Make Realm work with SwiftUI List(children:)

Question

I am trying to use Realm as a database with a parent/children relationship and show the data in a hierarchical SwiftUI List using the children: initializer. I oriented myself at the SwiftUI+Realm tutorial. My Realm class looks like this:

import Foundation
import RealmSwift
    
class BlockList: RealmSwift.Object, RealmSwift.ObjectKeyIdentifiable {
    @Persisted(primaryKey: true) var id: RealmSwift.ObjectId
    @Persisted var title: String
    @Persisted var childBlockLists = RealmSwift.List<BlockList>()
    @Persisted(originProperty: "childBlockLists") var parentBlockList: RealmSwift.LinkingObjects<BlockList>
    
    convenience init(title: String) {
        self.init()
        self.title = title
   }
}

Then my content view looks like this:

struct ContentView: View {
    @ObservedResults(BlockList.self) var blockLists
    
    var body: some View {
        let parentBlockLists = blockLists.where {
            ($0.parentBlockList.count == 0)
        }
        
        List(parentBlockLists) { blockList in
            Text(blockList.title)
        }
        .toolbar {
            ToolbarItem {
                Button(action: addItem) {
                    Label("Add Item", systemImage: "plus")
                }
            }
        }
    }
    
    func addItem() {
        let realm = try! Realm()

        let newBlockList = BlockList(title: "New List")
        let newSubBlockList = BlockList(title: "New SubList")
        newBlockList.childBlockLists.append(newSubBlockList)
        try! realm.write {
            realm.add(newBlockList)
        }
    }
}

This shows all lists that do not have a parent. The next step would be to use List(children:) to show the data hierarchically, ie show those lists that have sublists with a chevron to expand the list.

For this purpose, the children parameter expects the key paths to the child nodes. However, I cannot figure out how to provide that. I tried to write an extension to BlockList and provide a function that casts the children into something useful, but none of my approaches work:

extension BlockList {
    var childBlockListsArray: AnyRealmCollection<BlockList> {
        AnyRealmCollection(childBlockLists)
//        guard let set = childBlockLists as? Array<BlockList>, set.isEmpty == false else { return nil }
//        childBlockLists.count == 0 ? nil : AnyRealmCollection(childBlockLists)
//        childBlockLists.count == 0 ? nil : Array(childBlockLists)
    }
}

I feel I get closest with casting in AnyRealmCollection and then use

List(AnyRealmCollection(parentBlockLists), children: \.childBlockListsArray)

But I still get the error

Key path value type 'AnyRealmCollection' cannot be converted to contextual type 'AnyRealmCollection?'

How can I provide the correct Realm data and their children key paths to be shown in SwiftUIs List?

Answer 1

Let me try an answer as I think that everything you need already exists in your question without any additional code.

I believe the goal is to generate a list of parents, and then a chevron or disclosure triangles in the UI for the user to click on to then show their children - all done with one model.

Here's your (simplified) model

class BlockList: RealmSwift.Object {
    @Persisted var title: String
    @Persisted var childBlockLists = RealmSwift.List<BlockList>()
    @Persisted(originProperty: "childBlockLists") var parentBlockList: RealmSwift.LinkingObjects<BlockList>

    convenience init(title: String) {
        self.init()
        self.title = title
   }
}

Now populate realm with some data to test with; parents and children. In this case ParentA has two children while ParentB and ParentC have one.

let parentA = BlockList(title: "parent A")
let parentB = BlockList(title: "parent B")
let parentC = BlockList(title: "parent C")

let child0ofParentA = BlockList(title: "child 0 of Parent A")
let child1ofParentA = BlockList(title: "child 1 of Parent A")

let child0ofParentB = BlockList(title: "child 0 of Parent B")

let child0ofParentC = BlockList(title: "child 0 of Parent C")

parentA.childBlockLists.append(child0ofParentA)
parentA.childBlockLists.append(child1ofParentA)

parentB.childBlockLists.append(child0ofParentB)

parentC.childBlockLists.append(child0ofParentC)

try! realm.write {
    realm.add([parentA, parentB, parentC])
}

Then finally, let's read that data in and present a list - similar to what you'd do in the UI

//get all of the parents
let parents = realm.objects(BlockList.self).where { $0.childBlockLists.count > 0 }

parents.forEach { parent in
    print("parent title: \(parent.title)")

    parent.childBlockLists.forEach { child in
        print("   child title: \(child.title)")
    }
}

and the output - which would be the same as a user taps a discosure triangle for each parent

parent title: parent A
   child title: child 0 of Parent A
   child title: child 1 of Parent A
parent title: parent B
   child title: child 0 of Parent B
parent title: parent C
   child title: child 0 of Parent C

Note that I am not using parentBlockList at all so it could be removed in this use case. Perhaps you want a want to transverse the graph back to the parent from the child? If so, leave it in.

The SwiftUI part is just displaying a child objects just like the parent objects are displayed and you seems to know how to display a list of objects already.