shell/platform/linux/fl_key_embedder_responder.cc

// Copyright 2013 The Flutter 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 "flutter/shell/platform/linux/fl_key_embedder_responder.h"

#include <gtk/gtk.h>
#include <cinttypes>

#include "flutter/shell/platform/embedder/embedder.h"
#include "flutter/shell/platform/linux/fl_key_embedder_responder_private.h"
#include "flutter/shell/platform/linux/key_mapping.h"

constexpr uint64_t kMicrosecondsPerMillisecond = 1000;

static const FlutterKeyEvent kEmptyEvent{
    .struct_size = sizeof(FlutterKeyEvent),
    .timestamp = 0,
    .type = kFlutterKeyEventTypeDown,
    .physical = 0,
    .logical = 0,
    .character = nullptr,
    .synthesized = false,
};

// Look up a hash table that maps a uint64_t to a uint64_t.
//
// Returns 0 if not found.
//
// Both key and value should be directly hashed.
static uint64_t lookup_hash_table(GHashTable* table, uint64_t key) {
  return gpointer_to_uint64(
      g_hash_table_lookup(table, uint64_to_gpointer(key)));
}

static gboolean hash_table_find_equal_value(gpointer key,
                                            gpointer value,
                                            gpointer user_data) {
  return gpointer_to_uint64(value) == gpointer_to_uint64(user_data);
}

// Look up a hash table that maps a uint64_t to a uint64_t; given its key,
// find its value.
//
// Returns 0 if not found.
//
// Both key and value should be directly hashed.
static uint64_t reverse_lookup_hash_table(GHashTable* table, uint64_t value) {
  return gpointer_to_uint64(g_hash_table_find(
      table, hash_table_find_equal_value, uint64_to_gpointer(value)));
}

static uint64_t to_lower(uint64_t n) {
  constexpr uint64_t lower_a = 0x61;
  constexpr uint64_t upper_a = 0x41;
  constexpr uint64_t upper_z = 0x5a;

  constexpr uint64_t lower_a_grave = 0xe0;
  constexpr uint64_t upper_a_grave = 0xc0;
  constexpr uint64_t upper_thorn = 0xde;
  constexpr uint64_t division = 0xf7;

  // ASCII range.
  if (n >= upper_a && n <= upper_z) {
    return n - upper_a + lower_a;
  }

  // EASCII range.
  if (n >= upper_a_grave && n <= upper_thorn && n != division) {
    return n - upper_a_grave + lower_a_grave;
  }

  return n;
}

/* Define FlKeyEmbedderUserData */

/**
 * FlKeyEmbedderUserData:
 * The user_data used when #FlKeyEmbedderResponder sends message through the
 * embedder.SendKeyEvent API.
 */
#define FL_TYPE_EMBEDDER_USER_DATA fl_key_embedder_user_data_get_type()
G_DECLARE_FINAL_TYPE(FlKeyEmbedderUserData,
                     fl_key_embedder_user_data,
                     FL,
                     KEY_EMBEDDER_USER_DATA,
                     GObject);

struct _FlKeyEmbedderUserData {
  GObject parent_instance;

  FlKeyResponderAsyncCallback callback;
  gpointer user_data;
};

G_DEFINE_TYPE(FlKeyEmbedderUserData, fl_key_embedder_user_data, G_TYPE_OBJECT)

static void fl_key_embedder_user_data_dispose(GObject* object);

static void fl_key_embedder_user_data_class_init(
    FlKeyEmbedderUserDataClass* klass) {
  G_OBJECT_CLASS(klass)->dispose = fl_key_embedder_user_data_dispose;
}

static void fl_key_embedder_user_data_init(FlKeyEmbedderUserData* self) {}

static void fl_key_embedder_user_data_dispose(GObject* object) {
  // The following line suppresses a warning for unused function
  // FL_IS_KEY_EMBEDDER_USER_DATA.
  g_return_if_fail(FL_IS_KEY_EMBEDDER_USER_DATA(object));
}

