Make evented#addEventListener idempotent

mapbox · May 18, 2018 · bac1f67 · bac1f67
1 parent 6b82353
commit bac1f67
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Showing 2 changed files with 18 additions and 2 deletions.
diff --git a/src/util/evented.js b/src/util/evented.js
@@ -6,8 +6,11 @@ type Listener = (Object) => any;
 type Listeners = { [string]: Array<Listener> };
 
 function _addEventListener(type: string, listener: Listener, listenerList: Listeners) {
-    listenerList[type] = listenerList[type] || [];
-    listenerList[type].push(listener);
+    const listenerExists = listenerList[type] && listenerList[type].indexOf(listener) !== -1;
+    if (!listenerExists) {
+        listenerList[type] = listenerList[type] || [];
+        listenerList[type].push(listener);
+    }
 }
 
 function _removeEventListener(type: string, listener: Listener, listenerList: Listeners) {

diff --git a/test/unit/util/evented.test.js b/test/unit/util/evented.test.js
@@ -119,6 +119,19 @@ test('Evented', (t) => {
         t.end();
     });
 
+    t.test('on is idempotent', (t) => {
+        const evented = new Evented();
+        const listenerA = t.spy();
+        const listenerB = t.spy();
+        evented.on('a', listenerA);
+        evented.on('a', listenerB);
+        evented.on('a', listenerA);
+        evented.fire(new Event('a'));
+        t.ok(listenerA.calledOnce);
+        t.ok(listenerA.calledBefore(listenerB));
+        t.end();
+    });
+
     t.test('evented parents', (t) => {
 
         t.test('adds parents with "setEventedParent"', (t) => {