will scroll event for scrollable element

rebornix · rebornix · commit 9061a91fed1d · 2020-02-20T10:10:39.000-08:00
diff --git a/src/vs/base/browser/ui/list/listView.ts b/src/vs/base/browser/ui/list/listView.ts
@@ -197,6 +197,7 @@ export class ListView<T> implements ISpliceable<T>, IDisposable {
 	get contentHeight(): number { return this.rangeMap.size; }
 
 	get onDidScroll(): Event<ScrollEvent> { return this.scrollableElement.onScroll; }
+	get onWillScroll(): Event<ScrollEvent> { return this.scrollableElement.onWillScroll; }
 
 	constructor(
 		container: HTMLElement,
diff --git a/src/vs/base/browser/ui/scrollbar/scrollableElement.ts b/src/vs/base/browser/ui/scrollbar/scrollableElement.ts
@@ -167,13 +167,17 @@ export abstract class AbstractScrollableElement extends Widget {
 	private readonly _onScroll = this._register(new Emitter<ScrollEvent>());
 	public readonly onScroll: Event<ScrollEvent> = this._onScroll.event;
 
+	private readonly _onWillScroll = this._register(new Emitter<ScrollEvent>());
+	public readonly onWillScroll: Event<ScrollEvent> = this._onWillScroll.event;
+
 	protected constructor(element: HTMLElement, options: ScrollableElementCreationOptions, scrollable: Scrollable) {
 		super();
 		element.style.overflow = 'hidden';
 		this._options = resolveOptions(options);
 		this._scrollable = scrollable;
 
 		this._register(this._scrollable.onScroll((e) => {
+			this._onWillScroll.fire(e);
 			this._onDidScroll(e);
 			this._onScroll.fire(e);
 		}));
