SwiftUI & Core Data - How do I use a parent record in a predicate?

Question

Is it possible to use a core data record in a predicate inside the @FetchRequest property wrapper in SwiftUI?

I have a list of Project and a list of Tasks. I want to tap on a project and navigate to a list of related tasks for that project. I can't seem to find a way to pass in the parent project in a way that SwiftUI can see before the @FetcheRequest is initialized.

I tried placing the parent project in an EnvironmentObject. This is called when I navigate from the ProjectListView to the TaskListView.

TaskListView()
   .environment(\.managedObjectContext, self.managedObjectContext)
   .environmentObject(self.projectToEdit)

Then in the TaskListView I added tried this:

    @Environment(\.managedObjectContext) var managedObjectContext

    @EnvironmentObject var parentProject: Project

    @FetchRequest(
        entity: Task.entity(),
        sortDescriptors: [
            NSSortDescriptor(keyPath: \Task.name, ascending: true)
        ],
        predicate: NSPredicate(format: String(format: "%@%@", "taskProject", " == %@"), parentProject)
    ) var tasks: FetchedResults<Task>

I get the following error on the line with the predicate.

Cannot use instance member 'parentProject' within property initializer; property initializers run before 'self' is available

So is there a way to write a predicate in some way that can use the parent project? Passing the project to the task view does not seem like it's going to work. How else would I go about using a record in a predicate like this?

Answer 1

The FetchRequest can be dynamically created in the init method. That way you can vary predicate and sort conditions. Here is some sample code to achieve that.

// sample Project class
class Project:NSManagedObject {
    var id : String
    var name : String
}

// sample Task class
class Task:NSManagedObject {
    var id : String
    var prjId : String
    var name : String
}

// Task List View
struct TaskListView: View {
    @Environment(\.managedObjectContext) var managedObjectContext

    private var tasksRequest: FetchRequest<Task>
    private var tasks: FetchedResults<Task> { tasksRequest.wrappedValue }

    private var project:Project

    // init Task with Project
    init(_ project:Project) {
        self.project = project

        // create FetchRequest
        self.tasksRequest = FetchRequest(
            entity: Task.entity(),
            sortDescriptors: [NSSortDescriptor(key: "name", ascending:true)],
            predicate: NSPredicate(format: "prjId == %@", project.id))
    }

    var body: some View {
        VStack {
            Section(header: Text("Tasks under \(project.name):")) {
                // access the fetched objects
                ForEach(tasks, id:\.id) { task in
                    Text("\(task.name)")
                }
            }
        }
    }
}

Then the call to TaskListView() would look like:

// call to TaskListView
TaskListView(self.projectToEdit)
    .environment(\.managedObjectContext, self.managedObjectContext)