Application-specific information that is saved with the asset-download object.
- iOS 5.0+
This dictionary is for storing arbitrary key-value pairs associated with the asset download. The keys and values must be valid property-list types; if any are not, an exception is raised. Examples of download-related information you might store in the dictionary are the file (issue) name, the download URL, the encoding mechanisms, and custom identifiers. However, for performance reasons the dictionary content should be as minimal as possible. You can also assign
nil to this property.