// Creates a new FlKeyChannelUserData private class with all information.
//
// The callback and the user_data might be nullptr.
static FlKeyEmbedderUserData* fl_key_embedder_user_data_new(
    FlKeyResponderAsyncCallback callback,
    gpointer user_data) {
  FlKeyEmbedderUserData* self = FL_KEY_EMBEDDER_USER_DATA(
      g_object_new(FL_TYPE_EMBEDDER_USER_DATA, nullptr));

  self->callback = callback;
  self->user_data = user_data;
  return self;
}

/* Define FlKeyEmbedderResponder */

namespace {

typedef enum {
  kStateLogicUndecided,
  kStateLogicNormal,
  kStateLogicReversed,
} StateLogicInferrence;

}

struct _FlKeyEmbedderResponder {
  GObject parent_instance;

  EmbedderSendKeyEvent send_key_event;
  void* send_key_event_user_data;

  // Internal record for states of whether a key is pressed.
  //
  // It is a map from Flutter physical key to Flutter logical key.  Both keys
  // and values are directly stored uint64s.  This table is freed by the
  // responder.
  GHashTable* pressing_records;

  // Internal record for states of whether a lock mode is enabled.
  //
  // It is a bit mask composed of GTK mode bits.
  guint lock_records;

  // Internal record for the last observed key mapping.
  //
  // It stores the physical key last seen during a key down event for a logical
  // key. It is used to synthesize modifier keys and lock keys.
  //
  // It is a map from Flutter logical key to physical key.  Both keys and
  // values are directly stored uint64s.  This table is freed by the responder.
  GHashTable* mapping_records;

  // The inferred logic type indicating whether the CapsLock state logic is
  // reversed on this platform.
  //
  // For more information, see #update_caps_lock_state_logic_inferrence.
  StateLogicInferrence caps_lock_state_logic_inferrence;

  // Record if any events has been sent during a
  // |fl_key_embedder_responder_handle_event| call.
  bool sent_any_events;

  // A static map from GTK modifier bits to #FlKeyEmbedderCheckedKey to
  // configure the modifier keys that needs to be tracked and kept synchronous
  // on.
  //
  // The keys are directly stored guints.  The values must be freed with g_free.
  // This table is freed by the responder.
  GHashTable* modifier_bit_to_checked_keys;

  // A static map from GTK modifier bits to #FlKeyEmbedderCheckedKey to
  // configure the lock mode bits that needs to be tracked and kept synchronous
  // on.
  //
  // The keys are directly stored guints.  The values must be freed with g_free.
  // This table is freed by the responder.
  GHashTable* lock_bit_to_checked_keys;

  // A static map generated by reverse mapping lock_bit_to_checked_keys.
  //
  // It is a map from primary physical keys to lock bits.  Both keys and values
  // are directly stored uint64s.  This table is freed by the responder.
  GHashTable* logical_key_to_lock_bit;
};

static void fl_key_embedder_responder_iface_init(
    FlKeyResponderInterface* iface);
static void fl_key_embedder_responder_dispose(GObject* object);

#define FL_TYPE_EMBEDDER_RESPONDER_USER_DATA \
  fl_key_embedder_responder_get_type()
G_DEFINE_TYPE_WITH_CODE(
    FlKeyEmbedderResponder,
    fl_key_embedder_responder,
    G_TYPE_OBJECT,
    G_IMPLEMENT_INTERFACE(FL_TYPE_KEY_RESPONDER,
                          fl_key_embedder_responder_iface_init))

static void fl_key_embedder_responder_handle_event(
    FlKeyResponder* responder,
    FlKeyEvent* event,
    uint64_t specified_logical_key,
    FlKeyResponderAsyncCallback callback,
    gpointer user_data);

static void fl_key_embedder_responder_iface_init(
    FlKeyResponderInterface* iface) {
  iface->handle_event = fl_key_embedder_responder_handle_event;
}

// Initializes the FlKeyEmbedderResponder class methods.
static void fl_key_embedder_responder_class_init(
    FlKeyEmbedderResponderClass* klass) {
  G_OBJECT_CLASS(klass)->dispose = fl_key_embedder_responder_dispose;
}

// Initializes an FlKeyEmbedderResponder instance.
static void fl_key_embedder_responder_init(FlKeyEmbedderResponder* self) {}

// Disposes of an FlKeyEmbedderResponder instance.
static void fl_key_embedder_responder_dispose(GObject* object) {
  FlKeyEmbedderResponder* self = FL_KEY_EMBEDDER_RESPONDER(object);

  g_clear_pointer(&self->pressing_records, g_hash_table_unref);
  g_clear_pointer(&self->mapping_records, g_hash_table_unref);
  g_clear_pointer(&self->modifier_bit_to_checked_keys, g_hash_table_unref);
  g_clear_pointer(&self->lock_bit_to_checked_keys, g_hash_table_unref);
  g_clear_pointer(&self->logical_key_to_lock_bit, g_hash_table_unref);

