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

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)