SwiftUI selectable lists where selection variable is ordered - ios

I'll keep it simple, I have a List with UUID identified items. This list is selectable, when EditMode() is entered user can tap items, and these get added to the selection variable:
And the selection variable is:
#State var selection = Set<UUID?>()
However, the issue is that I care about the order in which the items are selected. And in the selection array, the items are not updated in a FIFO or LIFO way, they seem to be random or perhaps it depends on the value of the UUID but the point is it doesn't conserve the order in which they are added.
I considered using a stack to keep track of what is added, but the List + selection combination in SwiftUI doesn't appear to have built in methods to inform of new additions e.g. UUID 1234 has been added. I can thing of "not clean" ways to make it work like iterating through the whole selection Set every time selection.count changes and add the "new item" to the stack but I wouldn't want to come down to that.

So after some research, the List constructor that enables selection in .editMode does specify that the selection variable has to be a Set and nothing but a Set:
public init(selection: Binding<Set<SelectionValue>>?, #ViewBuilder content: () -> Content)
So I decided to implement my own array that keeps track of the selection order. I'll show how in case it helps anyone, but it is quite rudimentary:
#State var selectedItemsArray=[UUID]()
...
List(selection: $selection) {
...
}
.onChange(of: selection) { newValue in
if newValue.count>selectedItemsArray.count {
for uuid in newValue {
if !selectedItemsArray.contains(uuid) {
selectedItemsArray.append(uuid)
break
}
}
}
if newValue.count<selectedItemsArray.count {
for uuid in selectedItemsArray {
if !newValue.contains(uuid) {
selectedItemsArray.remove(at: selectedItemsArray.firstIndex(of: uuid)!)
break
}
}
}
}

Related

SwiftUI 4: navigationDestination()'s destination view isn't updated when state changes

While experiments with the new NavigationStack in SwiftUI 4, I find that when state changes, the destination view returned by navigationDestination() doesn't get updated. See code below.
struct ContentView: View {
#State var data: [Int: String] = [
1: "One",
2: "Two",
3: "Three",
4: "Four"
]
var body: some View {
NavigationStack {
List {
ForEach(Array(data.keys).sorted(), id: \.self) { key in
NavigationLink("\(key)", value: key)
}
}
.navigationDestination(for: Int.self) { key in
if let value = data[key] {
VStack {
Text("This is \(value)").padding()
Button("Modify It") {
data[key] = "X"
}
}
}
}
}
}
}
Steps to reproduce the issue:
Run the code and click on the first item in the list. That would bring you to the detail view of that item.
The detail view shows the value of the item. It also has a button to modify the value. Click on that button. You'll observe that the value in the detail view doesn't change.
I debugged the issue by setting breakpoints at different place. My observations:
When I clicked the button, the code in List get executed. That's as expected.
But the closure passed to navigationDestination() doesn't get executed, which explains why the detail view doesn't get updated.
Does anyone know if this is a bug or expected behavior? If it's not a bug, how can I program to get the value in detail view updated?
BTW, if I go back to root view and click on the first item to go to its detail view again, the closure passed to navigationDestination() get executed and the detail view shows the modified value correctly.
#NoeOnJupiter's solution and #Asperi's comment are very helpful. But as you see in my comments above, there were a few details I wasn't sure about. Below is a summary of my final understanding, which hopefully clarifies the confusion.
navigationDestination() takes a closure parameter. That closure captures an immutable copy of self.
BTW, SwiftUI takes advantage of property wrapper to make it possible to "modify" an immutable value, but we won't discuss the details here.
Take my above code as an example, due to the use of #State wrapper, different versions of ContentView (that is, the self captured in the closure) share the same data value.
The key point here is I think the closure actually has access to the up-to-date data value.
When an user clicks on the "Modify it" button, the data state changes, which causes body re-evaluted. Since navigationDestination() is a function in body, it get called too. But a modifier function is just shortcut to modifier(SomeModifier()). The actual work of a Modifier is in its body. Just because a modifier function is called doesn't necessarilly means the corresponding Modifier's body gets called. The latter is a mystery (an implementation detail that Apple don't disclose and is hard to guess). See this post for example (the author is a high reputation user in Apple Developer Forum):
In my opinion, it definitely is a bug, but not sure if Apple will fix it soon.
One workaround, pass a Binding instead of a value of #State variables.
BTW, I have a hypothesis on this. Maybe this is based on a similar approach as how SwiftUI determines if it recalls a child view's body? My guess is that it might be a design, instead of a bug. For some reason (performance?) the SwiftUI team decided to cache the view returned by navigationDestination() until the NavigationStack is re-constructed. As a user I find this behavior is confusing, but it's not the only example of the inconsistent behaviors in SwiftUI.
So, unlike what I had thought, this is not an issue with closure, but one with how modifier works. Fortunately there is a well known and robust workaround, as suggested by #NoeOnJupiter and #Asperi.
Update: an alternative solution is to use EnvironmentObject to cause the placeholder view's body get re-called whenever data model changes. I ended up using this approach and it's reliable. The binding approach worked in my simple experiments but didn't work in my app (the placeholder view's body didn't get re-called when data model changed. I spent more than one day on this but unfortunately I can't find any way to debug it when binding stopped working mysteriously).
The button is correctly changing the value. By default navigationDestination does't create a Binding relation between the parent & child making the passed values immutable.
So you should create a separate struct for the child in order to achieve Bindable behavior:
struct ContentView: View {
#State var data: [Int: String] = [
1: "One",
2: "Two",
3: "Three",
4: "Four"
]
var body: some View {
NavigationStack {
List {
ForEach(Array(data.keys).sorted(), id: \.self) { key in
NavigationLink("\(key)", value: key)
}
}
.navigationDestination(for: Int.self) { key in
SubContentView(key: key, data: $data)
}
}
}
}
struct SubContentView: View {
let key: Int
#Binding var data: [Int: String]
var body: some View {
if let value = data[key] {
VStack {
Text("This is \(value)").padding()
Button("Modify It") {
data[key] = "X"
}
}
}
}
}