  G_OBJECT_CLASS(fl_key_embedder_responder_parent_class)->dispose(object);
}

// Fill in #logical_key_to_lock_bit by associating a logical key with
// its corresponding modifier bit.
//
// This is used as the body of a loop over #lock_bit_to_checked_keys.
static void initialize_logical_key_to_lock_bit_loop_body(gpointer lock_bit,
                                                         gpointer value,
                                                         gpointer user_data) {
  FlKeyEmbedderCheckedKey* checked_key =
      reinterpret_cast<FlKeyEmbedderCheckedKey*>(value);
  GHashTable* table = reinterpret_cast<GHashTable*>(user_data);
  g_hash_table_insert(table,
                      uint64_to_gpointer(checked_key->primary_logical_key),
                      GUINT_TO_POINTER(lock_bit));
}

// Creates a new FlKeyEmbedderResponder instance.
FlKeyEmbedderResponder* fl_key_embedder_responder_new(
    EmbedderSendKeyEvent send_key_event,
    void* send_key_event_user_data) {
  FlKeyEmbedderResponder* self = FL_KEY_EMBEDDER_RESPONDER(
      g_object_new(FL_TYPE_EMBEDDER_RESPONDER_USER_DATA, nullptr));

  self->send_key_event = send_key_event;
  self->send_key_event_user_data = send_key_event_user_data;

  self->pressing_records = g_hash_table_new(g_direct_hash, g_direct_equal);
  self->mapping_records = g_hash_table_new(g_direct_hash, g_direct_equal);
  self->lock_records = 0;
  self->caps_lock_state_logic_inferrence = kStateLogicUndecided;

  self->modifier_bit_to_checked_keys =
      g_hash_table_new_full(g_direct_hash, g_direct_equal, NULL, g_free);
  initialize_modifier_bit_to_checked_keys(self->modifier_bit_to_checked_keys);

  self->lock_bit_to_checked_keys =
      g_hash_table_new_full(g_direct_hash, g_direct_equal, NULL, g_free);
  initialize_lock_bit_to_checked_keys(self->lock_bit_to_checked_keys);

  self->logical_key_to_lock_bit =
      g_hash_table_new(g_direct_hash, g_direct_equal);
  g_hash_table_foreach(self->lock_bit_to_checked_keys,
                       initialize_logical_key_to_lock_bit_loop_body,
                       self->logical_key_to_lock_bit);

  return self;
}

/* Implement FlKeyEmbedderUserData */

static uint64_t apply_id_plane(uint64_t logical_id, uint64_t plane) {
  return (logical_id & kValueMask) | plane;
}

static uint64_t event_to_physical_key(const FlKeyEvent* event) {
  auto found = xkb_to_physical_key_map.find(event->keycode);
  if (found != xkb_to_physical_key_map.end()) {
    return found->second;
  }
  return apply_id_plane(event->keycode, kGtkPlane);
}

static uint64_t event_to_logical_key(const FlKeyEvent* event) {
  guint keyval = event->keyval;
  auto found = gtk_keyval_to_logical_key_map.find(keyval);
  if (found != gtk_keyval_to_logical_key_map.end()) {
    return found->second;
  }
  // EASCII range
  if (keyval < 256) {
    return apply_id_plane(to_lower(keyval), kUnicodePlane);
  }
  // Auto-generate key
  return apply_id_plane(keyval, kGtkPlane);
}

static uint64_t event_to_timestamp(const FlKeyEvent* event) {
  return kMicrosecondsPerMillisecond * static_cast<double>(event->time);
}

// Returns a newly accocated UTF-8 string from event->keyval that must be
// freed later with g_free().
static char* event_to_character(const FlKeyEvent* event) {
  gunichar unicodeChar = gdk_keyval_to_unicode(event->keyval);
  glong items_written;
  gchar* result = g_ucs4_to_utf8(&unicodeChar, 1, NULL, &items_written, NULL);
  if (items_written == 0) {
    if (result != NULL) {
      g_free(result);
    }
    return nullptr;
  }
  return result;
}

