list: make onDragStart optional

src/vs/base/browser/ui/list/list.ts

@@ -88,7 +88,7 @@ export const ListDragOverReactions = {
 export interface IListDragAndDrop<T> {
 	getDragURI(element: T): string | null;
 	getDragLabel?(elements: T[]): string | undefined;
-	onDragStart(data: IDragAndDropData, originalEvent: DragEvent): void;
+	onDragStart?(data: IDragAndDropData, originalEvent: DragEvent): void;
 	onDragOver(data: IDragAndDropData, targetElement: T | undefined, targetIndex: number | undefined, originalEvent: DragEvent): boolean | IListDragOverReaction;
 	drop(data: IDragAndDropData, targetElement: T | undefined, targetIndex: number | undefined, originalEvent: DragEvent): void;
 }

src/vs/base/browser/ui/list/listView.ts

@@ -632,7 +632,9 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 		this.currentDragData = new ElementsDragAndDropData(elements);
 		StaticDND.CurrentDragAndDropData = new ExternalElementsDragAndDropData(elements);
 
-		this.dnd.onDragStart(this.currentDragData, event);
+		if (this.dnd.onDragStart) {
+			this.dnd.onDragStart(this.currentDragData, event);
+		}
 	}
 
 	private onDragOver(event: IListDragEvent<T>): boolean {

src/vs/base/browser/ui/list/listWidget.ts

@@ -956,11 +956,17 @@ class ListViewDragAndDrop<T> implements IListViewDragAndDrop<T> {
 	}
 
 	getDragLabel?(elements: T[]): string | undefined {
-		return this.dnd.getDragLabel && this.dnd.getDragLabel(elements);
+		if (this.dnd.getDragLabel) {
+			return this.dnd.getDragLabel(elements);
+		}
+
+		return undefined;
 	}
 
 	onDragStart(data: IDragAndDropData, originalEvent: DragEvent): void {
-		this.dnd.onDragStart(data, originalEvent);
+		if (this.dnd.onDragStart) {
+			this.dnd.onDragStart(data, originalEvent);
+		}
 	}
 
 	onDragOver(data: IDragAndDropData, targetElement: T, targetIndex: number, originalEvent: DragEvent): boolean | IListDragOverReaction {

src/vs/base/browser/ui/tree/abstractTree.ts

@@ -28,11 +28,17 @@ class TreeNodeListDragAndDrop<T, TFilterData, TRef> implements IListDragAndDrop<
 	}
 
 	getDragLabel(nodes: ITreeNode<T, TFilterData>[]): string | undefined {
-		return this.dnd.getDragLabel && this.dnd.getDragLabel(nodes.map(node => node.element));
+		if (this.dnd.getDragLabel) {
+			return this.dnd.getDragLabel(nodes.map(node => node.element));
+		}
+
+		return undefined;
 	}
 
 	onDragStart(data: IDragAndDropData, originalEvent: DragEvent): void {
-		this.dnd.onDragStart(data, originalEvent);
+		if (this.dnd.onDragStart) {
+			this.dnd.onDragStart(data, originalEvent);
+		}
 	}
 
 	onDragOver(data: IDragAndDropData, targetNode: ITreeNode<T, TFilterData> | undefined, targetIndex: number | undefined, originalEvent: DragEvent, raw = true): boolean | IListDragOverReaction {

src/vs/base/browser/ui/tree/asyncDataTree.ts

@@ -134,17 +134,23 @@ function asObjectTreeOptions<TInput, T, TFilterData>(options?: IAsyncDataTreeOpt
 			getDragURI(node) {
 				return options.dnd!.getDragURI(node.element as T);
 			},
-			getDragLabel: options.dnd!.getDragLabel && ((nodes) => {
-				return options.dnd!.getDragLabel!(nodes.map(node => node.element as T));
-			}),
+			getDragLabel(nodes) {
+				if (options.dnd!.getDragLabel) {
+					return options.dnd!.getDragLabel!(nodes.map(node => node.element as T));
+				}
+
+				return undefined;
+			},
 			onDragStart(data, originalEvent) {
-				return options.dnd!.onDragStart(data, originalEvent);
+				if (options.dnd!.onDragStart) {
+					options.dnd!.onDragStart!(data, originalEvent);
+				}
 			},
 			onDragOver(data, targetNode, targetIndex, originalEvent) {
 				return options.dnd!.onDragOver(data, targetNode && targetNode.element as T, targetIndex, originalEvent);
 			},
 			drop(data, targetNode, targetIndex, originalEvent) {
-				return options.dnd!.drop(data, targetNode && targetNode.element as T, targetIndex, originalEvent);
+				options.dnd!.drop(data, targetNode && targetNode.element as T, targetIndex, originalEvent);
 			}
 		},
 		multipleSelectionController: options.multipleSelectionController && {

src/vs/workbench/parts/debug/electron-browser/watchExpressionsView.ts

@@ -288,10 +288,6 @@ class WatchExpressionsDragAndDrop implements ITreeDragAndDrop<IExpression> {
 		return elements[0].name;
 	}
 
-	onDragStart(data: IDragAndDropData, originalEvent: DragEvent): void {
-		// noop
-	}
-
 	drop(data: IDragAndDropData, targetElement: IExpression, targetIndex: number, originalEvent: DragEvent): void {
 		const draggedData = data.getData();
 		if (Array.isArray(draggedData)) {