Update #StateObject every time that init() is invoked in SwiftUI - swiftui

Question

I'm writing a SwiftUI code for a carousel, and I have a CarouselModel class that goes like this:
CarouselModel.swift
import Foundation
import SwiftUI
class CarouselModel: ObservableObject {
//MARK: - Properties
#Published private var imagesCount: Int
#Published private var currentImage: Int
#Published private var offsetMainAxis: CGFloat
private var screenSize: CGFloat
private var isHorizontal: Bool
private var padding: CGFloat
private var nextImage: Int {
return (self.currentImage+1) % self.imagesCount
}
private var prevImage: Int {
return (self.currentImage-1+self.imagesCount) % self.imagesCount
}
private var centralIndex: CGFloat {
return CGFloat(self.imagesCount-1)/2
}
public var computedOffset: CGFloat {
return (centralIndex-CGFloat(currentImage))*(screenSize-padding) + offsetMainAxis
}
init(imagesCount: Int, isHorizontal: Bool = true, padding: CGFloat = 20, currentImage: Int = 0) {
self.imagesCount = imagesCount
self.currentImage = currentImage
self.offsetMainAxis = 0
self.padding = padding
self.isHorizontal = isHorizontal
if isHorizontal {
self.screenSize = UIScreen.main.bounds.width
} else {
self.screenSize = UIScreen.main.bounds.height
}
}
//MARK: - Methods
public func getCurrentImage() -> Int {
return self.currentImage
}
public func goNext() -> Void {
withAnimation(.easeOut(duration: 0.5)) {
currentImage = nextImage
}
}
public func goPrevious() -> Void {
withAnimation(.easeOut(duration: 0.5)) {
currentImage = prevImage
}
}
public func onDragChange(offset: CGFloat) -> Void {
self.offsetMainAxis = offset
}
public func onDragEnd() -> Void {
if abs(offsetMainAxis) > 0.2*screenSize {
if offsetMainAxis > 0 {
withAnimation(.easeOut(duration: 0.5)) {
offsetMainAxis = 0
currentImage = prevImage
}
} else {
withAnimation(.easeOut(duration: 0.5)) {
offsetMainAxis = 0
currentImage = nextImage
}
}
} else {
withAnimation(.easeOut(duration: 0.5)) {
offsetMainAxis = 0
}
}
}
public func skipTo(number i: Int) {
withAnimation(.easeOut(duration: 0.5)) {
currentImage = i
}
}
}
At this point every time I need to place a concrete Carousel in my app I create a CarouselView.swift file containing some View, and the thing goes more or less like this:
CarouselView.swift
import SwiftUI
struct Carousel: View {
#StateObject var carousel: CarouselModel
private var screenWidth: CGFloat = UIScreen.main.bounds.width
private var images: [String]
private let padding: CGFloat
private var descriptions: [String]?
#State private var imageSelected: [String:Bool]
init(images: [String], descriptions: [String]?, padding: CGFloat = 20) {
self.images = images
self.padding = padding
_carousel = StateObject(
wrappedValue:
CarouselModel(
imagesCount: images.count,
isHorizontal: true,
padding: padding
)
)
self.imageSelected = Dictionary(uniqueKeysWithValues: images.map{ ($0, false) })
guard let desc = descriptions else {
self.descriptions = nil
return
}
self.descriptions = desc
}
var body: some View {
// USE carousel TO DISPLAY SOME CAROUSEL IMPLEMENTATION
}
}
Now, a problem arises when inside some container I need to display a different set of images based on a #State private var selectedImgSet: Int and each set has a different amount of pictures in it. The point is, whenever I call CarouselView(...) inside the container and its parameters change, its init is invoked but the CarouselModel doesn't get updated with the new images count.
This is totally expected since it is a #StateObject but I'm not sure how the update should happen. I can't use the .onAppear(perform: ) hook in the CarouselView since that is executed only once, when the CarouselView first appears.
What is the correct way to update the carousel: CarouselModel variable whenever CarouselView's init parameters change?
EDIT: Here are pastes for a full working examples: ContentView, CarouselItem, CarouselModel, CarouselView, Items. Add the following images sets to your assets: item00, item01, item02, item10, item11, item20, item21, item22, item23. Any 1920x1080p placeholder will do the trick for now.

Answer 1

I eventually fixed my issue the following way: I introduced the following method in CarouselModel:
public func update(_ images: Int) {
print("Performing update with \(images) images")
self.imagesCount = images
self.currentImage = 0
self.offsetMainAxis = 0
}
Then I added to the outmost View inside CarouselView the following modifier:
.onChange(of: self.images) { v in
self.carousel.update(v.count)
}
Now the update happens as expected.

Answer 2

You've got things the wrong way round. Create the #StateObject in the View above Carousel View and pass it in as #ObservedObject. However, since your model does no loading or saving it doesn't need to be a reference type, i.e. an object. A value type will suffice, i.e. a struct, so it can be #State in the parent and pass in as #Binding so you can call its mutating functions.