Avoid repeatedly starting RepeatedEventHandler

Also adds a test for stopping promptly when asked.

AX-Relnotes: n/a.
Bug: None.
Change-Id: Id0dc61f10a7c08aa01658b98c0b429eb95ef753b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380585
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802817}

chrome/browser/resources/chromeos/accessibility/common/repeated_event_handler.js

@@ -63,6 +63,9 @@ class RepeatedEventHandler {
 
   /** Starts listening or handling events. */
   startListening() {
+    if (this.listening_) {
+      return;
+    }
     this.listening_ = true;
     for (const node of this.nodes_) {
       node.addEventListener(this.type_, this.handler_, this.capture_);
@@ -71,6 +74,9 @@ class RepeatedEventHandler {
 
   /** Stops listening or handling future events. */
   stopListening() {
+    if (!this.listening_) {
+      return;
+    }
     this.listening_ = false;
     for (const node of this.nodes_) {
       node.removeEventListener(this.type_, this.handler_, this.capture_);

chrome/browser/resources/chromeos/accessibility/common/repeated_event_handler_test.js

@@ -26,6 +26,30 @@ TEST_F('RepeatedEventHandlerTest', 'RepeatedEventHandledOnce', function() {
     repeatedHandler.onEvent_();
 
     // Yield before verify how many times the handler was called.
-    setTimeout(() => assertEquals(this.handlerCallCount, 1), 0);
+    setTimeout(
+        this.newCallback(() => assertEquals(this.handlerCallCount, 1)), 0);
   });
 });
+
+TEST_F(
+    'RepeatedEventHandlerTest', 'NoEventsHandledAfterStopListening',
+    function() {
+      this.runWithLoadedTree('', (root) => {
+        this.handlerCallCount = 0;
+        const handler = () => this.handlerCallCount++;
+
+        const repeatedHandler =
+            new RepeatedEventHandler(root, 'focus', handler);
+
+        // Simulate events being fired.
+        repeatedHandler.onEvent_();
+        repeatedHandler.onEvent_();
+        repeatedHandler.onEvent_();
+
+        repeatedHandler.stopListening();
+
+        // Yield before verifying how many times the handler was called.
+        setTimeout(
+            this.newCallback(() => assertEquals(this.handlerCallCount, 0)), 0);
+      });
+    });