// Handles a response from the embedder API to a key event sent to the framework
// earlier.
static void handle_response(bool handled, gpointer user_data) {
  g_autoptr(FlKeyEmbedderUserData) data = FL_KEY_EMBEDDER_USER_DATA(user_data);

  g_return_if_fail(data->callback != nullptr);

  data->callback(handled, data->user_data);
}

// Sends a synthesized event to the framework with no demand for callback.
static void synthesize_simple_event(FlKeyEmbedderResponder* self,
                                    FlutterKeyEventType type,
                                    uint64_t physical,
                                    uint64_t logical,
                                    double timestamp) {
  FlutterKeyEvent out_event;
  out_event.struct_size = sizeof(out_event);
  out_event.timestamp = timestamp;
  out_event.type = type;
  out_event.physical = physical;
  out_event.logical = logical;
  out_event.character = nullptr;
  out_event.synthesized = true;
  self->sent_any_events = true;
  self->send_key_event(&out_event, nullptr, nullptr,
                       self->send_key_event_user_data);
}

namespace {

// Context variables for the foreach call used to synchronize pressing states
// and lock states.
typedef struct {
  FlKeyEmbedderResponder* self;
  guint state;
  uint64_t event_logical_key;
  bool is_down;
  double timestamp;
} SyncStateLoopContext;

// Context variables for the foreach call used to find the physical key from
// a modifier logical key.
typedef struct {
  bool known_modifier_physical_key;
  uint64_t logical_key;
  uint64_t physical_key_from_event;
  uint64_t corrected_physical_key;
} ModifierLogicalToPhysicalContext;

}  // namespace

// Update the pressing record.
//
// If `logical_key` is 0, the record will be set as "released".  Otherwise, the
// record will be set as "pressed" with this logical key.  This function asserts
// that the key is pressed if the caller asked to release, and vice versa.
static void update_pressing_state(FlKeyEmbedderResponder* self,
                                  uint64_t physical_key,
                                  uint64_t logical_key) {
  if (logical_key != 0) {
    g_return_if_fail(lookup_hash_table(self->pressing_records, physical_key) ==
                     0);
    g_hash_table_insert(self->pressing_records,
                        uint64_to_gpointer(physical_key),
                        uint64_to_gpointer(logical_key));
  } else {
    g_return_if_fail(lookup_hash_table(self->pressing_records, physical_key) !=
                     0);
    g_hash_table_remove(self->pressing_records,
                        uint64_to_gpointer(physical_key));
  }
}

// Update the lock record.
//
// If `is_down` is false, this function is a no-op.  Otherwise, this function
// finds the lock bit corresponding to `physical_key`, and flips its bit.
static void possibly_update_lock_bit(FlKeyEmbedderResponder* self,
                                     uint64_t logical_key,
                                     bool is_down) {
  if (!is_down) {
    return;
  }
  const guint mode_bit = GPOINTER_TO_UINT(g_hash_table_lookup(
      self->logical_key_to_lock_bit, uint64_to_gpointer(logical_key)));
  if (mode_bit != 0) {
    self->lock_records ^= mode_bit;
  }
}

static void update_mapping_record(FlKeyEmbedderResponder* self,
                                  uint64_t physical_key,
                                  uint64_t logical_key) {
  g_hash_table_insert(self->mapping_records, uint64_to_gpointer(logical_key),
                      uint64_to_gpointer(physical_key));
}

// Synchronizes the pressing state of a key to its state from the event by
// synthesizing events.
//
// This is used as the body of a loop over #modifier_bit_to_checked_keys.
static void synchronize_pressed_states_loop_body(gpointer key,
                                                 gpointer value,
                                                 gpointer user_data) {
  SyncStateLoopContext* context =
      reinterpret_cast<SyncStateLoopContext*>(user_data);
  FlKeyEmbedderCheckedKey* checked_key =
      reinterpret_cast<FlKeyEmbedderCheckedKey*>(value);

  const guint modifier_bit = GPOINTER_TO_INT(key);
  FlKeyEmbedderResponder* self = context->self;
  // Each TestKey contains up to two logical keys, typically the left modifier
  // and the right modifier, that correspond to the same modifier_bit. We'd
  // like to infer whether to synthesize a down or up event for each key.
  //
  // The hard part is that, if we want to synthesize a down event, we don't know
  // which physical key to use. Here we assume the keyboard layout do not change
  // frequently and use the last physical-logical relationship, recorded in
  // #mapping_records.
  const uint64_t logical_keys[] = {
      checked_key->primary_logical_key,
      checked_key->secondary_logical_key,
  };
  const guint length = checked_key->secondary_logical_key == 0 ? 1 : 2;

  const bool any_pressed_by_state = (context->state & modifier_bit) != 0;

  bool any_pressed_by_record = false;

