diff --git a/extensions/ql-vscode/src/common/app.ts b/extensions/ql-vscode/src/common/app.ts
new file mode 100644
index 00000000000..03e06c208bb
--- /dev/null
+++ b/extensions/ql-vscode/src/common/app.ts
@@ -0,0 +1,5 @@
+import { AppEventEmitter } from './events';
+
+export interface App {
+  createEventEmitter<T>(): AppEventEmitter<T>;
+}
diff --git a/extensions/ql-vscode/src/common/events.ts b/extensions/ql-vscode/src/common/events.ts
new file mode 100644
index 00000000000..598fcf4d547
--- /dev/null
+++ b/extensions/ql-vscode/src/common/events.ts
@@ -0,0 +1,10 @@
+import { Disposable } from '../pure/disposable-object';
+
+export interface AppEvent<T> {
+  (listener: (event: T) => void): Disposable;
+}
+
+export interface AppEventEmitter<T> {
+  event: AppEvent<T>;
+  fire(data: T): void;
+}
diff --git a/extensions/ql-vscode/src/common/vscode/events.ts b/extensions/ql-vscode/src/common/vscode/events.ts
new file mode 100644
index 00000000000..399bfdfb2cf
--- /dev/null
+++ b/extensions/ql-vscode/src/common/vscode/events.ts
@@ -0,0 +1,7 @@
+import * as vscode from 'vscode';
+import { AppEventEmitter } from '../events';
+
+export class VSCodeAppEventEmitter<T>
+  extends vscode.EventEmitter<T>
+  implements AppEventEmitter<T> {
+}
diff --git a/extensions/ql-vscode/src/common/vscode/vscode-app.ts b/extensions/ql-vscode/src/common/vscode/vscode-app.ts
new file mode 100644
index 00000000000..ec99fe655ac
--- /dev/null
+++ b/extensions/ql-vscode/src/common/vscode/vscode-app.ts
@@ -0,0 +1,9 @@
+import { App } from '../app';
+import { AppEventEmitter } from '../events';
+import { VSCodeAppEventEmitter } from './events';
+
+export class ExtensionApp implements App {
+  public createEventEmitter<T>(): AppEventEmitter<T> {
+    return new VSCodeAppEventEmitter<T>();
+  }
+}
diff --git a/extensions/ql-vscode/src/pure/disposable-object.ts b/extensions/ql-vscode/src/pure/disposable-object.ts
index f351892b831..d8e39c085c7 100644
--- a/extensions/ql-vscode/src/pure/disposable-object.ts
+++ b/extensions/ql-vscode/src/pure/disposable-object.ts
@@ -1,7 +1,7 @@
 
 // Avoid explicitly referencing Disposable type in vscode.
 // This file cannot have dependencies on the vscode API.
-interface Disposable {
+export interface Disposable {
   dispose(): any;
 }