SectionedFetchResults

Overview 概要

Use a SectionedFetchResults instance to show or edit Core Data managed objects, grouped into sections, in your app’s user interface. If you don’t need sectioning, use FetchedResults instead.

You request a particular set of results by annotating the fetched results property declaration with a SectionedFetchRequest property wrapper. Indicate the type of the fetched entities with a Results type, and the type of the identifier that distinguishes the sections with a SectionIdentifier type. For example, you can create a request to list all Quake managed objects that the Loading and Displaying a Large Data Feed sample code project defines to store earthquake data, sorted by their time property and grouped by a string that represents the days when earthquakes occurred:

@SectionedFetchRequest<String, Quake>(
    sectionIdentifier: \.day,
    sortDescriptors: [SortDescriptor(\.time, order: .reverse)]
)
private var quakes: SectionedFetchResults<String, Quake>

The quakes property acts as a collection of SectionedFetchResults.Section instances, each containing a collection of Quake instances. The example above depends on the Quake model object declaring both time and day properties, either stored or computed. For best performance with large data sets, use stored properties.

The collection of sections, as well as the collection of managed objects in each section, conforms to the RandomAccessCollection protocol, so you can access them as you would any other collection. For example, you can create nested ForEach loops inside a List to iterate over the results:

List {
    ForEach(quakes) { section in
        Section(header: Text(section.id)) {
            ForEach(section) { quake in
                QuakeRow(quake: quake) // Displays information about a quake.
            }
        }
    }
}

Don’t confuse the Section view that you use to create a hierarchical display with the SectionedFetchResults.Section instances that hold the fetched results.

When you need to dynamically change the request’s section identifier, predicate, or sort descriptors, set the result instance’s sectionIdentifier, nsPredicate, and sortDescriptors or nsSortDescriptors properties, respectively. Be sure that the sorting and sectioning work together to avoid discontinguous sections.

The fetch request and its results use the managed object context stored in the environment, which you can access using the managedObjectContext environment value. To support user interface activity, you typically rely on the viewContext property of a shared NSPersistentContainer instance. For example, you can set a context on your top-level content view using a container that you define as part of your model:

ContentView()
    .environment(
        \.managedObjectContext,
        QuakesProvider.shared.container.viewContext)