[Switch Access] Use EventHandler to look for nodes

Code clean-up. Does not change the behavior of Switch Access.

AX-Relnotes: n/a.
Bug: None.
Change-Id: I49b5eefd69c9165f9729927f85779b8bdacfc224
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373290Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#801612}

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

@@ -12,7 +12,7 @@ class EventHandler {
    *         !Array<!chrome.automation.AutomationNode>} nodes
    * @param {!chrome.automation.EventType |
    *     !Array<!chrome.automation.EventType>} types
-   * @param {!function(!chrome.automation.AutomationEvent)} callback
+   * @param {?function(!chrome.automation.AutomationEvent)} callback
    * @param {{capture: (boolean|undefined), exactMatch: (boolean|undefined),
    *     listenOnce: (boolean|undefined), predicate:
    *     ((function(chrome.automation.AutomationEvent): boolean)|undefined)}}
@@ -32,7 +32,7 @@ class EventHandler {
     /** @private {!Array<!chrome.automation.EventType>} */
     this.types_ = types instanceof Array ? types : [types];
 
-    /** @private {!function(!chrome.automation.AutomationEvent)} */
+    /** @private {?function(!chrome.automation.AutomationEvent)} */
     this.callback_ = callback;
 
     /** @private {boolean} */
@@ -81,6 +81,11 @@ class EventHandler {
     this.listening_ = false;
   }
 
+  /** @param {?function(!chrome.automation.AutomationEvent)} callback */
+  setCallback(callback) {
+    this.callback_ = callback;
+  }
+
   /**
    * Changes what nodes are being listened to. Removes listeners from existing
    *     nodes before adding listeners on new nodes.
@@ -138,6 +143,8 @@ class EventHandler {
       this.stop();
     }
 
-    this.callback_(event);
+    if (this.callback_) {
+      this.callback_(event);
+    }
   }
 }

chrome/browser/resources/chromeos/accessibility/switch_access/switch_access.js

@@ -63,32 +63,27 @@ class SwitchAccess {
     }
     // If it's not currently in the tree, listen for changes to the desktop
     // tree.
-    const onDesktopChildrenChanged = (event) => {
+    const eventHandler = new EventHandler(
+        desktop, chrome.automation.EventType.CHILDREN_CHANGED,
+        null /** callback */);
+
+    const onEvent = (event) => {
       if (event.target.matches(findParams)) {
         // If the event target is the node we're looking for, we've found it.
-        desktop.removeEventListener(
-            chrome.automation.EventType.CHILDREN_CHANGED,
-            onDesktopChildrenChanged, false);
+        eventHandler.stop();
         foundCallback(event.target);
       } else if (event.target.children.length > 0) {
         // Otherwise, see if one of its children is the node we're looking for.
         node = event.target.find(findParams);
         if (node) {
-          desktop.removeEventListener(
-              chrome.automation.EventType.CHILDREN_CHANGED,
-              onDesktopChildrenChanged, false);
+          eventHandler.stop();
           foundCallback(node);
         }
       }
     };
 
-    // Note: Cannot use an EventHandler here because in some cases we want
-    // to run foundCallback on the event.target, and in some cases we want to
-    // run foundCallback on one of the target's children. We would need to
-    // recalculate the interesting child twice to use EventHandler.
-    desktop.addEventListener(
-        chrome.automation.EventType.CHILDREN_CHANGED, onDesktopChildrenChanged,
-        false);
+    eventHandler.setCallback(onEvent);
+    eventHandler.start();
   }
 
   /*