  // Traverse each logical key of this modifier bit for 2 purposes:
  //
  //  1. Perform the synthesization of release events: If the modifier bit is 0
  //     and the key is pressed, synthesize a release event.
  //  2. Prepare for the synthesization of press events: If the modifier bit is
  //     1, and no keys are pressed (discovered here), synthesize a press event
  //     later.
  for (guint logical_key_idx = 0; logical_key_idx < length; logical_key_idx++) {
    const uint64_t logical_key = logical_keys[logical_key_idx];
    g_return_if_fail(logical_key != 0);
    const uint64_t pressing_physical_key =
        reverse_lookup_hash_table(self->pressing_records, logical_key);
    const bool this_key_pressed_before_event = pressing_physical_key != 0;

    any_pressed_by_record =
        any_pressed_by_record || this_key_pressed_before_event;

    if (this_key_pressed_before_event && !any_pressed_by_state) {
      const uint64_t recorded_physical_key =
          lookup_hash_table(self->mapping_records, logical_key);
      // Since this key has been pressed before, there must have been a recorded
      // physical key.
      g_return_if_fail(recorded_physical_key != 0);
      // In rare cases #recorded_logical_key is different from #logical_key.
      const uint64_t recorded_logical_key =
          lookup_hash_table(self->pressing_records, recorded_physical_key);
      synthesize_simple_event(self, kFlutterKeyEventTypeUp,
                              recorded_physical_key, recorded_logical_key,
                              context->timestamp);
      update_pressing_state(self, recorded_physical_key, 0);
    }
  }
  // If the modifier should be pressed, synthesize a down event for its primary
  // key.
  if (any_pressed_by_state && !any_pressed_by_record) {
    const uint64_t logical_key = checked_key->primary_logical_key;
    const uint64_t recorded_physical_key =
        lookup_hash_table(self->mapping_records, logical_key);
    // The physical key is derived from past mapping record if possible.
    //
    // The event to be synthesized is a key down event. There might not have
    // been a mapping record, in which case the hard-coded #primary_physical_key
    // is used.
    const uint64_t physical_key = recorded_physical_key != 0
                                      ? recorded_physical_key
                                      : checked_key->primary_physical_key;
    if (recorded_physical_key == 0) {
      update_mapping_record(self, physical_key, logical_key);
    }
    synthesize_simple_event(self, kFlutterKeyEventTypeDown, physical_key,
                            logical_key, context->timestamp);
    update_pressing_state(self, physical_key, logical_key);
  }
}

// Find the stage # by the current record, which should be the recorded stage
// before the event.
static int find_stage_by_record(bool is_down, bool is_enabled) {
  constexpr int stage_by_record_index[] = {
      0,  // is_down: 0,  is_enabled: 0
      2,  //          0               1
      3,  //          1               0
      1   //          1               1
  };
  return stage_by_record_index[(is_down << 1) + is_enabled];
}

// Find the stage # by an event for the target key, which should be inferred
// stage before the event.
static int find_stage_by_self_event(int stage_by_record,
                                    bool is_down_event,
                                    bool is_state_on,
                                    bool reverse_state_logic) {
  if (!is_state_on) {
    return reverse_state_logic ? 2 : 0;
  }
  if (is_down_event) {
    return reverse_state_logic ? 0 : 2;
  }
  return stage_by_record;
}

// Find the stage # by an event for a non-target key, which should be inferred
// stage during the event.
static int find_stage_by_others_event(int stage_by_record, bool is_state_on) {
  g_return_val_if_fail(stage_by_record >= 0 && stage_by_record < 4,
                       stage_by_record);
  if (!is_state_on) {
    return 0;
  }
  if (stage_by_record == 0) {
    return 1;
  }
  return stage_by_record;
}

