From 96e97682e05f9cf6c3f155a891a59b6e50d06518 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Wed, 12 Apr 2023 14:33:09 +0200
Subject: [PATCH] handle event listener registration before mount; handle
 unregister

---
 src/runtime/internal/Component.ts           | 28 ++++++++++++++++++++-
 test/custom-elements/samples/events/test.js | 19 ++++++++++++--
 2 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/src/runtime/internal/Component.ts b/src/runtime/internal/Component.ts
index 0772d98f6c..b30ca481c5 100644
--- a/src/runtime/internal/Component.ts
+++ b/src/runtime/internal/Component.ts
@@ -152,6 +152,8 @@ if (typeof HTMLElement === 'function') {
 		private $$data = {};
 		private $$reflecting = false;
 		private $$props_definition: Record<string, CustomElementPropDefinition> = {};
+		private $$listeners: Record<string, Function[]> = {};
+		private $$listener_unsubscribe_fns = new Map<Function, Function>();
 
 		constructor(
 			private $$componentCtor: ComponentType,
@@ -165,10 +167,26 @@ if (typeof HTMLElement === 'function') {
 			// We can't determine upfront if the event is a custom event or not, so we have to
 			// listen to both. If someone uses a custom event with the same name as a regular
 			// browser event, this fires twice - we can't avoid that.
-			this.$$component!.$on(type, listener);
+			this.$$listeners[type] = this.$$listeners[type] || [];
+			this.$$listeners[type].push(listener);
+			if (this.$$component) {
+				const unsub = this.$$component!.$on(type, listener);
+				this.$$listener_unsubscribe_fns.set(listener, unsub);
+			}
 			super.addEventListener(type, listener, options);
 		}
 
+		removeEventListener(type: string, listener: any, options?: any): void {
+			super.removeEventListener(type, listener, options);
+			if (this.$$component) {
+				const unsub = this.$$listener_unsubscribe_fns.get(listener);
+				if (unsub) {
+					unsub();
+					this.$$listener_unsubscribe_fns.delete(listener);
+				}
+			}
+		}
+
 		async connectedCallback() {
 			this.$$connected = true;
 			if (!this.$$component) {
@@ -228,6 +246,14 @@ if (typeof HTMLElement === 'function') {
 						}
 					}
 				});
+
+				for (const type in this.$$listeners) {
+					for (const listener of this.$$listeners[type]) {
+						const unsub = this.$$component!.$on(type, listener);
+						this.$$listener_unsubscribe_fns.set(listener, unsub);
+					}
+				}
+				this.$$listeners = {};
 			}
 		}
 
diff --git a/test/custom-elements/samples/events/test.js b/test/custom-elements/samples/events/test.js
index ec4fcd2796..ba87fe8f57 100644
--- a/test/custom-elements/samples/events/test.js
+++ b/test/custom-elements/samples/events/test.js
@@ -4,10 +4,20 @@ import './main.svelte';
 
 export default async function (target) {
 	target.innerHTML = '<custom-element></custom-element>';
-	await tick();
 	const el = target.querySelector('custom-element');
 
 	const events = [];
+	const custom_before = () => {
+		events.push('before');
+	};
+	const click_before = () => {
+		events.push('click_before');
+	};
+	el.addEventListener('custom', custom_before);
+	el.addEventListener('click', click_before);
+
+	await tick();
+
 	el.addEventListener('custom', e => {
 		events.push(e.detail);
 	});
@@ -16,5 +26,10 @@ export default async function (target) {
 	});
 
 	el.shadowRoot.querySelector('button').click();
-	assert.deepEqual(events, ['foo', 'click']);
+	assert.deepEqual(events, ['before', 'foo', 'click_before', 'click']);
+
+	el.removeEventListener('custom', custom_before);
+	el.removeEventListener('click', click_before);
+	el.shadowRoot.querySelector('button').click();
+	assert.deepEqual(events, ['before', 'foo', 'click_before', 'click', 'foo', 'click']);
 }