I'm using a core data child context to separate temporary entities from my main context, but when I pass in the main context as an environment variable, I don't have access to it in order to initialize the child context and get a 'self' used before all stored properties are initialized error
Any suggestions?
struct CreatePlayerView: View {
#State private var newPlayer: PlayerEntity
#Environment(\.managedObjectContext) private var managedObjectContext
private let childManagedObjectContext: NSManagedObjectContext
init() {
childManagedObjectContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
\\ 'self' used before all stored properties are initialized
childManagedObjectContext.parent = managedObjectContext
newPlayer = PlayerEntity(context: childManagedObjectContext)
}
var body: some View {
EditPlayerViewDetail(
player: $newPlayer,
onDone: { player in
try? childManagedObjectContext.save()
}
)
}
}
Related
I am trying to create a Recpipe object (CoreData entity) and then pass that object to a ChildView after it's created. What's the best way to do this?
My first attempt was to create a #StateObject in the MainView and then pass that to ChildViews as an #ObservedObject, but this seems to only work great for objects that already exist. The #StateObject is a 'get only' property so I can't modify it based on the function return. Perhaps I don't really want to create a #StateObject in this circumstance?
struct MainView: View {
#State private var presentChildView: Bool = false
#StateObject private var recipe: Recipe = Recipe()
var body: some View {
VStack {
NavigationLink(destination: ChildView(recipe: recipe), isActive: $presentChildView) {
EmptyView()
}
Button("Action", action: {
recipe = functionThatReturnsRecipe()
presentChildView = true
})
}
}
}
struct ChildView: View {
#ObservedObject private var recipe: Recipe
var body: some View {
Text(recipe.title)
}
}
Normally you hold the object in #State or even better inside a struct to hold all the ChildView's related vars so it can be tested independently, e.g.
struct ChildViewConfig {
var recipe: Recipe?
var isPresented = false
mutating func present(viewContext: NSManagedObjectContext) {
recipe = Recipe(context: viewContext)
isPresented = true
}
// might have other save or dismiss mutating funcs here.
}
struct MainView: View {
#Environment(\.managedObjectContext) private var viewContext
#State private var config = ChildViewConfig()
var body: some View {
VStack {
Button("Present") {
config.present(context: viewContext)
}
}
.sheet(isPresented: $config.isPresented, onDismiss: nil) {
ChildView(recipe: config.recipe!)
}
}
}
struct ChildView: View {
#ObservedObject private var recipe: Recipe
var body: some View {
Text(recipe.title)
}
}
This answer to another question uses the more advanced .sheet(item:onDismiss) that you may prefer because is designed for this use case of editing an item and allows for the use of a child context scratch pad because when setting the item to nil means the item.childContext will be discarded if the editing in the sheet is cancelled.
Learn more about this #State Config struct pattern at 4:18 in Data Essentials in SwiftUI (WWDC 2020)
You can have instead some concept of app state or manager (anything represented business logic) which would hold/update your recipes, like
class AppState: ObservableObject {
#Published var recipe: Recipe = Recipe()
func refreshRecipe() {
self.recipe = functionThatReturnsRecipe() // << now works !!
}
private func functionThatReturnsRecipe() -> Recipe {
Recipe()
}
}
struct MainView: View {
#State private var presentChildView: Bool = false
#StateObject private var appState = AppState() // << non-changeable
var body: some View {
VStack {
NavigationLink(destination: ChildView(recipe: appState.recipe), isActive: $presentChildView) {
EmptyView()
}
Button("Action", action: {
appState.refreshRecipe()
presentChildView = true
})
}
}
}
I'm having trouble wrapping my head around how to use Combine in SwiftUI. I'm accustomed to using key-value observation in AppKit an UIKit because view controllers don't need to know about each other and can just react to some global objects that help determine state.
For example, in an AppKit/UIKit app, I would create a global state object like this:
//Global State file
#objc class AppState: NSObject {
#objc dynamic var project: Project?
}
//Create an instance I can access anywhere in my app
let app = AppState()
Then in a view controller, I can get notified of any changes to my app-wide project instance and react accordingly:
//View Controller
class MyViewController: NSViewController{
var observerProject: NSKeyValueObservation?
override func viewDidLoad() {
observerProject = app.observe(\.project) { object, change in
self.refreshData()
}
}
func refreshData(){
//Query my persistent store and update my UI
}
}
What is the Combine/SwiftUI analog to this?
Do I need to create a Publisher and then listen to my global object changes? If so, how do I make my Core Data #FetchRequest (whose predicate includes my global Project object) respond in real-time?
I've done things the old way for so long that this transition to SwiftUI/Combine is rather confusing to me. 🙂
#FetchRequest doesn't work well with a dynamic predicate (There are some workarounds in SO) you will have to use the "old school" NSFetchedResultsController for that and put it into an ObservableObject. Here is a video with a sample. It is a lot of setup and code.
import SwiftUI
import Combine
class AppState: ObservableObject {
static let shared = AppState()
#Published var project: String?
