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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 http://mozilla.org/MPL/2.0/. */
#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 "mozilla/layers/FocusTarget.h" // for FocusTarget
#include "mozilla/layers/ScrollableLayerGuid.h" // for ViewID
#include "mozilla/Mutex.h" // for Mutex
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 {
public:
FocusState();
/**
* 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;
/**
* 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(LayersId aRootLayerTreeId, LayersId aOriginatingLayersId,
const FocusTarget& aTarget);
/**
* Removes a focus target by its layer tree ID.
*/
void RemoveFocusTarget(LayersId aLayersId);
/**
* Gets the scrollable layer that should be horizontally scrolled for a key
* event, if any. The returned ScrollableLayerGuid doesn't contain a
* presShellId, and so it should not be used in comparisons.
*
* No scrollable layer is returned if any of the following are true:
* 1. We don't have a current focus target
* 2. There are event listeners that could change the focus
* 3. The target has not been layerized
*/
Maybe<ScrollableLayerGuid> GetHorizontalTarget() const;
/**
* The same as GetHorizontalTarget() but for vertical scrolling.
*/
Maybe<ScrollableLayerGuid> GetVerticalTarget() const;
/**
* Gets whether it is safe to not increment the focus sequence number for an
* unmatched keyboard event.
*/
bool CanIgnoreKeyboardShortcutMisses() const;
private:
/**
* 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.
* This can only be called by methods that have already acquired mMutex; they
* have to pass their lock as compile-time proof.
*/
bool IsCurrent(const MutexAutoLock& aLock) const;
private:
// All methods should hold this lock, since this class is accessed via both
// the updater and controller threads.
mutable Mutex mMutex;
// The set of focus targets received indexed by their layer tree ID
std::unordered_map<LayersId, FocusTarget, LayersId::HashFn> 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;
// A flag that is false until the first call to Update().
bool mReceivedUpdate;
// The layer tree ID which contains the scrollable frame of the focused
// element
LayersId 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.
ScrollableLayerGuid::ViewID mFocusHorizontalTarget;
ScrollableLayerGuid::ViewID mFocusVerticalTarget;
};
} // namespace layers
} // namespace mozilla
#endif // mozilla_layers_FocusState_h