SwiftUI: Execute functions in Preview Provider - swiftui

Question

I think I'm going about this SwiftUI thing all wrong. It's clear that we're just defining the layout as a structs and there can be limited conventional programming embroiled in the layout. I'm having difficulties thinking like this. What is the best way of doing this?
Take the example below. Project is an NSManagedObject. All I want to do is pass in example record so the SwiftUI will render. Nothing I try works.
struct ProjectView: View
{
#State var project: Project //NSManagedObject
var body: some View
{
TextField("", text: Binding<String>($project.projectName)!)
}
}
struct ProjectView_Previews: PreviewProvider
{
static var previews: some View
{
var p:Project
p = getFirstProject() //returns a Project
return ProjectView(project: p)
}
}
If I try returning the struct it says it cannot preview in the file.
If I don't return the struct I get a Function declares an opaque return type, but has no return statements in its body from which to infer an underlying type error.
UPDATE:
var app = UIApplication.shared.delegate as! AppDelegate
#UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
lazy var persistentContainer: NSPersistentCloudKitContainer = {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
return container
}()
lazy var managedObjectContext: NSManagedObjectContext =
{
return persistentContainer.viewContext
}()
}
And the rest of the code:
func allRecords<T: NSManagedObject>(_ type : T.Type, sort: NSSortDescriptor? = nil) -> [T]
{
let context = app.managedObjectContext
let request = T.fetchRequest()
if let sortDescriptor = sort
{
request.sortDescriptors = [sortDescriptor]
}
do
{
let results = try context.fetch(request)
return results as! [T]
}
catch
{
print("Error with request: \(error)")
return []
}
}
func getCount() -> String
{
let r = allRecords(Project.self)
return String(r.count)
}
struct ProjectView: View
{
// #ObservedObject var project: Project
var body: some View
{
Text(getCount())
// TextField("", text: Binding<String>($project.projectName)!)
}
}
struct ProjectView_Previews: PreviewProvider
{
static var previews: some View
{
ProjectView()
}
}
r.count is returning 0, but in the main application thread it is returning 8. Has app.managedObjectContext not been defined properly? I think this has just got too complicated too quickly.

Answer 1

Assuming getFirstProject works correctly the following should work
struct ProjectView_Previews: PreviewProvider
{
static var previews: some View
{
ProjectView(project: getFirstProject())
}
}
However there are concerns about the following...
struct ProjectView: View
{
#State var project: Project //NSManagedObject
because #State is designed to be internal view state-only thing, but Project in your case is a model, so the recommended scenario for this is to use ObservableObject view model either by conforming Project or as standalone clue class holding Project instance(s).