feat(core events): Support once-events in add_event_listener.

Add support for once-events in add_event_listener and unregister them
when called from the event_listener_map.

Author: thet

src/core/events.js

@@ -3,7 +3,7 @@
 // Event listener registration for easy-to-remove event listeners.
 // once Safari supports the ``signal`` option for addEventListener we can abort
 // event handlers by calling AbortController.abort().
-const event_listener_map = {};
+export const event_listener_map = {};
 
 /**
  * Add an event listener to a DOM element under a unique id.
@@ -22,11 +22,21 @@ const add_event_listener = (el, event_type, id, cb, opts = {}) => {
     }
     remove_event_listener(el, id); // do not register one listener twice.
 
+    // Create event_listener_map entry if not existent.
     if (!event_listener_map[el]) {
         event_listener_map[el] = {};
     }
-    event_listener_map[el][id] = [event_type, cb, opts.capture ? opts : undefined]; // prettier-ignore
-    el.addEventListener(event_type, cb, opts);
+    let _cb = cb;
+    if (opts?.once === true) {
+        // For `once` events, also remove the entry from the event_listener_map.
+        _cb = (e) => {
+            delete event_listener_map[el][id];
+            cb(e);
+        };
+    }
+    // Only `capture` option is necessary for `removeEventListener`.
+    event_listener_map[el][id] = [event_type, _cb, opts.capture ? opts : undefined];
+    el.addEventListener(event_type, _cb, opts);
 };
 
 /**

src/core/events.test.js

@@ -50,6 +50,21 @@ describe("core.events tests", () => {
             done();
         });
 
+        it("Supports once-events and unregisters them from the event_listener_map", async () => {
+            const event_listener_map = (await import("./events")).event_listener_map;
+            const el = document.createElement("div");
+
+            // register the once-event handler
+            events.add_event_listener(el, "test", "test_once_event", () => {}, {
+                once: true,
+            });
+
+            expect(event_listener_map[el].test_once_event).toBeDefined();
+            el.dispatchEvent(new Event("test"));
+
+            expect(event_listener_map[el].test_once_event).not.toBeDefined();
+        });
+
         it("Awaits an event to happen", async () => {
             const el = document.createElement("div");