// Infer the logic type of CapsLock on the current platform if applicable.
//
// In most cases, when a lock key is pressed or released, its event has the
// key's state as 0-1-1-1 for the 4 stages (as documented in
// #synchronize_lock_states_loop_body) respectively.  But in very rare cases it
// produces 1-1-0-1, which we call "reversed state logic".  This is observed
// when using Chrome Remote Desktop on macOS (likely a bug).
//
// To detect whether the current platform behaves normally or reversed, this
// function is called on the first down event of CapsLock before calculating
// stages.  This function then store the inferred mode as
// self->caps_lock_state_logic_inferrence.
//
// This does not help if the same app session is used alternatively between a
// reversed platform and a normal platform.  But this is the best we can do.
static void update_caps_lock_state_logic_inferrence(
    FlKeyEmbedderResponder* self,
    bool is_down_event,
    bool enabled_by_state,
    int stage_by_record) {
  if (self->caps_lock_state_logic_inferrence != kStateLogicUndecided) {
    return;
  }
  if (!is_down_event) {
    return;
  }
  const int stage_by_event = find_stage_by_self_event(
      stage_by_record, is_down_event, enabled_by_state, false);
  if ((stage_by_event == 0 && stage_by_record == 2) ||
      (stage_by_event == 2 && stage_by_record == 0)) {
    self->caps_lock_state_logic_inferrence = kStateLogicReversed;
  } else {
    self->caps_lock_state_logic_inferrence = kStateLogicNormal;
  }
}

// Synchronizes the lock state of a key to its state from the event by
// synthesizing events.
//
// This is used as the body of a loop over #lock_bit_to_checked_keys.
//
// This function might modify #caps_lock_state_logic_inferrence.
static void synchronize_lock_states_loop_body(gpointer key,
                                              gpointer value,
                                              gpointer user_data) {
  SyncStateLoopContext* context =
      reinterpret_cast<SyncStateLoopContext*>(user_data);
  FlKeyEmbedderCheckedKey* checked_key =
      reinterpret_cast<FlKeyEmbedderCheckedKey*>(value);

  guint modifier_bit = GPOINTER_TO_INT(key);
  FlKeyEmbedderResponder* self = context->self;

  const uint64_t logical_key = checked_key->primary_logical_key;
  const uint64_t recorded_physical_key =
      lookup_hash_table(self->mapping_records, logical_key);
  // The physical key is derived from past mapping record if possible.
  //
  // If the event to be synthesized is a key up event, then there must have
  // been a key down event before, which has updated the mapping record.
  // If the event to be synthesized is a key down event, then there might
  // not have been a mapping record, in which case the hard-coded
  // #primary_physical_key is used.
  const uint64_t physical_key = recorded_physical_key != 0
                                    ? recorded_physical_key
                                    : checked_key->primary_physical_key;

  // A lock mode key can be at any of a 4-stage cycle, depending on whether it's
  // pressed and enabled. The following table lists the definition of each
  // stage (TruePressed and TrueEnabled), the event of the lock key between
  // every 2 stages (SelfType and SelfState), and the event of other keys at
  // each stage (OthersState). On certain platforms SelfState uses a reversed
  // rule for certain keys (SelfState(rvsd), as documented in
  // #update_caps_lock_state_logic_inferrence).
  //
  //               #    [0]         [1]          [2]           [3]
  //     TruePressed: Released    Pressed      Released      Pressed
  //     TrueEnabled: Disabled    Enabled      Enabled       Disabled
  //        SelfType:         Down         Up           Down            Up
  //       SelfState:          0           1             1              1
  // SelfState(rvsd):          1           1             0              1
  //     OthersState:    0           1            1              1
  //
  // When the exact stage can't be derived, choose the stage that requires the
  // minimal synthesization.

  const uint64_t pressed_logical_key =
      recorded_physical_key == 0
          ? 0
          : lookup_hash_table(self->pressing_records, recorded_physical_key);

  g_return_if_fail(pressed_logical_key == 0 ||
                   pressed_logical_key == logical_key);
  const int stage_by_record = find_stage_by_record(
      pressed_logical_key != 0, (self->lock_records & modifier_bit) != 0);

  const bool enabled_by_state = (context->state & modifier_bit) != 0;
  const bool this_key_is_event_key = logical_key == context->event_logical_key;
  if (this_key_is_event_key && checked_key->is_caps_lock) {
    update_caps_lock_state_logic_inferrence(self, context->is_down,
                                            enabled_by_state, stage_by_record);
    g_return_if_fail(self->caps_lock_state_logic_inferrence !=
                     kStateLogicUndecided);
  }
  const bool reverse_state_logic =
      checked_key->is_caps_lock &&
      self->caps_lock_state_logic_inferrence == kStateLogicReversed;
  const int stage_by_event =
      this_key_is_event_key
          ? find_stage_by_self_event(stage_by_record, context->is_down,
                                     enabled_by_state, reverse_state_logic)
          : find_stage_by_others_event(stage_by_record, enabled_by_state);

