Add RepeatedTreeChangeHandler to accessibility/common

AX-Relnotes: n/a.
Bug: None
Change-Id: Ia65bacff95aed253d5ef7fd1bb286d0f19723e5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302737
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789330}

chrome/browser/resources/chromeos/accessibility/common/BUILD.gn

@@ -30,6 +30,7 @@ run_jsbundler("accessibility_common_copied_files") {
     "closure_shim.js",
     "constants.js",
     "repeated_event_handler.js",
+    "repeated_tree_change_handler.js",
     "tree_walker.js",
   ]
   rewrite_rules = [ rebase_path(".", root_build_dir) + ":" ]
@@ -83,6 +84,10 @@ js_library("repeated_event_handler") {
   externs_list = [ "$externs_path/automation.js" ]
 }
 
+js_library("repeated_tree_change_handler") {
+  externs_list = [ "$externs_path/automation.js" ]
+}
+
 source_set("browser_tests") {
   testonly = true
   assert(enable_extensions)
@@ -114,6 +119,7 @@ js2gtest("accessibility_tests") {
     "array_util_test.js",
     "automation_util_test.js",
     "repeated_event_handler_test.js",
+    "repeated_tree_change_handler_test.js",
     "tree_walker_test.js",
   ]
   gen_include_files = [

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

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * This class assists with processing repeated tree changes in nontrivial ways
+ * by allowing only the most recent tree change to be processed.
+ */
+class RepeatedTreeChangeHandler {
+  /**
+   * @param {!chrome.automation.TreeChangeObserverFilter} filter
+   * @param {!function(!chrome.automation.TreeChange)} callback
+   * @param {{predicate: (function(!chrome.automation.TreeChange): boolean)}}
+   *     options
+   *         predicate A generic predicate that filters for changes of interest.
+   */
+  constructor(filter, callback, options = {}) {
+    /** @private {!Array<!chrome.automation.TreeChange>} */
+    this.changeStack_ = [];
+
+    /** @private {!function(!chrome.automation.TreeChange)} */
+    this.callback_ = callback;
+
+    /**
+     * A predicate for which tree changes are of interest. If none is provided,
+     * default to always return true.
+     * @private {!function(!chrome.automation.TreeChange)}
+     */
+    this.predicate_ = options.predicate || ((c) => true);
+
+    /** @private {!function(!chrome.automation.TreeChange)} */
+    this.handler_ = this.onChange_.bind(this);
+
+    chrome.automation.addTreeChangeObserver(filter, this.handler_);
+  }
+
+  /**
+   * @param {!chrome.automation.TreeChange} change
+   * @private
+   */
+  onChange_(change) {
+    if (this.predicate_(change)) {
+      this.changeStack_.push(change);
+      setTimeout(this.handleChange_.bind(this), 0);
+    }
+  }
+
+  /** @private */
+  handleChange_() {
+    if (this.changeStack_.length === 0) {
+      return;
+    }
+
+    const change = this.changeStack_.pop();
+    this.changeStack_ = [];
+
+    this.callback_(change);
+  }
+}

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

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Include test fixture.
+GEN_INCLUDE([
+  '../chromevox/testing/chromevox_next_e2e_test_base.js',
+  'repeated_tree_change_handler.js'
+]);
+
+/** Test fixture for array_util.js. */
+RepeatedTreeChangeHandlerTest = class extends ChromeVoxNextE2ETest {};
+
+TEST_F(
+    'RepeatedTreeChangeHandlerTest', 'RepeatedTreeChangeHandledOnce',
+    function() {
+      this.runWithLoadedTree('', (root) => {
+        this.handlerCallCount = 0;
+        const handler = () => this.handlerCallCount++;
+
+        const repeatedHandler =
+            new RepeatedTreeChangeHandler('allTreeChanges', handler);
+
+        // Simulate events being fired.
+        repeatedHandler.onChange_();
+        repeatedHandler.onChange_();
+        repeatedHandler.onChange_();
+        repeatedHandler.onChange_();
+        repeatedHandler.onChange_();
+
+        // Yield before verifying how many times the handler was called.
+        setTimeout(() => assertEquals(this.handlerCallCount, 1), 0);
+      });
+    });
+
+TEST_F('RepeatedTreeChangeHandlerTest', 'Predicate', function() {
+  this.runWithLoadedTree('', (root) => {
+    this.handlerCallCount = 0;
+    const handler = () => this.handlerCallCount++;
+
+    const repeatedHandler = new RepeatedTreeChangeHandler(
+        'allTreeChanges', handler,
+        {predicate: (c) => c.type === 'nodeRemoved'});
+
+    // Simulate events being fired.
+    repeatedHandler.onChange_({type: 'nodeAdded'});
+    repeatedHandler.onChange_({type: 'nodeAdded'});
+    repeatedHandler.onChange_({type: 'nodeAdded'});
+    repeatedHandler.onChange_({type: 'nodeRemoved'});
+    repeatedHandler.onChange_({type: 'nodeRemoved'});
+    repeatedHandler.onChange_({type: 'nodeRemoved'});
+    repeatedHandler.onChange_({type: 'nodeRemoved'});
+
+    // Verify that nodes that don't satisfy the predicate aren't added to the
+    // change stack.
+    assertEquals(repeatedHandler.changeStack_.length, 4);
+
+    // Yield before verifying how many times the handler was called.
+    setTimeout(() => assertEquals(this.handlerCallCount, 1), 0);
+  });
+});