event.once

src/vs/base/common/event.ts

@@ -179,6 +179,17 @@ export function fromPromise<T>(promise: TPromise<Event<T>>): Event<T> {
 	return emitter.event;
 }
 
+export function once<T>(event: Event<T>): Event<T> {
+	return (listener, thisArgs = null, disposables?) => {
+		const result = event(e => {
+			result.dispose();
+			return listener.call(thisArgs, e);
+		}, null, disposables);
+
+		return result;
+	};
+}
+
 export function mapEvent<I,O>(event: Event<I>, map: (i:I)=>O): Event<O> {
 	return (listener, thisArgs = null, disposables?) => event(i => listener.call(thisArgs, map(i)), null, disposables);
 }

src/vs/base/test/common/event.test.ts

@@ -5,7 +5,7 @@
 'use strict';
 
 import * as assert from 'assert';
-import Event, {Emitter, fromEventEmitter, debounceEvent, EventBufferer} from 'vs/base/common/event';
+import Event, {Emitter, fromEventEmitter, debounceEvent, EventBufferer, once} from 'vs/base/common/event';
 import {IDisposable} from 'vs/base/common/lifecycle';
 import {EventEmitter} from 'vs/base/common/eventEmitter';
 import Errors = require('vs/base/common/errors');
@@ -254,4 +254,32 @@ suite('EventBufferer', () => {
 
 		listener.dispose();
 	});
+
+	test('once', () => {
+		const emitter = new Emitter<void>();
+
+		let counter1 = 0, counter2 = 0, counter3 = 0;
+
+		const listener1 = emitter.event(() => counter1++);
+		const listener2 = once(emitter.event)(() => counter2++);
+		const listener3 = once(emitter.event)(() => counter3++);
+
+		assert.equal(counter1, 0);
+		assert.equal(counter2, 0);
+		assert.equal(counter3, 0);
+
+		listener3.dispose();
+		emitter.fire();
+		assert.equal(counter1, 1);
+		assert.equal(counter2, 1);
+		assert.equal(counter3, 0);
+
+		emitter.fire();
+		assert.equal(counter1, 2);
+		assert.equal(counter2, 1);
+		assert.equal(counter3, 0);
+
+		listener1.dispose();
+		listener2.dispose();
+	});
 });