implement drag and drop

MultiChrono/ContentView.swift

@@ -7,6 +7,7 @@
 
 import SwiftUI
 import Combine
+import UniformTypeIdentifiers
 
 class ContentViewModel: ObservableObject {
     @Published var stopwatches: [Stopwatch] = []
@@ -35,6 +36,11 @@ class ContentViewModel: ObservableObject {
         }
     }
     
+    func moveStopwatch(fromOffsets source: IndexSet, toOffset destination: Int) {
+        stopwatches.move(fromOffsets: source, toOffset: destination)
+        save()
+    }
+    
     func save() {
         if let encoded = try? JSONEncoder().encode(stopwatches) {
             UserDefaults.standard.set(encoded, forKey: saveKey)
@@ -55,28 +61,23 @@ struct ContentView: View {
     @State private var isShowingAddSheet = false
     @State private var isShowingSettings = false
     @State private var selectedStopwatch: Stopwatch?
+    @State private var draggingStopwatch: Stopwatch?
     @Environment(\.scenePhase) private var scenePhase
     
     var body: some View {
         NavigationStack {
-            List {
+            ScrollView {
+                LazyVStack(spacing: 0) {
                     ForEach(viewModel.stopwatches) { stopwatch in
-                    Button {
-                        selectedStopwatch = stopwatch
-                    } label: {
-                        StopwatchRow(stopwatch: stopwatch, onDelete: {
-                            viewModel.deleteStopwatch(id: stopwatch.id)
-                        })
-                    }
-                    .buttonStyle(.plain) // Preserves the row layout and interactions
-                }
-                .onDelete { indexSet in
-                    for index in indexSet {
-                        viewModel.deleteStopwatch(at: index)
+                        StopwatchListItem(
+                            stopwatch: stopwatch,
+                            viewModel: viewModel,
+                            selectedStopwatch: $selectedStopwatch,
+                            draggingStopwatch: $draggingStopwatch
+                        )
                     }
                 }
             }
-            .listStyle(.plain)
             .navigationTitle("MultiChrono")
             .toolbar {
                 ToolbarItem(placement: .topBarLeading) {
@@ -126,7 +127,6 @@ struct ContentView: View {
                 }
             }
         }
-
         .onChange(of: scenePhase) { newPhase in
             if newPhase == .background || newPhase == .inactive {
                 viewModel.save()
@@ -135,6 +135,69 @@ struct ContentView: View {
     }
 }
 
+struct StopwatchListItem: View {
+    @ObservedObject var stopwatch: Stopwatch
+    @ObservedObject var viewModel: ContentViewModel
+    @Binding var selectedStopwatch: Stopwatch?
+    @Binding var draggingStopwatch: Stopwatch?
+    
+    var body: some View {
+        Button {
+            selectedStopwatch = stopwatch
+        } label: {
+            StopwatchRow(stopwatch: stopwatch, onDelete: {
+                withAnimation {
+                    viewModel.deleteStopwatch(id: stopwatch.id)
+                }
+            })
+        }
+        .buttonStyle(.plain)
+        .padding(.horizontal)
+        .padding(.vertical, 8)
+        .background(Color(UIColor.systemBackground))
+        .contentShape(Rectangle())
+        .onDrag {
+            self.draggingStopwatch = stopwatch
+            return NSItemProvider(object: stopwatch.id.uuidString as NSString)
+        }
+        .onDrop(of: [UTType.text], delegate: StopwatchDropDelegate(item: stopwatch, viewModel: viewModel, draggingItem: $draggingStopwatch))
+        
+        Divider()
+            .padding(.leading)
+    }
+}
+
+struct StopwatchDropDelegate: DropDelegate {
+    let item: Stopwatch
+    let viewModel: ContentViewModel
+    @Binding var draggingItem: Stopwatch?
+    
+    func dropEntered(info: DropInfo) {
+        guard let draggingItem = draggingItem else { return }
+        
+        if draggingItem.id != item.id {
+            if let from = viewModel.stopwatches.firstIndex(where: { $0.id == draggingItem.id }),
+               let to = viewModel.stopwatches.firstIndex(where: { $0.id == item.id }) {
+                
+                withAnimation {
+                    viewModel.stopwatches.move(fromOffsets: IndexSet(integer: from),
+                                               toOffset: to > from ? to + 1 : to)
+                }
+            }
+        }
+    }
+    
+    func performDrop(info: DropInfo) -> Bool {
+        viewModel.save()
+        draggingItem = nil
+        return true
+    }
+    
+    func dropUpdated(info: DropInfo) -> DropProposal? {
+        return DropProposal(operation: .move)
+    }
+}
+
 #Preview {
     ContentView()
 }