Add maxSimultaneousDrags parameter to TreeDraggable

example/lib/src/examples/drag_and_drop.dart

@@ -85,7 +85,7 @@ class _DragAndDropTreeViewState extends State<DragAndDropTreeView> {
     populateExampleTree(root);
 
     treeController = TreeController<Node>(
-      roots: root.children,
+      root: root,
       childrenProvider: (Node node) => node.children,
 
       // The parentProvider is extremely important when automatically expanding

example/lib/src/examples/filterable.dart

@@ -85,7 +85,7 @@ class _FilterableTreeViewState extends State<FilterableTreeView> {
     populateExampleTree(root);
 
     treeController = TreeController<Node>(
-      roots: root.children,
+      root: root,
       childrenProvider: getChildren,
     )..expandAll();
 

example/lib/src/examples/lazy_loading.dart

@@ -98,7 +98,7 @@ class _LazyLoadingTreeViewState extends State<LazyLoadingTreeView> {
   void initState() {
     super.initState();
     treeController = TreeController<Data>(
-      roots: childrenProvider(Data.root),
+      root: Data.root,
       childrenProvider: childrenProvider,
     );
   }

example/lib/src/examples/minimal.dart

@@ -32,7 +32,7 @@ class _MinimalTreeViewState extends State<MinimalTreeView> {
     });
 
     treeController = TreeController<Node>(
-      roots: root.children,
+      root: root,
       childrenProvider: (Node node) => node.children,
     );
   }

lib/src/drag_and_drop.dart

@@ -60,6 +60,7 @@ class TreeDraggable<T extends Object> extends StatefulWidget {
     this.hitTestBehavior = HitTestBehavior.deferToChild,
     this.longPressDelay,
     this.longPressHapticFeedbackOnStart = true,
+    this.maxSimultaneousDrags = 1,
   });
 
   /// The widget below this widget in the tree.
@@ -256,6 +257,16 @@ class TreeDraggable<T extends Object> extends StatefulWidget {
   /// Defaults to `null`.
   final Duration? longPressDelay;
 
+  /// How many simultaneous drags to support.
+  ///
+  /// When null, no limit is applied. Set this to 1 if you want to only allow
+  /// the drag source to have one item dragged at a time. Set this to 0 if you
+  /// want to prevent the draggable from actually being dragged.
+  ///
+  /// If you set this property to 1, consider supplying an "empty" widget for
+  /// [childWhenDragging] to create the illusion of actually moving [child].
+  final int? maxSimultaneousDrags;
+
   @override
   State<TreeDraggable<T>> createState() => _TreeDraggableState<T>();
 }
@@ -378,7 +389,7 @@ class _TreeDraggableState<T extends Object> extends State<TreeDraggable<T>>
     if (widget.longPressDelay != null) {
       return LongPressDraggable<T>(
         data: widget.node,
-        maxSimultaneousDrags: 1,
+        maxSimultaneousDrags: widget.maxSimultaneousDrags,
         onDragStarted: onDragStarted,
         onDragUpdate: onDragUpdate,
         onDraggableCanceled: onDraggableCanceled,
@@ -399,7 +410,7 @@ class _TreeDraggableState<T extends Object> extends State<TreeDraggable<T>>
 
     return Draggable<T>(
       data: widget.node,
-      maxSimultaneousDrags: 1,
+      maxSimultaneousDrags: widget.maxSimultaneousDrags,
       onDragStarted: onDragStarted,
       onDragUpdate: onDragUpdate,
       onDraggableCanceled: onDraggableCanceled,

lib/src/tree_controller.dart

@@ -97,11 +97,11 @@ class TreeController<T extends Object> with ChangeNotifier {
   /// [TreeController.parentProvider] is set to a callback that always returns
   /// null.
   TreeController({
-    required Iterable<T> roots,
+    required T root,
     required this.childrenProvider,
     ParentProvider<T>? parentProvider,
     this.defaultExpansionState = false,
-  }) : _roots = roots {
+  }) : _root = root {
     assert(() {
       _debugHasParentProvider = parentProvider != null;
       return true;
@@ -112,11 +112,16 @@ class TreeController<T extends Object> with ChangeNotifier {
   /// The roots of the tree.
   ///
   /// These nodes are used as a starting point when traversing the tree.
-  Iterable<T> get roots => _roots;
-  Iterable<T> _roots;
-  set roots(Iterable<T> nodes) {
-    if (nodes == _roots) return;
-    _roots = nodes;
+  Iterable<T> get roots => childrenProvider(root);
+
+  /// The root of the tree.
+  ///
+  /// This node is used as a starting point when traversing the tree.
+  T get root => _root;
+  T _root;
+  set root(T node) {
+    if (node == _root) return;
+    _root = node;
     rebuild();
   }
 
@@ -615,7 +620,6 @@ class TreeController<T extends Object> with ChangeNotifier {
 
   @override
   void dispose() {
-    _roots = const Iterable.empty();
     toggledNodes.clear();
     super.dispose();
   }

lib/src/tree_indentation.dart

@@ -152,6 +152,7 @@ class IndentGuide {
     PathModifier? pathModifier,
     bool roundCorners,
     bool connectBranches,
+    double? fixedVerticalOffset,
   }) = ConnectingLinesGuide;
 
   /// Convenient constructor to create a [ScopingLinesGuide].
@@ -522,6 +523,7 @@ class ConnectingLinesGuide extends AbstractLineGuide {
     super.pathModifier,
     this.roundCorners = false,
     this.connectBranches = false,
+    this.fixedVerticalOffset,
   });
 
   /// Decides if the connection between vertical and horizontal lines should be
@@ -543,6 +545,9 @@ class ConnectingLinesGuide extends AbstractLineGuide {
   /// Defaults to `false`.
   final bool connectBranches;
 
+  /// TODO
+  final double? fixedVerticalOffset;
+
   @override
   CustomPainter createPainter(BuildContext context, TreeEntry<Object> entry) {
     return _ConnectingLinesPainter(
@@ -567,6 +572,7 @@ class ConnectingLinesGuide extends AbstractLineGuide {
     PathModifier? Function()? pathModifier,
     bool? roundCorners,
     bool? connectBranches,
+    double? fixedVerticalOffset,
   }) {
     return ConnectingLinesGuide(
       indent: indent ?? this.indent,
@@ -580,6 +586,7 @@ class ConnectingLinesGuide extends AbstractLineGuide {
       pathModifier: pathModifier != null ? pathModifier() : this.pathModifier,
       roundCorners: roundCorners ?? this.roundCorners,
       connectBranches: connectBranches ?? this.connectBranches,
+      fixedVerticalOffset: fixedVerticalOffset ?? fixedVerticalOffset,
     );
   }
 
@@ -671,7 +678,7 @@ class _ConnectingLinesPainter extends CustomPainter {
     }
 
     // Add connections
-    final double y = size.height * 0.5;
+    final double y = guide.fixedVerticalOffset ?? size.height * 0.5;
     path.moveTo(connectionStart, 0.0);
 
     if (guide.roundCorners) {