How to make a drag and drop viewmodifier accept any object - swiftui

Question

I have created a drag-and-drop viewmodifier that works as expected, but now I would like to make it accept any object. I can add <T: Identifiable> to all the functions, structs, and view-modifiers, but when I try to do add it to my singleton class, I get "Static stored properties not supported in generic types".
I need the singleton class, so I can put the .dropObjectOutside viewmodifier anywhere in my view-hierarchy, so I've tried downcasting the ID to a String, but I can't seem to make that work.
Is there a way to downcast or make this code accept any object?
import SwiftUI
// I want this to be any object
struct StopContent: Identifiable {
var id: String = UUID().uuidString
}
// Singleton class to hold drag state
class DragToReorderController: ObservableObject {
// Make it a singleton, so it can be accessed from any view
static let shared = DragToReorderController()
private init() { }
#Published var draggedID: String? // How do I make this a T.ID or downcast T.ID to string everywhere else?
#Published var dragActive:Bool = false
}
// Add ViewModifier to view
extension View {
func dragToReorder(_ item: StopContent, array: Binding<[StopContent]>) -> some View {
self.modifier(DragToReorderObject(sourceItem: item, contentArray: array))
}
func dropOutside() -> some View {
self.onDrop(of: [UTType.text], delegate: DropObjectOutsideDelegate())
}
}
import UniformTypeIdentifiers
// MARK: View Modifier
struct DragToReorderObject: ViewModifier {
let sourceItem: StopContent
#Binding var contentArray: [StopContent]
#ObservedObject private var dragReorder = DragToReorderController.shared
func body(content: Content) -> some View {
content
.onDrag {
dragReorder.draggedID = sourceItem.id
dragReorder.dragActive = false
return NSItemProvider(object: String(sourceItem.id) as NSString)
}
.onDrop(of: [UTType.text], delegate: DropObjectDelegate(sourceItem: sourceItem, listData: $contentArray, draggedItem: $dragReorder.draggedID, dragActive: $dragReorder.dragActive))
.onChange(of: dragReorder.dragActive, perform: { value in
if value == false {
// Drag completed
}
})
.opacity(dragReorder.draggedID == sourceItem.id && dragReorder.dragActive ? 0 : 1)
}
}
// MARK: Drop and reorder
struct DropObjectDelegate: DropDelegate {
let sourceItem: StopContent
#Binding var listData: [StopContent]
#Binding var draggedItem: String?
#Binding var dragActive: Bool
func dropEntered(info: DropInfo) {
if draggedItem == nil { draggedItem = sourceItem.id }
dragActive = true
// Make sure the dragged item has moved and that it still exists
if sourceItem.id != draggedItem {
if let draggedItemValid = draggedItem {
if let from = listData.firstIndex(where: { $0.id == draggedItemValid } ) {
// If that is true, move it to the new location
let to = listData.firstIndex(where: { $0.id == sourceItem.id } )!
if listData[to].id != draggedItem! {
listData.move(fromOffsets: IndexSet(integer: from),
toOffset: to > from ? to + 1 : to)
}
}
}
}
}
func dropUpdated(info: DropInfo) -> DropProposal? {
return DropProposal(operation: .move)
}
func performDrop(info: DropInfo) -> Bool {
dragActive = false
draggedItem = nil
return true
}
}
// MARK: Drop and cancel
struct DropObjectOutsideDelegate: DropDelegate {
// Using a singleton so we can drop anywhere
#ObservedObject private var dragReorder = DragToReorderController.shared
func dropEntered(info: DropInfo) {
dragReorder.dragActive = true
}
func performDrop(info: DropInfo) -> Bool {
dragReorder.dragActive = false
dragReorder.draggedID = nil
return true
}
}

Answer 1

For this, you have to add Identifiable generic constraint everywhere. Also, use Int for draggedID instead of String.
Here is the demo code.
// Singleton class to hold drag state
class DragToReorderController: ObservableObject {
// Make it a singleton, so it can be accessed from any view
static let shared = DragToReorderController()
private init() { }
#Published var draggedID: Int?
#Published var dragActive: Bool = false
}
// Add ViewModifier to view
extension View {
func dragToReorder<T: Identifiable>(_ item: T, array: Binding<[T]>) -> some View {
self.modifier(DragToReorderObject(sourceItem: item, contentArray: array))
}
func dropOutside() -> some View {
self.onDrop(of: [UTType.text], delegate: DropObjectOutsideDelegate())
}
}
import UniformTypeIdentifiers
// MARK: View Modifier
struct DragToReorderObject<T: Identifiable>: ViewModifier {
let sourceItem: T
#Binding var contentArray: [T]
#ObservedObject private var dragReorder = DragToReorderController.shared
func body(content: Content) -> some View {
content
.onDrag {
dragReorder.draggedID = sourceItem.id.hashValue
dragReorder.dragActive = false
return NSItemProvider(object: String(sourceItem.id.hashValue) as NSString)
}
.onDrop(of: [UTType.text], delegate: DropObjectDelegate(sourceItem: sourceItem, listData: $contentArray, draggedItem: $dragReorder.draggedID, dragActive: $dragReorder.dragActive))
.onChange(of: dragReorder.dragActive, perform: { value in
if value == false {
// Drag completed
}
})
.opacity((dragReorder.draggedID == sourceItem.id.hashValue) && dragReorder.dragActive ? 0 : 1)
}
}
// MARK: Drop and reorder
struct DropObjectDelegate<T: Identifiable>: DropDelegate {
let sourceItem: T
#Binding var listData: [T]
#Binding var draggedItem: Int?
#Binding var dragActive: Bool
func dropEntered(info: DropInfo) {
if draggedItem == nil { draggedItem = sourceItem.id.hashValue }
dragActive = true
// Make sure the dragged item has moved and that it still exists
if sourceItem.id.hashValue != draggedItem {
if let draggedItemValid = draggedItem {
if let from = listData.firstIndex(where: { $0.id.hashValue == draggedItemValid } ) {
// If that is true, move it to the new location
let to = listData.firstIndex(where: { $0.id == sourceItem.id } )!
if listData[to].id.hashValue != draggedItem! {
listData.move(fromOffsets: IndexSet(integer: from),
toOffset: to > from ? to + 1 : to)
}
}
}
}
}
func dropUpdated(info: DropInfo) -> DropProposal? {
return DropProposal(operation: .move)
}
func performDrop(info: DropInfo) -> Bool {
dragActive = false
draggedItem = nil
return true
}
}
// MARK: Drop and cancel
struct DropObjectOutsideDelegate: DropDelegate {
// Using a singleton so we can drop anywhere
#ObservedObject private var dragReorder = DragToReorderController.shared
func dropEntered(info: DropInfo) {
dragReorder.dragActive = true
}
func performDrop(info: DropInfo) -> Bool {
dragReorder.dragActive = false
dragReorder.draggedID = nil
return true
}
}