssetReferenceRestrictionForbidCrossSiteReference
    Indicates that references from a remote asset to remote media data stored at a different site should not be followed.
  @constant     AVAssetReferenceRestrictionForbidLocalReferenceToLocal
    Indicates that references from a local asset to local media data stored outside the asset's container file should not be followed.
  @constant     AVAssetReferenceRestrictionForbidAll
    Indicates that only references to media data stored within the asset's container file should be allowed.
*/
typedef NS_OPTIONS(NSUInteger, AVAssetReferenceRestrictions) {
    AVAssetReferenceRestrictionForbidNone = 0UL,
    AVAssetReferenceRestrictionForbidRemoteReferenceToLocal = (1UL << 0),
    AVAssetReferenceRestrictionForbidLocalReferenceToRemote = (1UL << 1),
    AVAssetReferenceRestrictionForbidCrossSiteReference = (1UL << 2),
    AVAssetReferenceRestrictionForbidLocalReferenceToLocal = (1UL << 3),
    AVAssetReferenceRestrictionForbidAll = 0xFFFFUL,

    AVAssetReferenceRestrictionDefaultPolicy = AVAssetReferenceRestrictionForbidLocalReferenceToRemote
};

而AVAsset的属性定义为：

@property       referenceRestrictions
@abstract       Indicates the reference restrictions being used by the receiver.
@discussion
    For AVURLAsset, this property reflects the value passed in for AVURLAssetReferenceRestrictionsKey, if any. See AVURLAssetReferenceRestrictionsKey below for a full discussion of reference restrictions. The default value for this property is AVAssetReferenceRestrictionForbidNone.
@property (nonatomic, readonly) AVAssetReferenceRestrictions referenceRestrictions API_AVAILABLE(macos(10.7), ios(5.0), tvos(9.0)) API_UNAVAILABLE(watchos);

因此，尽管该属性的文档明确指出默认值为AVAssetReferenceRestrictionForbidNone，但AVAssetReferenceRestrictionDefaultPolicy 当前为AVAssetReferenceRestrictionForbidLocalReferenceToRemote。是文档没有正确更新，还是 AVAssetReferenceRestrictionDefaultPolicy 与我预期的不同？

第 2 部分：

在对应的 Swift 中，它是这样定义（转换的？）：

  @enum         AVAssetReferenceRestrictions
  @abstract     These constants can be passed in to AVURLAssetReferenceRestrictionsKey to control the resolution of references to external media data.

  @constant     AVAssetReferenceRestrictionForbidNone
    Indicates that all types of references should be followed.
  @constant     AVAssetReferenceRestrictionForbidRemoteReferenceToLocal
    Indicates that references from a remote asset (e.g. referenced via http URL) to local media data (e.g. stored in a local file) should not be followed.
  @constant     AVAssetReferenceRestrictionForbidLocalReferenceToRemote
    Indicates that references from a local asset to remote media data should not be followed.
  @constant     AVAssetReferenceRestrictionForbidCrossSiteReference
    Indicates that references from a remote asset to remote media data stored at a different site should not be followed.
  @constant     AVAssetReferenceRestrictionForbidLocalReferenceToLocal
    Indicates that references from a local asset to local media data stored outside the asset's container file should not be followed.
  @constant     AVAssetReferenceRestrictionForbidAll
    Indicates that only references to media data stored within the asset's container file should be allowed.

public struct AVAssetReferenceRestrictions : OptionSet {

    public init(rawValue: UInt)

    public static var forbidRemoteReferenceToLocal: AVAssetReferenceRestrictions { get }

    public static var forbidLocalReferenceToRemote: AVAssetReferenceRestrictions { get }

    public static var forbidCrossSiteReference: AVAssetReferenceRestrictions { get }

    public static var forbidLocalReferenceToLocal: AVAssetReferenceRestrictions { get }

    public static var forbidAll: AVAssetReferenceRestrictions { get }

    public static var defaultPolicy: AVAssetReferenceRestrictions { get }
}

这里没有AVAssetReferenceRestrictionForbidNone 的选项。但是文档明确表示确实如此。那么，到底是谁的错，还是我的误解？

第 3 部分：我是如何在这里结束的？

我不得不检查这些 因为我们能够从UIImagePickerController 中选择一个视频， 然后以AVURLAsset 形式导出它以使用 AVAssetExportSession。 一切都很好，直到 iOS 13.0 都可以无缝运行。该功能适用​​于 iOS 12.4.1 及更低版本的设备。

AVAssetReferenceRestrictions 的用法是在初始化器中 AVURLAsset，我相信默认值已经改变了 不允许我们再导出。

Answer 1

这不是你的错。

直到 2019 年 6 月 19 日，Apple referenceRestrictions documentation 中的默认值为 forbidNone：

此属性的默认值为 禁止无 .看 AVURLAssetReferenceRestrictionsKey 有关参考限制的完整讨论。

后来这个页面改成了this：

此属性的默认值为 defaultPolicy。有关引用限制的完整讨论，请参阅 AVURLAssetReferenceRestrictionsKey。

defaultPolicy 现在是forbidLocalReferenceToRemote，仅在 iOS 13 中可用。

但是，我们有这样一段：

对于 AVURLAsset，此属性反映为 AVURLAssetReferenceRestrictionsKey 传入的值，如果有。

因此，显然，不传递限制键应该与 forbidNone 一样工作。

你试过了吗？

let asset = AVURLAsset(url: url, options: nil)