# AVPartialAsyncProperty An asynchronous property that constrains its type. ``` class AVPartialAsyncProperty ``` ## Overview This class defines the [`AVAsyncProperty`](/documentation/AVFoundation/AVAsyncProperty) constants for various AVFoundation types, including [`AVAsset`](/documentation/AVFoundation/AVAsset), [`AVAssetTrack`](/documentation/AVFoundation/AVAssetTrack), and [`AVMetadataItem`](/documentation/AVFoundation/AVMetadataItem). ## Topics ### Loading properties [AVAsset](/documentation/AVFoundation/avasset-async-properties) Asynchronous properties for assets. [AVAssetTrack](/documentation/AVFoundation/avassettrack-async-properties) Asynchronous properties for asset tracks. [AVURLAsset](/documentation/AVFoundation/avurlasset-async-properties) Asynchronous properties for URL assets. [AVFragmentedAsset](/documentation/AVFoundation/avfragmentedasset-async-properties) Asynchronous properties for fragmented assets. [AVMetadataItem](/documentation/AVFoundation/avmetadataitem-async-properties) Asynchronous properties for metadata items. [AVComposition](/documentation/AVFoundation/avcomposition-async-properties) Asynchronous properties for compositions. [AVMutableComposition](/documentation/AVFoundation/avmutablecomposition-async-properties) Asynchronous properties for mutable compositions. [AVMovie](/documentation/AVFoundation/avmovie-async-properties) Asynchronous properties for movies. [AVMutableMovie](/documentation/AVFoundation/avmutablemovie-async-properties) Asynchronous properties for mutable movies. [AVFragmentedMovie](/documentation/AVFoundation/avfragmentedmovie-async-properties) Asynchronous properties for fragmented movies. ### Describing a property [`var description: String`](/documentation/AVFoundation/AVPartialAsyncProperty/description) A description of the object. [`static var sidecarURL: AVAsyncProperty`](/documentation/AVFoundation/AVPartialAsyncProperty/sidecarURL) The sidecar URL used by the MediaExtension. The sidecar URL is returned only if the MediaExtension format reader supports sidecar files, and implements this property [MEFileInfo setSidecarFilename:]. Will return nil otherwise. ### Type Properties [`static var constituentFileURLs: AVAsyncProperty`](/documentation/AVFoundation/AVPartialAsyncProperty/constituentFileURLs) The list of file URLs used by the MediaExtension that constitute the asset. The list of file URLs that constitute the asset are returned only for QuickTime reference movies, or if the MediaExtension format reader implements this property [MEFileInfo setConstituentFileNames:]. ## Relationships ### Conforms To [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`Sendable`](/documentation/Swift/Sendable) ### Inherits From [`AVAnyAsyncProperty`](/documentation/AVFoundation/AVAnyAsyncProperty) ### Inherited By [`AVAsyncProperty`](/documentation/AVFoundation/AVAsyncProperty) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)