How to manually reload any row in swiftUI

I wanted to create List in SwiftUI with following requirements:
Each row has some detail, and Toggle which represents its isEnabled status.
If someone enables or disables that toggle, then based on some logic, Either I have to allow its to be enabled, OR not allow it to be enabled OR allow it to be enabled, but have to disable other object from other row.
I have my ViewModel and Model as below
I have my model and View model as somewhat like this
class MyViewModel:ObservableObject{
#Published myObjArray:[MyObject]
....
}
class MyObject:Identifiable{
var id:..
var isEnabled:Bool
var title:...
....
....
}
As MyObject is used at multiple places in project, so, I dont want to touch It.
For implementing Toggle, we have to provide binding, so, I created one state object in its RowView and on enable and disable of that toggle, I get its event like this:
struct MyRowView: View {
var myObj:MyObject
#State var isEnabled: Bool
....
....
init(myObj:MyObject) {
self.myObj = myObj
self.isEnabled = myObj.isEnabled
}
var body: some View {
....
Toggle("", isOn: $isEnabled)
.labelsHidden()
.onChange(of: isEnabled) { value in
self.MyViewModelAsEnvironmentObject.toggleValueUpdated()
}
....
}
As value of isEnabled changes, I am passing that to above mentioned MyViewModel object, where it checks for some conflicts, if conflicting, I am showing some alert, asking user to to forcefully enable this or not, if he says yes, then I have to disable row in other row, if user says no, then I have to disable this row.
Question is, How I can reload individual cell to enable or disable given toggle in given rows? If I print value directly in Row, then it is updating properly, but here isEnabled variable is assigned when row initiated, now its not possible to change its value from view model.
How to deal with this situation?

How to cancel a change made to a projectedValue

So I have a view that allows a user to update a CoreData property.
They then have the option to cancel or save
The TextField is linked with the projectedValue of a CoreData class
TextField("New Value", text: $coreDataClass.value)
Its very nice that CoreData has been linked with Combine so that we can attach $ to get the two-way binding feature. However on this particular view is causing me some problems because if a user hits cancel after inserting any value then the value persists even after the view is dismissed.
How would I cancel the change if the user hits cancel... I thought using a lazy var would be perfect since its only read once but that didn't work. I thought about using a class to capture the initial value and then use it in the cancel method but that did not work either.
Any guidance on the issue would be greatly appreciated!
Answer for question in comment Unrelated
import SwiftUI
extension Text {
func buttonView(type: ButtonType) -> some View {
func selectColor(type: ButtonType) -> Color {
switch type {
case .default:
return .blue
case .destructive:
return .red
case .utility:
return .purple
}
}
return self.bold()
.foregroundColor(.white)
.padding(.horizontal)
.padding(.vertical, 5)
.background(selectColor(type: type))
.cornerRadius(8)
}
}
enum ButtonType {
case `default`
case destructive
case utility
}
As you may know, when you edit CoreData field, it's not getting saved to the database until you run viewContext.save().
In case you need to revert changes there's another one: viewContext.rollback()
But note, that it'll discard all changes made for all objects since the last save. So if you have many changes and wanna discard only a single one, you need to do it manually: in the init fill #State value with a value from your object field, and on Save write #State to your object field.

