QueryPreview

struct QueryPreview<Operation: Relay.Operation, Content: View>: View

A QueryPreview is a view that makes it easier to create SwiftUI previews for views that use @Fragment or @PaginationFragment to load their data.

All data loaded by Relay needs to orignally come from a query, which can make it tricky to preview leaf fragment views in isolation from the rest of your app. You can define a new query specifically for your preview and provide data for it using previewPayload(), but then you still need a view that fetches the query's data using @Query.

You can use QueryPreview for that rather than defining a new View type.

struct ToDoItem_Previews: PreviewProvider {

static let op = ToDoItemPreviewQuery()

static var previews: some View {

QueryPreview(op) { data in

List(data.user!.todos!) { todoItem in

ToDoItem(todo: todoItem)

}

}

.previewPayload(op, resource: "ToDoItemPreview")

}

}

QueryPreview handles the loading, error, and missing data cases for the query by showing a Text view with information about them, because unless you've made a mistake, these cases shouldn't happen for previews. If the query is able to load its data, the data will be passed on to you to render your preview. This is a handy shortcut to render one or more fragment views in a preview using the results of a query.

See the documentation for previewPayload() for a more complete example of how to use this.

Creating a QueryPreview

init(_:_:)

init(

_ operation: Operation,

_ content: @escaping (Operation.Data) -> Content

)

Creates a new QueryPreview for a given query operation.

Parameters

• operation: The query that the view should load.
• content: The view that should be rendered if the query is able to successfully load its data.