SwiftUI: How to update element in ForEach without necessity to update all elements? - swift

Question

Imagine that you have some parent view that generate some number of child views:
struct CustomParent: View {
var body: some View {
HStack {
ForEach(0..<10, id: \.self) { index in
CustomChild(index: index)
}
}
}
}
struct CustomChild: View {
#State var index: Int
#State private var text: String = ""
var body: some View {
Button(action: {
// Here should be some update of background/text/opacity or whatever.
// So how can I update background/text/opacity or whatever for button with index for example 3 from button with index for example 1?
}) {
Text(text)
}
.onAppear {
text = String(index)
}
}
}
Question is included in the code as comment.
Thanks!
UPDATE:
First of all really thanks for all of your answers, but now imagine that you use mentioned advanced approach.
struct CustomParent: View {
#StateObject var customViewModel = CustomViewModel()
var body: some View {
HStack {
ForEach(0..<10, id: \.self) { index in
CustomChild(index: index, customViewModel: customViewModel)
}
}
}
}
If I use let _ = Self._printChanges() method in CustomChildView, to catch UI updates/changes, it'll print that every element in ForEach was updated/changed on button action.
struct CustomChild: View {
let index: Int
#ObservedObject var customViewModel: CustomViewModel
var body: some View {
let _ = Self._printChanges() // This have been added to code
Button(action: {
customViewModel.buttonPushed(at: index)
}) {
Text(customViewModel.childTexts[index])
}
}
}
class CustomViewModel: ObservableObject {
#Published var childTexts = [String](repeating: "", count: 10)
init() {
for i in 0..<childTexts.count {
childTexts[i] = String(i)
}
}
func buttonPushed(at index: Int) {
//button behaviors goes here
//for example:
childTexts[index + 1] = "A"
}
}
And now imagine that you have for example 1000 custom elements which have some background, opacity, shadow, texts, fonts and so on. Now I change text in any of the elements.
Based on log from let _ = Self._printChanges() method, it goes through all elements, and all elements are updated/changed what can cause delay.
Q1: Why did update/change all elements, if I change text in only one element?
Q2: How can I prevent update/change all elements, if I change only one?
Q3: How to update element in ForEach without necessity to update all elements?

Answer 1

Simpler Approach:
Although child views cannot access things that the host views have, it's possible to declare the child states in the host view and pass that state as a binding variable to the child view. In the code below, I have passed the childTexts variable to the child view, and (for your convenience) initialized the text so that it binds to the original element in the array (so that your onAppear works properly). Every change performed on the text and childTexts variable inside the child view reflects on the host view.
I strongly suggest not to do this though, as more elegant approaches exist.
struct CustomParent: View {
#State var childTexts = [String](repeating: "", count: 10)
var body: some View {
HStack {
ForEach(0..<10, id: \.self) { index in
CustomChild(index: index, childTexts: $childTexts)
}
}
}
}
struct CustomChild: View {
let index: Int
#Binding private var text: String
#Binding private var childTexts: [String]
init(index: Int, childTexts: Binding<[String]>) {
self.index = index
self._childTexts = childTexts
self._text = childTexts[index]
}
var body: some View {
Button(action: {
//button behaviors goes here
//for example
childTexts[index + 1] = "A"
}) {
Text(text)
}
.onAppear {
text = String(index)
}
}
}
Advanced Approach:
By using the Combine framework, all your logics can be moved into an ObservableObject view model. This is much better as the button logic is no longer inside the view. In simplest terms, the #Published variable in the ObservableObject will publish a change when it senses its own mutation, while the #StateObjectand the #ObservedObject will listen and recalculate the view for you.
struct CustomParent: View {
#StateObject var customViewModel = CustomViewModel()
var body: some View {
HStack {
ForEach(0..<10, id: \.self) { index in
CustomChild(index: index, customViewModel: customViewModel)
}
}
}
}
struct CustomChild: View {
let index: Int
#ObservedObject var customViewModel: CustomViewModel
var body: some View {
Button(action: {
customViewModel.buttonPushed(at: index)
}) {
Text(customViewModel.childTexts[index])
}
}
}
class CustomViewModel: ObservableObject {
#Published var childTexts = [String](repeating: "", count: 10)
init() {
for i in 0..<childTexts.count {
childTexts[i] = String(i)
}
}
func buttonPushed(at index: Int) {
//button behaviors goes here
//for example:
childTexts[index + 1] = "A"
}
}