Actually, you do not need an [NSItemProvider] to process a drag and drop with multiple items in SwiftUI. Since you must keep track of the multiple selected Items in your own selection manager anyway, use that selection when generating a custom dragging preview and when processing the drop.
Replace the ContentView of a new MacOS App project with all of the code below. This is a complete working sample of how to drag and drop multiple items using SwiftUI.
To use it, you must select one or more items in order to initiate a drag and then it/they may be dragged onto any other unselected item. The results of what would happen during the drop operation is printed on the console.
I threw this together fairly quickly, so there may be some inefficiencies in my sample, but it does seem to work well.
import SwiftUI
import Combine
struct ContentView: View {
private let items = ["Item 0", "Item 1", "Item 2", "Item 3", "Item 4", "Item 5", "Item 6", "Item 7"]
@StateObject var selection = StringSelectionManager()
@State private var refreshID = UUID()
@State private var dropTargetIndex: Int? = nil
var body: some View {
VStack(alignment: .leading) {
ForEach(0 ..< items.count, id: \.self) { index in
HStack {
Image(systemName: "folder")
Text(items[index])
}
.opacity((dropTargetIndex != nil) && (dropTargetIndex == index) ? 0.5 : 1.0)
// This id must change whenever the selection changes, otherwise SwiftUI will use a cached preview
.id(refreshID)
.onDrag { itemProvider(index: index) } preview: {
DraggingPreview(selection: selection)
}
.onDrop(of: [.text], delegate: MyDropDelegate(items: items,
selection: selection,
dropTargetIndex: $dropTargetIndex,
index: index) )
.padding(2)
.onTapGesture { selection.toggle(items[index]) }
.background(selection.isSelected(items[index]) ?
Color(NSColor.selectedContentBackgroundColor) : Color(NSColor.windowBackgroundColor))
.cornerRadius(5.0)
}
}
.onReceive(selection.objectWillChange, perform: { refreshID = UUID() } )
.frame(width: 300, height: 300)
}
private func itemProvider(index: Int) -> NSItemProvider {
// Only allow Items that are part of a selection to be dragged
if selection.isSelected(items[index]) {
return NSItemProvider(object: items[index] as NSString)
} else {
return NSItemProvider()
}
}
}
struct DraggingPreview: View {
var selection: StringSelectionManager
var body: some View {
VStack(alignment: .leading, spacing: 1.0) {
ForEach(selection.items, id: \.self) { item in
HStack {
Image(systemName: "folder")
Text(item)
.padding(2.0)
.background(Color(NSColor.selectedContentBackgroundColor))
.cornerRadius(5.0)
Spacer()
}
}
}
.frame(width: 300, height: 300)
}
}
struct MyDropDelegate: DropDelegate {
var items: [String]
var selection: StringSelectionManager
@Binding var dropTargetIndex: Int?
var index: Int
func dropEntered(info: DropInfo) {
dropTargetIndex = index
}
func dropExited(info: DropInfo) {
dropTargetIndex = nil
}
func validateDrop(info: DropInfo) -> Bool {
// Only allow non-selected Items to be drop targets
if !selection.isSelected(items[index]) {
return info.hasItemsConforming(to: [.text])
} else {
return false
}
}
func dropUpdated(info: DropInfo) -> DropProposal? {
// Sets the proper DropOperation
if !selection.isSelected(items[index]) {
let dragOperation = NSEvent.modifierFlags.contains(NSEvent.ModifierFlags.option) ? DropOperation.copy : DropOperation.move
return DropProposal(operation: dragOperation)
} else {
return DropProposal(operation: .forbidden)
}
}
func performDrop(info: DropInfo) -> Bool {
// Only allows non-selected Items to be drop targets & gets the "operation"
let dropProposal = dropUpdated(info: info)
if dropProposal?.operation != .forbidden {
let dropOperation = dropProposal!.operation == .move ? "Move" : "Copy"
if selection.selection.count > 1 {
for item in selection.selection {
print("\(dropOperation): \(item) Onto: \(items[index])")
}
} else {
// https://stackoverflow.com/a/69325742/899918
if let item = info.itemProviders(for: ["public.utf8-plain-text"]).first {
item.loadItem(forTypeIdentifier: "public.utf8-plain-text", options: nil) { (data, error) in
if let data = data as? Data {
let item = NSString(data: data, encoding: 4)
print("\(dropOperation): \(item ?? "") Onto: \(items[index])")
}
}
}
return true
}
}
return false
}
}
class StringSelectionManager: ObservableObject {
@Published var selection: Set<String> = Set<String>()
let objectWillChange = PassthroughSubject<Void, Never>()
// Helper for ForEach
var items: [String] {
return Array(selection)
}
func isSelected(_ value: String) -> Bool {
return selection.contains(value)
}
func toggle(_ value: String) {
if isSelected(value) {
deselect(value)
} else {
select(value)
}
}
func select(_ value: String?) {
if let value = value {
objectWillChange.send()
selection.insert(value)
}
}
func deselect(_ value: String) {
objectWillChange.send()
selection.remove(value)
}
}
