Boolean inside ObservableObject Array does not trigger re-render

In my current project, i am having a hard time getting SwiftUI to recognize a change in a nested ObservableObject, here a demo / debug code

import SwiftUI
import SwiftData

class Collection: ObservableObject, Identifiable, Hashable {
    
    static func == (lhs: Collection, rhs: Collection) -> Bool {
        return lhs.title == rhs.title && lhs.items == rhs.items
    }

    func hash(into hasher: inout Hasher) {
        hasher.combine(title)
    }
    var id = UUID()
    @Published var title : String = ""
    @Published var items : [String] = []
    @Published var isCollapsed: Bool = true
    
    init(title: String, items: [String]) {
        self.title = title
        self.items = items
    }
}



class GridViewModel: ObservableObject {
    @Published var collections : [Collection] = []
}


struct ContentView: View {
    
//    @EnvironmentObject var gridViewModel : GridViewModel
    @StateObject var  gridViewModel = GridViewModel()

    var body: some View {
        VStack {
            Text("Toggle Visibility with observed Objects")
            Button("Add Collection", action: {
                addCollection()
            })
            
            ScrollView {
                ForEach(gridViewModel.collections, id: \.self) { coll in
                    Button("toggle visibility for:", action: {
                        coll.isCollapsed.toggle()     // does not work
                    })
                    Button("edit", action: {
                        changeItem()
                    })
                    Text(coll.title)
                    
                    if coll.isCollapsed {
                        ForEach(coll.items, id: \.self) { item in
                            Text(item)
                        }
                    }
                    Spacer(minLength: 20)
                }
            }
            
        }
        .onAppear {
            let collection1 = Collection(title: "first Collection", items: ["apple", "banana", "citrus"])
            let collection2 = Collection(title: "second Collection", items: ["banana", "citrus", "apple"])
            let collection3 = Collection(title: "third Collection", items: ["citrus", "banana", "apple"])

            gridViewModel.collections = [collection1, collection2, collection3]
        }
    }
    
    func addCollection() {
        let collectionNew = Collection(title: "new_vierte Collection", items: ["apple", "banana", "citrus"])
        gridViewModel.collections.append(collectionNew)
        gridViewModel.objectWillChange.send()   // does work
    }
    
    func changeItem() {
        gridViewModel.collections[0].items[0] = "---edit------edit------edit---"
        print("done: changed to: \(gridViewModel.collections[0].items[0])")
        gridViewModel.objectWillChange.send()   // works
    }



}

#Preview {
    var gridViewModel = GridViewModel()
    return ContentView().environmentObject(gridViewModel)
    #if os(macOS)
        .frame(width: 700, height: 500)
    #endif

}

As you can see, the change in the first level, which is inside GridViewModel, triggers a re-render, but a change inside Collection does not..

i need SwiftUI to recognize that isCollapsed boolean has changed, and the UI needs to be updated, any suggestions how to make that work?

Try using gridViewModel.objectWillChange.send() as shown in this code

Button("toggle visibility for:", action: {
    gridViewModel.objectWillChange.send() // <--- here
    coll.isCollapsed.toggle()
})

EDIT-1:

As mentioned you could also use a struct Collection to include into the older style class GridViewModel: ObservableObject, such as:

struct Collection: Identifiable, Hashable {  //<--- here
    let id = UUID()
    
    var title: String = ""
    var items: [String] = []
    var isCollapsed: Bool = true
}

class GridViewModel: ObservableObject {
    @Published var collections : [Collection] = []
}

struct ContentView: View {
    //    @EnvironmentObject var gridViewModel : GridViewModel
    @StateObject private var gridViewModel = GridViewModel()
    
    var body: some View {
        VStack {
            Text("Toggle Visibility with observed Objects")
            Button("Add Collection") {
                addCollection()
            }
            
            ScrollView {
                ForEach($gridViewModel.collections) { $coll in  //<--- here $
                    Button("toggle visibility for:", action: {
                        coll.isCollapsed.toggle()
                    })
                    Button("edit", action: {
                        changeItem()
                    })
                    Text(coll.title)
                    
                    if coll.isCollapsed {
                        ForEach(coll.items, id: \.self) { item in
                            Text(item)
                        }
                    }
                    Spacer(minLength: 20)
                }
            }
        }
        .onAppear {
            let collection1 = Collection(title: "first Collection", items: ["apple", "banana", "citrus"])
            let collection2 = Collection(title: "second Collection", items: ["banana", "citrus", "apple"])
            let collection3 = Collection(title: "third Collection", items: ["citrus", "banana", "apple"])
            
            gridViewModel.collections = [collection1, collection2, collection3]
        }
    }
    
    func addCollection() {
        let collectionNew = Collection(title: "new_vierte Collection", items: ["apple", "banana", "citrus"])
        gridViewModel.collections.append(collectionNew) 
    }
    
    func changeItem() {
        gridViewModel.collections[0].items[0] = "---edit------edit------edit---" 
        print("done: changed to: \(gridViewModel.collections[0].items[0])")
    }
    
}

EDIT-2:

As mentioned in my comments, using the recommended more modern Observable framework

@Observable class Collection: Identifiable {  // <--- here
    let id = UUID()
    
    var title: String
    var items: [String]
    var isCollapsed: Bool
    
    init(title: String, items: [String], isCollapsed: Bool = true) {
        self.title = title
        self.items = items
        self.isCollapsed = isCollapsed
    }
}

@Observable class GridViewModel {  // <--- here
    var collections : [Collection] = []
}

struct ContentView: View {
    // when passing from parent ...  .environment(gridViewModel)
    // @Environment(GridViewModel.self) private var gridViewModel
    
    @State private var  gridViewModel = GridViewModel() // <---here
    
    var body: some View {
        VStack {
            Text("Toggle Visibility with observed Objects")
            Button("Add Collection") {
                addCollection()
            }
            
            ScrollView {
                ForEach(gridViewModel.collections) { coll in
                    Button("toggle visibility for:", action: {
                        coll.isCollapsed.toggle()
                    })
                    Button("edit", action: {
                        changeItem()
                    })
                    Text(coll.title)
                    
                    if coll.isCollapsed {
                        ForEach(coll.items, id: \.self) { item in
                            Text(item)
                        }
                    }
                    Spacer(minLength: 20)
                }
            }
        }
        .onAppear {
            let collection1 = Collection(title: "first Collection", items: ["apple", "banana", "citrus"])
            let collection2 = Collection(title: "second Collection", items: ["banana", "citrus", "apple"])
            let collection3 = Collection(title: "third Collection", items: ["citrus", "banana", "apple"])
            
            gridViewModel.collections = [collection1, collection2, collection3]
        }
    }
    
    func addCollection() {
        let collectionNew = Collection(title: "new_vierte Collection", items: ["apple", "banana", "citrus"])
        gridViewModel.collections.append(collectionNew) //<--- here
    }
    
    func changeItem() {
        gridViewModel.collections[0].items[0] = "---edit------edit------edit---" //<--- here
        print("done: changed to: \(gridViewModel.collections[0].items[0])")
    }
    
}

Note, don't use your static func == ... and func hash..., remove them.

Note also, using ForEach(coll.items, id: \.self) is bad practice, make sure your coll.items do not contain multiple same Strings by using for example a struct ItemName: Identifiable {....}