My apologies if this is not the right question to ask, as I am completely new to SwiftUI and iOS programming in general. The question indicates what I want to do, and the error I'm getting I believe is a red herring because of the SwiftUI compiler. It's likely that I am taking the incorrect approach to solving this problem altogether.
I am using XCode Version 11.2.1 (11B500)
View utilizing the ObservedObject:
struct Results: View {
var jobId: String
#ObservedObject var jobDetailService: JobDetailService
init(jobId: String) {
self.jobId = jobId
jobDetailService = JobDetailService(jobId: jobId)
}
var body: some View {
//... view code here
}
}
And it is within this view that I am getting the error (at the ZStack line) "Generic parameter 'C0' could not be inferred". When I comment out the NavigationLink block, the error goes away. Further, when the Results view does not depend on the jobId parameter (and we construct JobDetailService inline with #ObservedObject var jobDetailService = JobDetailService(), this all works. However, I need to be able to pass the jobId parameter to the JobDetailService in order to make the network call to fetch and publish the data.
struct JobList: View {
#ObservedObject var jobListService = JobListService()
var body: some View {
NavigationView {
List(jobListService.jobs) {job in
ZStack {
JobCard(name: job.fullName, date: job.lastUpdated)
NavigationLink(destination: Results(jobId: job.jobId)) {
EmptyView()
}
}
}
}
}
}
After reading this article, and thinking about Asperi's advice on not solely relying on initialization, I opted to do the following:
Remove custom initializer from JobDetailService and instead instantiate the service inside my Results view. Then in an .onAppear method on the Results view, call the getJobDetail method from JobDetailService which in turn makes the network call that populates the #ObservedObject. This allows me to pass in the parameters I need and control when the network call is made. Maybe not the right pattern for this problem but it works for my use case for now.
I assume the following should help you:
Declaration...
struct Results: View {
#ObservedObject var jobDetailService: JobDetailService
var body: some View {
//... view code here
}
}
... and usage
NavigationLink(destination: Results(jobDetailService: JobDetailService(jobId: jobId))) {
EmptyView()
}
Related
struct NotesView: View {
#State var notesArray = [Note]()
public var deleteid: String
var body: some View {
List{
ForEach(notesArray, id: \._id) { notesArray in
NavigationLink(destination: AddNotesView(addNotesViewIdentifier: "updateNote", id: notesArray._id, title: notesArray.title, note: notesArray.note, noteDate: notesArray.date)){
HStack {
Text(notesArray.title)
deleteid = notesArray._id //ERROR - Cannot assign to property: 'self' is immutable
}
}
}
.onDelete(perform: deleteNoteAtIndex)
}
}
func deleteNoteAtIndex(at offsets: IndexSet){ APIFunctions.functions.DeleteNote(id: _id) }
I was expecting the variable "deleteid" to update.
I assumed you can modify any variable by calling that variable and set it equal to a new value.
Like this
First declare variable:
var deleteid: String
next modify variables string valve
deleteid = notesArray._id
A couple of things:
This isn't directly related to your question, but may help you navigation your own code better... When you create a ForEach view to iterate over an array, you should use a different name for the value that represents each element in the iteration. Here, you're using the name notesArray for your array of notes, then creating a second local variable called notesArray for the loop. That variable inside the block will be an instance of Note, so I'd name it note, e.g.:
ForEach(notesArray, id: \._id) { note in
NavigationLink(destination: AddNotesView(addNotesViewIdentifier: note._id, // etc
}
If you want variables to be modifiable inside views, they should be #State variables. This is important due to the way Swift struct lifecycles work, and how the SwiftUI rendering system loads and reloads structs as it works out what has changed.
I'm not entirely sure what deleteid is supposed to represent here, and it's possible you don't need it at all. If you're using the onDelete modifier to implement SwiftUI's native swipe-to-delete system, SwiftUI will give you an IndexSet, which is a collection (usually of just one) of the positions of the item(s) to delete in the array.
From there, you can find the item(s) at each index and then either remove them, or lookup some other value (e.g., their _id attribute) and do some other operation on them.
So the method you might call in onDelete could look something like:
func deleteNoteAtIndex(offsets: IndexSet) {
// get the array objects that the offsets point to
let notes = offsets.map { noteArray[$0] }
for note in notes {
APIFunctions.functions.deleteNote(id: note._id)
}
}
I'm following a Kodeco tutorial(https://www.kodeco.com/4161005-mvvm-with-combine-tutorial-for-ios) for SwiftUI and Combine which hits an API and shows the data in a list. The tutorial doesn't explain how I can navigate to a detail view from the list, and I would like to implement this, but I'm having trouble adapting my code to do this.
My List is set up like this:
var body: some View {
NavigationView {
List {
characterSection
}
.navigationTitle("Characters")
}
}
Character Section:
var characterSection: some View {
Section {
ForEach(viewModel.dataSource, content: CharacterListRowView.init(viewModel:))
}
}
I would like to be able to do something like the following to navigation to a detail screen, but I'm getting errors:
ForEach(viewModel.dataSource, content: CharacterListRowView.init(viewModel:)) { item in
NavigationLink(destination: viewModel.characterDetailView) {
}
}
This obviously does not work but hopefully you can see what I am trying to do. I'm receiving these errors:
Generic parameter 'ID' could not be inferred,
Incorrect argument label in call (have ':content::', expected '_:id:content:')
Trailing closure passed to parameter of type 'KeyPath<Array.Element, ID>' that does not accept a closure
Here is my row view model in case the issue is with this, could be something to do with the ID?:
import Foundation
import SwiftUI
struct CharacterRowViewModel: Identifiable {
private let item: CharacterListResponse
var id: Int {
return item.char_id
}
var title: String {
guard let title = item.name?.description else { return "" }
return title
}
init(item: CharacterListResponse) {
self.item = item
}
}
Thanks for any help, I am new to SwiftUI.
