ios – What assured behaviour does onAppear and onDisappear have?

I’ve created a repo with one demonstration venture

https://github.com/PavelKatunin/SectionsAndItems

The entire thought and necessities are described within the readme file.

Lengthy story quick: it’s an infinity vertical feed, and I need to have the ability to rely reliably what number of sections and gadgets within the feed a consumer has seen.

I’ve concepts for 3 variations of implementation of this performance,
however I began with because it appeared in the mean time probably the most easy one.

I wished to trace look and disappearance of Sections and Gadgets by dealing with
onAppear and onDisappear callbacks.

And I discovered that typically onDisappear isn’t referred to as when a view really disappears from the display screen and likewise there isn’t any logical order of the calls (because it appears to me).

So I’m wondering if I am doing one thing incorrect right here?

Here’s a log after I scroll via the feed:

SiwftUI occasion: Part 1 appeared
SiwftUI occasion: Merchandise 1 appeared
SiwftUI occasion: Merchandise 2 appeared
SiwftUI occasion: Merchandise 1 disappeared
SiwftUI occasion: Merchandise 3 appeared
SiwftUI occasion: Merchandise 2 disappeared
SiwftUI occasion: Merchandise 4 appeared
SiwftUI occasion: Part 2 appeared
SiwftUI occasion: Merchandise 1 appeared
SiwftUI occasion: Merchandise 4 disappeared
SiwftUI occasion: Merchandise 3 disappeared
SiwftUI occasion: Merchandise 2 appeared
SiwftUI occasion: Merchandise 1 disappeared
SiwftUI occasion: Merchandise 3 appeared
SiwftUI occasion: Part 1 disappeared
SiwftUI occasion: Part 3 appeared
SiwftUI occasion: Merchandise 1 appeared
SiwftUI occasion: Merchandise 3 disappeared
SiwftUI occasion: Merchandise 2 disappeared
SiwftUI occasion: Merchandise 2 appeared
SiwftUI occasion: Merchandise 1 disappeared
SiwftUI occasion: Merchandise 3 appeared
SiwftUI occasion: Part 2 disappeared
SiwftUI occasion: Merchandise 2 disappeared
SiwftUI occasion: Merchandise 4 appeared

Right here is my 3 easy views:

struct ItemView: View {
    
    @State var textual content: String
    
    var physique: some View {
        ZStack {
            Textual content("(textual content)").font(.title)
        }
        .onAppear() {
            print("SiwftUI occasion: Merchandise (textual content) appeared")
        }
        .onDisappear() {
            print("SiwftUI occasion: Merchandise (textual content) disappeared")
        }
    }
    
}

struct SectionView: View {
    
    @State var shade: Shade
    @State var gadgets: [String]
    var sectionId: String
    
    var physique: some View {
        LazyVStack(spacing: 0) {
            ForEach(gadgets, id: .hashValue) { merchandise in
                ItemView(textual content: merchandise)
                    .body(top: UIScreen.fundamental.bounds.dimension.top)
            }
        }
        .onAppear() {
            print("SiwftUI occasion: Part (sectionId) appeared")
        }
        .onDisappear() {
            print("SiwftUI occasion: Part (sectionId) disappeared")
        }
        .background(shade)
    }
    
}

struct ScrollOffsetFeedView: View {
    
    static func createLocalSections() -> [Section] {
        return [Section(id: "1", color: .green, items: ["1", "2", "3", "4"]),
                Part(id: "2", shade: .crimson, gadgets: ["1", "2", "3"]),
                Part(id: "3", shade: .blue, gadgets: ["1", "2", "3", "4", "5"])]
    }
    
    @State var sections: [Section]
    
    init(sections: [Section]) {
        self.sections = sections
    }
    
    var physique: some View {
        ZStack {
            ScrollView {
                LazyVStack(spacing: 0) {
                    ForEach(sections, id: .id) { part in
                        SectionView(shade: part.shade,
                                    gadgets: part.gadgets,
                                    sectionId: part.id)
                    }
                }
                .scrollTargetLayout()
            }
            .scrollTargetBehavior(.paging)
            .ignoresSafeArea()
            .scrollIndicators(.hidden)
            VStack {
                ZStack {
                    makeSectionsCounter()
                        .background(Shade(white: 0.0, opacity: 0.5))
                        .cornerRadius(10)
                        .padding()
                }
                Spacer()
            }
        }
    }
    
    @ViewBuilder func makeSectionsCounter() -> some View {
        ZStack {
            Textual content("Sections accomplished 0, merchandise 1 / 4")
        }
        .padding()
    }
    
}

How does onAppear and onDisappear work in ScrollView and LazyVStack?