SwiftUI load very long list in async way

I've a navigation and detail view that is sending a dictionary of date (key) and array of an struct (but it's not important the struct, it contains array of string and other stuff.
If I send a very very long dictionary, the app is freezing in the selected row and the detail appears once the List finished to load each record.
struct DetailView: View {
var selectedChat: [Date: [TextStruct]]? // you can try with [Date: [String]]?
var body: some View {
List
{
ForEach(self.selectedChat.keys.sorted(), id: \.self)
{ key in //section data
Section(header:
Text("\(self.selectedChat[key]![0].date)
{
ForEach(self.selectedChat[key]!, id:\.self) {sText in
// my ChatView(sText) ....
}
}
}
}
I've tried to load some rows at the start by adding this var
#State private var dateAndText: [Date: [TextStruct]] = [:]
substitute the code above (self.selectedChat) whit self.dateAndText and on .onAppear:
.onAppear {
if let chat = self.selectedChat {
let keysDateSorted = chat.allText.keys.sorted()
self.chatLeader = chat.chatLeader
for key in keysDateSorted.prefix(30) {
self.dateAndText[key] = chat.allText[key]
}
DispatchQueue.global(qos: .utility).async {
self.dateAndText = chat.allText
self.progressBarValue = 1
}
}
}
With this solution, once I push the row, immediately I can see the first 30 records, and it is ok, but I can't scroll until all the records are loaded. I know there is a way to load the array only if the user is scrolling at the end of the list, but I want to load all the list also if the user don't scroll at the end.
So, there is a way to load the list partially (like send and update the array each 100 records) and in async way (in order to don't freeze the display for bad user experience)?
You are almost certainly running into the issues described and fixed here by Paul Hudson.
SwiftUI is trying to animate all of the changes so if you use his hack around the issue it should work but you will lost all animations between changes of the list.
Apple devs responded to him and Dave DeLong who were discussing it on Twitter, they said that it is definitely an issue on their end that they hope to fix.
tldr of the article:
Add .id(UUID()) to the end of your List's initializer.

SwiftUI: NavigationLink pops immediately if used within ForEach

I'm using a NavigationLink inside of a ForEach in a List to build a basic list of buttons each leading to a separate detail screen.
When I tap on any of the list cells, it transitions to the detail view of that cell but then immediately pops back to the main menu screen.
Not using the ForEach helps to avoid this behavior, but not desired.
Here is the relevant code:
struct MainMenuView: View {
...
private let menuItems: [MainMenuItem] = [
MainMenuItem(type: .type1),
MainMenuItem(type: .type2),
MainMenuItem(type: .typeN),
]
var body: some View {
List {
ForEach(menuItems) { item in
NavigationLink(destination: self.destination(item.destination)) {
MainMenuCell(menuItem: item)
}
}
}
}
// Constructs destination views for the navigation link
private func destination(_ destination: ScreenDestination) -> AnyView {
switch destination {
case .type1:
return factory.makeType1Screen()
case .type2:
return factory.makeType2Screen()
case .typeN:
return factory.makeTypeNScreen()
}
}
If you have a #State, #Binding or #ObservedObject in MainMenuView, the body itself is regenerated (menuItems get computed again) which causes the NavigationLink to invalidate (actually the id change does that). So you must not modify the menuItems arrays id-s from the detail view.
If they are generated every time consider setting a constant id or store in a non modifying part, like in a viewmodel.
Maybe I found the reason of this bug...
if you use iOS 15 (not found iOS 14),
and you write the code NavigationLink to go to same View in different locations in your projects, then this bug appear.
So I simply made another View that has different destination View name but the same contents... then it works..
you can try....
sorry for my poor English...

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