  // The destination stage is equal to stage_by_event but shifted cyclically to
  // be no less than stage_by_record.
  constexpr int kNumStages = 4;
  const int destination_stage = stage_by_event >= stage_by_record
                                    ? stage_by_event
                                    : stage_by_event + kNumStages;

  g_return_if_fail(stage_by_record <= destination_stage);
  if (stage_by_record == destination_stage) {
    return;
  }
  for (int current_stage = stage_by_record; current_stage < destination_stage;
       current_stage += 1) {
    if (current_stage == 9) {
      return;
    }

    const int standard_current_stage = current_stage % kNumStages;
    const bool is_down_event =
        standard_current_stage == 0 || standard_current_stage == 2;
    if (is_down_event && recorded_physical_key == 0) {
      update_mapping_record(self, physical_key, logical_key);
    }
    FlutterKeyEventType type =
        is_down_event ? kFlutterKeyEventTypeDown : kFlutterKeyEventTypeUp;
    update_pressing_state(self, physical_key, is_down_event ? logical_key : 0);
    possibly_update_lock_bit(self, logical_key, is_down_event);
    synthesize_simple_event(self, type, physical_key, logical_key,
                            context->timestamp);
  }
}

// Find if a given physical key is the primary physical of one of the known
// modifier keys.
//
// This is used as the body of a loop over #modifier_bit_to_checked_keys.
static void is_known_modifier_physical_key_loop_body(gpointer key,
                                                     gpointer value,
                                                     gpointer user_data) {
  ModifierLogicalToPhysicalContext* context =
      reinterpret_cast<ModifierLogicalToPhysicalContext*>(user_data);
  FlKeyEmbedderCheckedKey* checked_key =
      reinterpret_cast<FlKeyEmbedderCheckedKey*>(value);

  if (checked_key->primary_physical_key == context->physical_key_from_event) {
    context->known_modifier_physical_key = true;
  }
}

// Return the primary physical key of a known modifier key which matches the
// given logical key.
//
// This is used as the body of a loop over #modifier_bit_to_checked_keys.
static void find_physical_from_logical_loop_body(gpointer key,
                                                 gpointer value,
                                                 gpointer user_data) {
  ModifierLogicalToPhysicalContext* context =
      reinterpret_cast<ModifierLogicalToPhysicalContext*>(user_data);
  FlKeyEmbedderCheckedKey* checked_key =
      reinterpret_cast<FlKeyEmbedderCheckedKey*>(value);

  if (checked_key->primary_logical_key == context->logical_key ||
      checked_key->secondary_logical_key == context->logical_key) {
    context->corrected_physical_key = checked_key->primary_physical_key;
  }
}

static uint64_t corrected_modifier_physical_key(
    GHashTable* modifier_bit_to_checked_keys,
    uint64_t physical_key_from_event,
    uint64_t logical_key) {
  ModifierLogicalToPhysicalContext logical_to_physical_context;
  logical_to_physical_context.known_modifier_physical_key = false;
  logical_to_physical_context.physical_key_from_event = physical_key_from_event;
  logical_to_physical_context.logical_key = logical_key;
  // If no match is found, defaults to the physical key retrieved from the
  // event.
  logical_to_physical_context.corrected_physical_key = physical_key_from_event;

  // Check if the physical key is one of the known modifier physical key.
  g_hash_table_foreach(modifier_bit_to_checked_keys,
                       is_known_modifier_physical_key_loop_body,
                       &logical_to_physical_context);

