ui/accessibility/ax_tree_update_util.cc

// Copyright 2022 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "ui/accessibility/ax_tree_update_util.h"

#include "base/metrics/histogram_macros.h"
#include "ui/accessibility/ax_tree_update.h"

namespace ui {

// Two tree updates can be merged into one if the second one
// doesn't clear a subtree, doesn't have new tree data, and
// doesn't have a new root id - in other words the second tree
// update consists of only changes to nodes.
bool AXTreeUpdatesCanBeMerged(const AXTreeUpdate& u1, const AXTreeUpdate& u2) {
  if (u1.node_id_to_clear != kInvalidAXNodeID ||
      u2.node_id_to_clear != kInvalidAXNodeID) {
    return false;
  }

  if (u2.has_tree_data && u2.tree_data != u1.tree_data) {
    return false;
  }

  if (u2.root_id != u1.root_id) {
    return false;
  }

  if (u1.event_from != u2.event_from) {
    return false;
  }

  if (u1.event_from_action != u2.event_from_action) {
    return false;
  }

  if (u1.event_intents != u2.event_intents) {
    return false;
  }

  return true;
}

bool MergeAXTreeUpdates(std::vector<AXTreeUpdate>& src,
                        std::vector<AXTreeUpdate>* dst) {
  SCOPED_UMA_HISTOGRAM_TIMER_MICROS(
      "Accessibility.Performance.MergeAXTreeUpdates");

  size_t merge_count = 0;
  for (size_t i = 1; i < src.size(); i++) {
    if (AXTreeUpdatesCanBeMerged(src[i - 1], src[i]))
      merge_count++;
  }

  // Doing a single merge isn't necessarily worth it because
  // copying the tree updates takes time too so the total
  // savings is less. But two more more merges is probably
  // worth the overhead of copying.
  if (merge_count < 2)
    return false;

  dst->resize(src.size() - merge_count);
  (*dst)[0] = std::move(src[0]);
  size_t dst_index = 0;
  for (size_t i = 1; i < src.size(); i++) {
    // Use (*dst)[dst_index] over src[i-1] for the merge because src[i-1] may
    // be in an undetermined state after being merged.
    if (AXTreeUpdatesCanBeMerged((*dst)[dst_index], src[i])) {
      std::vector<AXNodeData>& dst_nodes = (*dst)[dst_index].nodes;
      std::vector<AXNodeData>& src_nodes = src[i].nodes;
      for (auto& src_node : src_nodes) {
        dst_nodes.emplace_back(src_node);
      }
    } else {
      dst_index++;
      (*dst)[dst_index] = src[i];
    }
  }

  return true;
}

}  // namespace ui