Dynamic Keys

Overview

Sharing uses the SharedKey protocol to express the functionality of loading, saving and subscribing to changes of an external storage system, such as user defaults, the file system and more. Often one needs to dynamically update the key so that the shared state can load different data. A common example of this is using @SharedReader to load data from a SQLite database, and needing to update the query with info specified by the user, such as a search string.

Learn about the techniques to do this below.

Loading a new key

There are two ways to load a new key into @Shared and @SharedReader. If the change is due to the user changing something, such as a search string, you can use load(_:):

.task(id: searchText) {
  do {
    try await $items.load(.search(searchText))
  } catch {
    // Handle error
  }
}

If the change is not due to user action, such as the first appearance of the view, then you can re-assign the projected value of the shared state directly:

init() {
  $items = SharedReader(.search(searchText))
}

SwiftUI views

There is one nuance to be aware of when using @Shared and @SharedReader directly in a SwiftUI view. When the view is recreated (which can happen many times and is an intentional design of SwiftUI), the corresponding @Shared and @SharedReader wrappers can also be created.

If you dynamically change the key of the property wrapper in the view, for example like this:

$value.load(.newKey)
// or...
$value = Shared(.newKey)

…then this key may be reset when the view is recreated. In order to prevent this you can use the version of Shared and SharedReader that works like @State in views:

@State.Shared(.key) var value

See SwiftUICore/State/Shared and SwiftUICore/State/SharedReader for more info.