One of your biggest problems is that your ForEach view is defining its contents twice, and the compiler (quite rightly) doesn't understand.
In the form you're using, ForEach takes two arguments:
the items to loop through
what view to generate for each item in the loop
That second argument is called content and it expects to be given something that receives a single item, and returns the View to use for that item.
When you use the form
ForEach(viewModel.dataSource, content: CharacterListRowView.init(viewModel:))
you're passing in a function – in this case CharacterListRowView.init - that receives a row view model, and returns a CharacterListRowView.
That's convenient when the init method matches the expected function type. But the more common form is the type you're trying to switch to - a block that receives an item, and inside which you build the view for the item. When we use that form, we usually use Swift's trailing block syntax. This allows us to omit the argument name. So when we say
ForEach(items) { item in
// view code here
}
it's the same as
ForEach(items, content: { item in
// view code here
})
Now, look again at the code you've amended:
ForEach(viewModel.dataSource, content: CharacterListRowView.init(viewModel:)) { item in
NavigationLink(destination: viewModel.characterDetailView) {
}
}
Here, you're not replacing the first form of content: with a block form, you're attempting to provide a second content block. The compiler is having a meltdown because you're providing two items to something that only expects one. When this happens, sometimes the compiler's error messages can get very confusing -- it reports how what it's tried to do to make your code work hasn't worked, rather than the direct cause of the failure.
So in your ForEach loop, remove the explicit content::
ForEach(viewModel.dataSource) { item in
NavigationLink(destination: viewModel.characterDetailView) {
}
}
You may still have problems after this step, but hopefully any error messages will be a little bit easier to understand.
Starting to learn SwiftUI I am a bit confused on how Viewss are implemented. View is actually not a type but a protocol which requires a var body of type View.
Problem 1: So the protocol requires it self. Is this not an infinite, recursive loop? Implementation of View requires a var body that implements View which requires a var body... How does this work?
Problem 2: var body is usually implemented as computed property. This is nothing unusual. However, the implementation does not return anything but only "creates" subviews which are not explicitly added to their parent view. Initializing the sub views is enough.
This is not only the case with the body var but with all other views which include other views like HStack, VStack, etc.
struct SomeView: View {
var body: some View {
ViewA()
OtherView()
VStack {
Sub1()
Sub2()
}
}
}
How can this work? Is this valid Swift syntax? I mean a "normal" computed property would look like this, wouldn't it?
var someValue: Int {
let value1 = getValue()
let value2 = SomeOtherValue()
return value1 + value2
}
And not like this:
var someValue: Int {
getValue()
SomeOtherValue()
}
I'm trying to inject my core data viewcontext into several view models, so I don't need to pass one big view model throughout the whole app.
I'm using the SwiftUI lifecycle, so the NSManagedObjectContext is generated here:
#main
struct Core_Data_VM_TestApp: App {
let persistenceController = PersistenceController.shared
var body: some Scene {
WindowGroup {
ContentView()
.environment(\.managedObjectContext, persistenceController.container.viewContext)
}
}
}
Following along this answer I didn't succeed, also after playing along with different initializers.
What I want to do is something like this, which isn't working ('Cannot use instance member 'viewContext' within property initializer; property initializers run before 'self' is available')
Content View
struct ContentView: View {
#Environment(\.managedObjectContext) private var viewContext
#StateObject var itemVM = ItemViewModel(viewContext: viewContext)
(...)
Viewmodel:
class ItemViewModel: ObservableObject {
var viewContext: NSManagedObjectContext
init(viewContext: NSManagedObjectContext) {
self.viewContext = viewContext
}
}
Thanks for any help!
Just get it from your controller. The variable in the ViewModel will look something like
var viewContext: NSManagedObjectContext = PersistenceController.shared.container.viewContext
There is no need to pass it in the init.
Also, as a best practice if several anything are using something you should create an object for that something and put the common code there.
All the ViewModels can reference this single object. It will make changing things and debugging a million times easier as you app grows.
One approach is to not instantiate the #StateObject in the declaration, but in the view's init method. Note that the StateObject struct that wraps the itemVM property is stored as _itemVM:
struct ContentView: View {
#Environment(\.managedObjectContext) private var viewContext
#StateObject var itemVM: ItemViewModel
init() {
_itemVM = StateObject(wrappedValue: ItemViewModel(viewContext: viewContext))
}
// ...
}
There are #State, #ObservedObject and #EnvironmentObject bindings in SwfitUI to share data between views and other objects. Each has its designated usage but #EnvironmentObject seems to be the most powerful and easiest to use. So, can I use it for all state variables and shared data? Are there any downsides to this?
First, #EnvironmentObject is for classes. So if you want to bind primitive type like Int - you can only use Binding.
Second, I think it will couse a problems when you try to define more then one #EnvironmentObject of same type. So, when you can use Binding - you should do that. Thats only my appinion.
class SomeClass: ObservableObject{
#Published var value: Int
init(value: Int){
self.value = value
}
}
struct ContentView: View {
#State var one: SomeClass = SomeClass(value: 1)
#State var two: SomeClass = SomeClass(value: 2)
var body: some View {
Adss().environmentObject(one).environmentObject(two)
}
}
struct Adss: View{
#EnvironmentObject var two: SomeClass
var body: some View{
Text("there must be two: \(two.value)")//prints "1"
}
}
you will have to define all the objects of needed type in straight order even if you don't need them