fix #29641

src/vs/vscode.proposed.d.ts

@@ -7,9 +7,14 @@
 
 declare module 'vscode' {
 
+	export interface WorkspaceFoldersChangeEvent {
+		readonly addedFolders: Uri[];
+		readonly removedFolders: Uri[];
+	}
+
 	export namespace workspace {
 
-		export const onDidChangeWorkspaceFolders: Event<Uri[] | undefined>;
+		export const onDidChangeWorkspaceFolders: Event<WorkspaceFoldersChangeEvent>;
 
 		export let workspaceFolders: Uri[] | undefined;
 	}

src/vs/workbench/api/node/extHostWorkspace.ts

@@ -7,7 +7,7 @@
 import URI from 'vs/base/common/uri';
 import Event, { Emitter } from 'vs/base/common/event';
 import { normalize } from 'vs/base/common/paths';
-import { isFalsyOrEmpty } from 'vs/base/common/arrays';
+import { isFalsyOrEmpty, delta } from 'vs/base/common/arrays';
 import { relative } from 'path';
 import { Workspace } from 'vs/platform/workspace/common/workspace';
 import { IThreadService } from 'vs/workbench/services/thread/common/threadService';
@@ -16,16 +16,17 @@ import { TPromise } from 'vs/base/common/winjs.base';
 import { fromRange, EndOfLine } from 'vs/workbench/api/node/extHostTypeConverters';
 import { IWorkspaceData, ExtHostWorkspaceShape, MainContext, MainThreadWorkspaceShape } from './extHost.protocol';
 import * as vscode from 'vscode';
+import { compare } from "vs/base/common/strings";
 
 export class ExtHostWorkspace extends ExtHostWorkspaceShape {
 
 	private static _requestIdPool = 0;
 
-	private readonly _onDidChangeWorkspace = new Emitter<URI[]>();
+	private readonly _onDidChangeWorkspace = new Emitter<vscode.WorkspaceFoldersChangeEvent>();
 	private readonly _proxy: MainThreadWorkspaceShape;
 	private _workspace: Workspace;
 
-	readonly onDidChangeWorkspace: Event<URI[]> = this._onDidChangeWorkspace.event;
+	readonly onDidChangeWorkspace: Event<vscode.WorkspaceFoldersChangeEvent> = this._onDidChangeWorkspace.event;
 
 	constructor(threadService: IThreadService, data: IWorkspaceData) {
 		super();
@@ -95,8 +96,24 @@ export class ExtHostWorkspace extends ExtHostWorkspaceShape {
 	}
 
 	$acceptWorkspaceData(data: IWorkspaceData): void {
+
+		// compute delta
+		const oldRoots = this._workspace ? this._workspace.roots.sort(ExtHostWorkspace._compareUri) : [];
+		const newRoots = data ? data.roots.sort(ExtHostWorkspace._compareUri) : [];
+		const { added, removed } = delta(oldRoots, newRoots, ExtHostWorkspace._compareUri);
+
+		// update state
 		this._workspace = data ? new Workspace(data.id, data.name, data.roots) : null;
-		this._onDidChangeWorkspace.fire(this.getRoots());
+
+		// send event
+		this._onDidChangeWorkspace.fire(Object.freeze({
+			addedFolders: Object.freeze<vscode.Uri[]>(added),
+			removedFolders: Object.freeze<vscode.Uri[]>(removed)
+		}));
+	}
+
+	private static _compareUri(a: vscode.Uri, b: vscode.Uri): number {
+		return compare(a.toString(), b.toString());
 	}
 
 	// --- search ---

src/vs/workbench/test/electron-browser/api/extHostWorkspace.test.ts

@@ -65,4 +65,57 @@ suite('ExtHostWorkspace', function () {
 		ws = new ExtHostWorkspace(new TestThreadService(), { id: 'foo', name: 'Test', roots: [URI.file('/Folder')] });
 		assert.equal(ws.getPath().replace(/\\/g, '/'), '/Folder');
 	});
+
+	test('Multiroot change event should have a delta, #29641', function () {
+		let ws = new ExtHostWorkspace(new TestThreadService(), { id: 'foo', name: 'Test', roots: [] });
+
+		let sub = ws.onDidChangeWorkspace(e => {
+			assert.deepEqual(e.addedFolders, []);
+			assert.deepEqual(e.removedFolders, []);
+		});
+		ws.$acceptWorkspaceData({ id: 'foo', name: 'Test', roots: [] });
+		sub.dispose();
+
+		sub = ws.onDidChangeWorkspace(e => {
+			assert.deepEqual(e.removedFolders, []);
+			assert.equal(e.addedFolders.length, 1);
+			assert.equal(e.addedFolders[0].toString(), 'foo:bar');
+		});
+		ws.$acceptWorkspaceData({ id: 'foo', name: 'Test', roots: [URI.parse('foo:bar')] });
+		sub.dispose();
+
+		sub = ws.onDidChangeWorkspace(e => {
+			assert.deepEqual(e.removedFolders, []);
+			assert.equal(e.addedFolders.length, 1);
+			assert.equal(e.addedFolders[0].toString(), 'foo:bar2');
+		});
+		ws.$acceptWorkspaceData({ id: 'foo', name: 'Test', roots: [URI.parse('foo:bar'), URI.parse('foo:bar2')] });
+		sub.dispose();
+
+		sub = ws.onDidChangeWorkspace(e => {
+			assert.equal(e.removedFolders.length, 2);
+			assert.equal(e.removedFolders[0].toString(), 'foo:bar');
+			assert.equal(e.removedFolders[1].toString(), 'foo:bar2');
+
+			assert.equal(e.addedFolders.length, 1);
+			assert.equal(e.addedFolders[0].toString(), 'foo:bar3');
+		});
+		ws.$acceptWorkspaceData({ id: 'foo', name: 'Test', roots: [URI.parse('foo:bar3')] });
+		sub.dispose();
+
+	});
+
+	test('Multiroot change event is immutable', function () {
+		let ws = new ExtHostWorkspace(new TestThreadService(), { id: 'foo', name: 'Test', roots: [] });
+		let sub = ws.onDidChangeWorkspace(e => {
+			assert.throws(() => {
+				e.addedFolders = [];
+			});
+			assert.throws(() => {
+				(<any>e.addedFolders)[0] = null;
+			});
+		});
+		ws.$acceptWorkspaceData({ id: 'foo', name: 'Test', roots: [] });
+		sub.dispose();
+	});
 });