SwiftUI: Drag & Drop between Lists crashes sometimes - swiftui

Question

I'm using Xcode 12 beta and trying to create a view where items from a left list can be dragged onto a right list and dropped there.
This crashes in the following situations:
The list is empty.
The list is not empty, but the item is dragged behind the last list element, after dragging it onto other list elements first. The crash already appears while the item is dragged, not when it is dropped (i.e., the .onInsert is not called yet).
The crash message tells:
SwiftUI`generic specialization <SwiftUI._ViewList_ID.Views> of (extension in Swift):Swift.RandomAccessCollection< where A.Index: Swift.Strideable, A.Indices == Swift.Range<A.Index>, A.Index.Stride == Swift.Int>.index(after: A.Index) -> A.Index:
Are there any ideas why this happens and how it can be avoided?
The left list code:
struct AvailableBuildingBricksView: View {
#StateObject var buildingBricksProvider: BuildingBricksProvider = BuildingBricksProvider()
var body: some View {
List {
ForEach(buildingBricksProvider.availableBuildingBricks) { buildingBrickItem in
Text(buildingBrickItem.title)
.onDrag {
self.provider(buildingBrickItem: buildingBrickItem)
}
}
}
}
private func provider(buildingBrickItem: BuildingBrickItem) -> NSItemProvider {
let image = UIImage(systemName: buildingBrickItem.systemImageName) ?? UIImage()
let provider = NSItemProvider(object: image)
provider.suggestedName = buildingBrickItem.title
return provider
}
}
final class BuildingBricksProvider: ObservableObject {
#Published var availableBuildingBricks: [BuildingBrickItem] = []
init() {
self.availableBuildingBricks = [
TopBrick.personalData,
TopBrick.education,
TopBrick.work,
TopBrick.overviews
].map({ return BuildingBrickItem(title: $0.title,
systemImageName: "stop") })
}
}
struct BuildingBrickItem: Identifiable {
var id: UUID = UUID()
var title: String
var systemImageName: String
}
The right list code:
struct DocumentStructureView: View {
#StateObject var documentStructureProvider: DocumentStructureProvider = DocumentStructureProvider()
var body: some View {
List {
ForEach(documentStructureProvider.documentSections) { section in
Text(section.title)
}
.onInsert(of: ["public.image"]) {
self.insertSection(position: $0,
itemProviders: $1,
top: true)
}
}
}
func insertSection(position: Int, itemProviders: [NSItemProvider], top: Bool) {
for item in itemProviders.reversed() {
item.loadObject(ofClass: UIImage.self) { image, _ in
if let _ = image as? UIImage {
DispatchQueue.main.async {
let section = DocumentSectionItem(title: item.suggestedName ?? "Unknown")
self.documentStructureProvider.insert(section: section, at: position)
}
}
}
}
}
}
final class DocumentStructureProvider: ObservableObject {
#Published var documentSections: [DocumentSectionItem] = []
init() {
documentSections = [
DocumentSectionItem(title: "Dummy")
]
}
func insert(section: DocumentSectionItem, at position: Int) {
if documentSections.count == 0 {
documentSections.append(section)
return
}
documentSections.insert(section, at: position)
}
}
struct DocumentSectionItem: Identifiable {
var id: UUID = UUID()
var title: String
}

Answer 1

Well, I succeeded to make the problem reproducable, code below.
Steps to reproduce:
Drag "A" on "1" as first item on the right.
Drag another "A" on "1", hold it dragged, draw it slowly down after "5" -> crash.
The drop function is not called before the crash.
struct ContentView: View {
var body: some View {
HStack {
LeftList()
Divider()
RightList()
}
}
}
import SwiftUI
import UniformTypeIdentifiers
struct LeftList: View {
var list: [String] = ["A", "B", "C", "D", "E"]
var body: some View {
List(list) { item in
Text(item)
.onDrag {
let stringItemProvider = NSItemProvider(object: item as NSString)
return stringItemProvider
}
}
}
}
import SwiftUI
import UniformTypeIdentifiers
struct RightList: View {
#State var list: [String] = ["1", "2", "3", "4", "5"]
var body: some View {
List {
ForEach(list) { item in
Text(item)
}
.onInsert(
of: [UTType.text],
perform: drop)
}
}
private func drop(at index: Int, _ items: [NSItemProvider]) {
debugPrint(index)
for item in items {
_ = item.loadObject(ofClass: NSString.self) { text, _ in
debugPrint(text)
DispatchQueue.main.async {
debugPrint("dispatch")
text.map { self.list.insert($0 as! String, at: index) }
}
}
}
}
}