How to create a background context
How to create a background context 관련
Updated for Xcode 15
Although SwiftData doesn't have the concept of child contexts, you can create as many peer or sibling contexts as you need, all attached to the same model container, and mixing foreground and background tasks freely.
Important
ModelContext and all SwiftData models do not conform to Sendable, which means they can't be transferred between Swift actors. ModelContainer does conform to Sendable, so please use that to create your background context.
Creating a new model context is as simple as calling its initializer with a model container, like this:
class BackgroundDataHander {
private var context: ModelContext
init(with container: ModelContainer) {
context = ModelContext(container)
}
As we've used a class above, it's important to create it away from the main actor by using async let, Task.detached(), or other concurrency options. If you use an actor rather than a class, the context will be bound to its actor rather than the main actor used for your main context.
Important
If you intend to create a background context using an actor in order to do bulk data imports, I would recommend against storing the context as a property because the extra actor synchronization will slow down your code dramatically.
When a save is triggered on your background context, its changes will automatically be reflected in your other contexts, including updating any @Query properties in your SwiftUI views.
Apple's official view is that autosave is enabled for the main context, but not for model context created by hand. In my experience this is inconsistent: yes, the main context always has autosave enabled, but any extra contexts you create by hand may or may not.
Very roughly – and this is simply through observation rather than actual facts that will stand the test of time – if you create a new model context on the main actor, it will have autosave enabled, but if you create it to run on a separate actor then autosave will be disabled.
Tips
Until we have more certainty around when autosave is actually enabled or not, I would recommend being explicit when creating your background contexts.
Because SwiftData models are not sendable, you should transfer them between tasks using their id property. This allows you to load the object on your background context, without worrying about data races.