//Just to mimic updates
var count = 0
private init() {
//Mimic getting updates to project
Timer.scheduledTimer(withTimeInterval: 2, repeats: true){ timer in
print("Timer")
self.project = self.count.description
self.count += 1
if self.count >= 15{
timer.invalidate()
}
}
}
}
class MyViewModel: ObservableObject{
#Published var refreshedData: String = "init"
let appState: AppState = AppState.shared
var projectCancellable: AnyCancellable?
init() {
projectCancellable = appState.$project.sink(receiveValue: {
value in
print(value ?? "nil")
self.refreshData()
})
}
func refreshData() {
refreshedData = Int.random(in: 0...100).description
}
}
struct MyView: View {
#StateObject var vm: MyViewModel = MyViewModel()
var body: some View {
VStack{
Text(vm.refreshedData)
}
}
}
struct MyView_Previews: PreviewProvider {
static var previews: some View {
MyView()
}
}
I'm trying to use a ViewModel between the ContentView and Core Data in SwiftUI. Xcode builder runs the App but I get an immediate error: Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) for var recList.
Can anyone help?
Following a simple example of what I'm doing:
ListViewModel:
class ListViewModel: ObservableObject {
var recRequest: FetchRequest<Newdb>
var recList: FetchedResults<Newdb>{recRequest.wrappedValue} <-------- error appears here
#Published var records = [ViewModel]()
init() {
self.recRequest = FetchRequest(entity: Newdb.entity(), sortDescriptors: [])
fetchEntries()
}
func fetchEntries() {
self.records = recList.map(ViewModel.init)
}
}
ViewModel:
class ViewModel {
var name: String = ""
init(db: Newdb) {
self.name = db.name!
}
}
ContentView:
struct ContentView: View {
#ObservedObject var listViewModel: ListViewModel
init() {
self.listViewModel = ListViewModel()
}
var body: some View {
ForEach(listViewModel.records, id: \.name) { index in
Text(index.name)
}
}
}
two things I noticed; your ListViewModel is an ObservableObject but you do not have any #Published var ...
Also when creating a class such as ListViewModel you cannot use "recRequest" as you do in recList, because it is not created yet. It is created in the init() method not before.
Do your "recList = FetchedResults{recRequest.wrappedValue}" somewhere else, like in the fetchEntries().
From what I can tell, FetchRequest is a property wrapper.
It is supposed to wrap something, e.g.;
#FetchRequest(
entity: User.entity(),
sortDescriptors: []
) var users: FetchedResults<User> // users are 'wrapped' in a FetchRequest instance
It makes sense that wrappedValue is nil because there's nothing to be wrapped in
self.recRequest = FetchRequest(entity: Newdb.entity(), sortDescriptors: [])
You might want to double-check its usage.
I saw same type of error but with different kind of code here, so I think it's better to ask a new question on this context. I have attempted to "find a specific entity" from core data by trying to pass a string variable (which use as a key to find that entity) called title into #FetchRequest. This is part of the code I have used
struct AccountMainPage: View {
//*** User input ***
var title: String
//*** Core data enviroment initialisation ***
#Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
#Environment(\.managedObjectContext) var managedObjectContext
#FetchRequest(fetchRequest: Accounts.getSpecificAccounts(findTitle: title)) var fetchedAccount: FetchedResults<Accounts>
var body: some View {
//SOME CODE HERE
}
}
The public class Accounts has the extension:
extension Accounts {
static func getSpecificAccounts(findTitle: String) -> NSFetchRequest<Accounts> {
let request: NSFetchRequest<Accounts> = Accounts.fetchRequest() as! NSFetchRequest<Accounts>
let findDescriptor = NSPredicate(format: "title == %#",findTitle)
request.predicate = findDescriptor
return request
}
}
However, the line with #FetchRequest(fetchRequest: Accounts.getSpecificAccounts(findTitle: title)) var fetchedAccount: FetchedResults<Accounts> has a syntax error:
Cannot use instance member 'title' within property initializer; property initializers run before 'self' is available
Is there something wrong with my code?
#FetchRequest is dynamic property which is initialised, as any other property, before your AccountMainPage init is called, so self is not available yet, that is why you cannot use title property which is a member of self, and that is about what compiler error tells.
So here is a possible solution: we initialise fetch request property with stub request and then in init, which is called later, reinitialise it with real fetch request.
Here is an approach demo (all unrelated things cut):
struct ContentView: View {
var title: String
#Environment(\.managedObjectContext) var managedObjectContext
#FetchRequest(fetchRequest: Accounts.fetchRequest()) var fetchedAccount: FetchedResults<Accounts>
init(title: String) {
self.title = title
_fetchedAccount = FetchRequest<Accounts>(fetchRequest: Accounts.getSpecificAccounts(findTitle: title))
}
...
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?
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)