author Ryan Hunt <>
Mon, 05 Jun 2017 19:45:31 -0500
changeset 366287 70136b7cb54b75cd105f7d6e6a92d9dd0240162f
parent 366283 310bd86a94f512df05f52dd84a3576c39a0defe3
child 366291 c3110a86cddfddc0a1334f6f71a585cd7d0e0be5
permissions -rw-r--r--
Bug 1351783 part 12 - Create and sync focus sequence numbers. r=kats,botond,dvander Focus can change at any moment in a document. This causes non-determinism and correctness problems for doing keyboard apz scrolling. To get around this, we will maintain deterministic behavior for focus changes initiated by input events and see if we can get away with more non-determinism for things like `setTimeout` In order to do this, we disable async keyboard scrolling when an input event is processed that could have a event listener. We then attach a sequence number to that input event and dispatch it to content. In content, we record the highest sequence number that we have processed from an event, and send that on each focus update. Using this, we can determine in APZ if we have a current focus target or if we are still waiting for an input event to be processed and focus to be reconfirmed. MozReview-Commit-ID: CWcu8YEFQz4

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at */

#ifndef mozilla_layers_FocusState_h
#define mozilla_layers_FocusState_h

#include <unordered_map>    // for std::unordered_map
#include <unordered_set>    // for std::unordered_set

#include "FrameMetrics.h"   // for FrameMetrics::ViewID

#include "mozilla/layers/FocusTarget.h" // for FocusTarget

namespace mozilla {
namespace layers {

 * This class is used for tracking chrome and content focus targets and calculating
 * global focus information from them for use by APZCTreeManager for async keyboard
 * scrolling.
 * # Calculating the element to scroll
 * Chrome and content processes have independently focused elements. This makes it
 * difficult to calculate the global focused element and its scrollable frame from
 * the chrome or content side. So instead we send the local focus information from
 * each process to here and then calculate the global focus information. This
 * local information resides in a `focus target`.
 * A focus target indicates that either:
 *    1. The focused element is a remote browser along with its layer tree ID
 *    2. The focused element is not scrollable
 *    3. The focused element is scrollable along with the ViewID's of its
         scrollable layers
 * Using this information we can determine the global focus information by
 * starting at the focus target of the root layer tree ID and following remote
 * browsers until we reach a scrollable or non-scrollable focus target.
 * # Determinism and sequence numbers
 * The focused element in content can be changed within any javascript code. And
 * javascript can run in response to an event or at any moment from `setTimeout`
 * and others. This makes it impossible to always have the current focus
 * information in APZ as it can be changed asynchronously at any moment. If we
 * don't have the latest focus information, we may incorrectly scroll a target
 * when we shouldn't.
 * A tradeoff is designed here whereby we will maintain deterministic focus
 * changes for user input, but not for other javascript code. The reasoning
 * here is that `setTimeout` and others are already non-deterministic and so it
 * might not be as breaking to web content.
 * To maintain deterministic focus changes for a given stream of user inputs, we
 * invalidate our focus state whenever we receive a user input that may trigger
 * event listeners. We then attach a new sequence number to these events and
 * dispatch them to content. Content will then include the latest sequence number
 * it has processed to every focus update. Using this we can determine whether
 * any potentially focus changing events have yet to be handled by content.
 * Once we have received the latest focus sequence number from content, we know
 * that all event listeners triggered by user inputs, and their resulting focus
 * changes, have been processed and so we have a current target that we can use
 * again.
class FocusState final

   * The sequence number of the last potentially focus changing event processed
   * by APZ. This number starts at one and increases monotonically. This number
   * will never be zero as that is used to catch uninitialized focus sequence
   * numbers on input events.
  uint64_t LastAPZProcessedEvent() const { return mLastAPZProcessedEvent; }

   * Whether the current focus state is known to be current or else if an event
   * has been processed that could change the focus but we have not received an
   * update with a new confirmed target.
  bool IsCurrent() const;

   * Notify focus state of a potentially focus changing event. This will
   * increment the current focus sequence number. The new value can be gotten
   * from LastAPZProcessedEvent().
  void ReceiveFocusChangingEvent();

   * Update the internal focus tree and recalculate the global focus target for
   * a focus target update received from chrome or content.
   * @param aRootLayerTreeId the layer tree ID of the root layer for the
                             parent APZCTreeManager
   * @param aOriginatingLayersId the layer tree ID that this focus target
                                 belongs to
  void Update(uint64_t aRootLayerTreeId,
              uint64_t aOriginatingLayersId,
              const FocusTarget& aTarget);

   * Collects a set of the layer tree IDs that we have a focus target for.
  std::unordered_set<uint64_t> GetFocusTargetLayerIds() const;

   * Removes a focus target by its layer tree ID.
  void RemoveFocusTarget(uint64_t aLayersId);

  // The set of focus targets received indexed by their layer tree ID
  std::unordered_map<uint64_t, FocusTarget> mFocusTree;

  // The focus sequence number of the last potentially focus changing event
  // processed by APZ. This number starts at one and increases monotonically.
  // We don't worry about wrap around here because at a pace of 100 increments/sec,
  // it would take 5.85*10^9 years before we would wrap around. This number will
  // never be zero as that is used to catch uninitialized focus sequence numbers
  // on input events.
  uint64_t mLastAPZProcessedEvent;
  // The focus sequence number last received in a focus update.
  uint64_t mLastContentProcessedEvent;

  // A flag whether there is a key listener on the event target chain for the
  // focused element
  bool mFocusHasKeyEventListeners;

  // The layer tree ID which contains the scrollable frame of the focused element
  uint64_t mFocusLayersId;
  // The scrollable layer corresponding to the scrollable frame that is used to
  // scroll the focused element. This depends on the direction the user is
  // scrolling.
  FrameMetrics::ViewID mFocusHorizontalTarget;
  FrameMetrics::ViewID mFocusVerticalTarget;

} // namespace layers
} // namespace mozilla

#endif // mozilla_layers_FocusState_h