How to store SwiftData attributes in an external file
How to store SwiftData attributes in an external file êŽë š
Updated for Xcode 15
SwiftData is great for storing information such as strings, integers, dates, and custom Codable objects, but if youâre storing larger data such as images or even movies itâs a good idea to have SwiftData store them in external, peripheral files, then reference those filenames in your SwiftData object.
Helpfully, SwiftData makes this trivial using the .externalStorage attribute, which suggests to SwiftData that this data is best stored separately. It will, if needed, stash that data externally from your main SwiftData storage, store only the filename to the external data, then transparently connect the two so you donât need to take any further action â for the most part external data behaves no differently from internal data.
For example, if you were storing a game playerâs avatar, you might have code like this:
@Model class User {
var name: String
var score: Int
@Attribute(.externalStorage) var avatar: Data
init(name: String, score: Int, avatar: Data) {
self.name = name
self.score = score
self.avatar = avatar
}
}
You can combine this with other attributes, such as encryption, like this:
@Attribute(.externalStorage, .allowsCloudEncryption) var avatar: Data
There are three important provisos when dealing with external storage, and itâs important that youâre aware of them.
First, the .externalStorage attributes merely suggests to SwiftData that some information is best stored outside the main SQLite data store, but it doesnât have to honor that request. In my tests with this attribute, SwiftData seems happy to store up to about 128K of a Data object right inside its main storage area, with larger data automatically being saved externally.
Second, if youâre using String rather than Data, SwiftData seems happy to store strings of unlimited size without using external files at all no matter whether you use the attribute or not.
Third, for the most part all this is an implementation detail: whether the data is stored right inside the database or not doesnât matter to us, because itâs loaded and saved the same as internal data. However, you canât use externally stored properties inside predicates, because the external files arenât visible to the underlying data store â if you see an error message like [Foundation.__NSSwiftData _bytesPtrForStore]: unrecognized selector sent to instance 0x600000c74ae0 with userInfo of (null) chances are you might have fallen into this trap.
