diff --git a/src/system/event.ts b/src/system/event.ts
index 84f1f94..72ecf8c 100644
--- a/src/system/event.ts
+++ b/src/system/event.ts
@@ -65,8 +65,10 @@ export function promisifyDeferred<T, U>(
 	let cancel: ((reason?: any) => void) | undefined;
 	let disposable: Disposable;
 
+	let pending = true;
 	const promise = new Promise<U>((resolve, reject) => {
 		cancel = () => {
+			pending = false;
 			cancel = undefined;
 			reject();
 		};
@@ -74,7 +76,9 @@ export function promisifyDeferred<T, U>(
 		disposable = event(async (value: T) => {
 			try {
 				await executor(value, resolve, reject);
+				pending = false;
 			} catch (ex) {
+				pending = false;
 				reject(ex);
 			}
 		});
@@ -90,6 +94,9 @@ export function promisifyDeferred<T, U>(
 	);
 
 	return {
+		get pending() {
+			return pending;
+		},
 		promise: promise,
 		cancel: () => cancel?.(),
 	};
diff --git a/src/system/promise.ts b/src/system/promise.ts
index 120ce61..fa208c0 100644
--- a/src/system/promise.ts
+++ b/src/system/promise.ts
@@ -126,16 +126,28 @@ export function cancellable<T>(
 }
 
 export interface Deferred<T> {
-	promise: Promise<T>;
+	readonly pending: boolean;
+	readonly promise: Promise<T>;
 	fulfill: (value: T) => void;
 	cancel(): void;
 }
 
 export function defer<T>(): Deferred<T> {
-	const deferred: Deferred<T> = { promise: undefined!, fulfill: undefined!, cancel: undefined! };
+	const deferred: Mutable<Deferred<T>> = {
+		pending: true,
+		promise: undefined!,
+		fulfill: undefined!,
+		cancel: undefined!,
+	};
 	deferred.promise = new Promise((resolve, reject) => {
-		deferred.fulfill = resolve;
-		deferred.cancel = reject;
+		deferred.fulfill = function (value) {
+			deferred.pending = false;
+			return resolve(value);
+		};
+		deferred.cancel = function () {
+			deferred.pending = false;
+			reject();
+		};
 	});
 	return deferred;
 }