  // If the physical key matches a known modifier key, find the modifier
  // physical key from the logical key.
  if (logical_to_physical_context.known_modifier_physical_key) {
    g_hash_table_foreach(modifier_bit_to_checked_keys,
                         find_physical_from_logical_loop_body,
                         &logical_to_physical_context);
  }

  return logical_to_physical_context.corrected_physical_key;
}

static void fl_key_embedder_responder_handle_event_impl(
    FlKeyResponder* responder,
    FlKeyEvent* event,
    uint64_t specified_logical_key,
    FlKeyResponderAsyncCallback callback,
    gpointer user_data) {
  FlKeyEmbedderResponder* self = FL_KEY_EMBEDDER_RESPONDER(responder);

  g_return_if_fail(event != nullptr);
  g_return_if_fail(callback != nullptr);

  const uint64_t logical_key = specified_logical_key != 0
                                   ? specified_logical_key
                                   : event_to_logical_key(event);
  const uint64_t physical_key_from_event = event_to_physical_key(event);
  const uint64_t physical_key = corrected_modifier_physical_key(
      self->modifier_bit_to_checked_keys, physical_key_from_event, logical_key);
  const double timestamp = event_to_timestamp(event);
  const bool is_down_event = event->is_press;

  SyncStateLoopContext sync_state_context;
  sync_state_context.self = self;
  sync_state_context.state = event->state;
  sync_state_context.timestamp = timestamp;
  sync_state_context.is_down = is_down_event;
  sync_state_context.event_logical_key = logical_key;

  // Update lock mode states
  g_hash_table_foreach(self->lock_bit_to_checked_keys,
                       synchronize_lock_states_loop_body, &sync_state_context);

  // Update pressing states
  g_hash_table_foreach(self->modifier_bit_to_checked_keys,
                       synchronize_pressed_states_loop_body,
                       &sync_state_context);

  // Construct the real event
  const uint64_t last_logical_record =
      lookup_hash_table(self->pressing_records, physical_key);

  FlutterKeyEvent out_event;
  out_event.struct_size = sizeof(out_event);
  out_event.timestamp = timestamp;
  out_event.physical = physical_key;
  out_event.logical =
      last_logical_record != 0 ? last_logical_record : logical_key;
  out_event.character = nullptr;
  out_event.synthesized = false;

  g_autofree char* character_to_free = nullptr;
  if (is_down_event) {
    if (last_logical_record) {
      // A key has been pressed that has the exact physical key as a currently
      // pressed one. This can happen during repeated events.
      out_event.type = kFlutterKeyEventTypeRepeat;
    } else {
      out_event.type = kFlutterKeyEventTypeDown;
    }
    character_to_free = event_to_character(event);  // Might be null
    out_event.character = character_to_free;
  } else {  // is_down_event false
    if (!last_logical_record) {
      // The physical key has been released before. It might indicate a missed
      // event due to loss of focus, or multiple keyboards pressed keys with the
      // same physical key. Ignore the up event.
      callback(true, user_data);
      return;
    } else {
      out_event.type = kFlutterKeyEventTypeUp;
    }
  }

  if (out_event.type != kFlutterKeyEventTypeRepeat) {
    update_pressing_state(self, physical_key, is_down_event ? logical_key : 0);
  }
  possibly_update_lock_bit(self, logical_key, is_down_event);
  if (is_down_event) {
    update_mapping_record(self, physical_key, logical_key);
  }
  FlKeyEmbedderUserData* response_data =
      fl_key_embedder_user_data_new(callback, user_data);
  self->sent_any_events = true;
  self->send_key_event(&out_event, handle_response, response_data,
                       self->send_key_event_user_data);
}

// Sends a key event to the framework.
static void fl_key_embedder_responder_handle_event(
    FlKeyResponder* responder,
    FlKeyEvent* event,
    uint64_t specified_logical_key,
    FlKeyResponderAsyncCallback callback,
    gpointer user_data) {
  FlKeyEmbedderResponder* self = FL_KEY_EMBEDDER_RESPONDER(responder);
  self->sent_any_events = false;
  fl_key_embedder_responder_handle_event_impl(
      responder, event, specified_logical_key, callback, user_data);
  if (!self->sent_any_events) {
    self->send_key_event(&kEmptyEvent, nullptr, nullptr,
                         self->send_key_event_user_data);
  }
}

void fl_key_embedder_responder_sync_modifiers_if_needed(
    FlKeyEmbedderResponder* responder,
    guint state,
    double event_time) {
  const double timestamp = event_time * kMicrosecondsPerMillisecond;

  SyncStateLoopContext sync_state_context;
  sync_state_context.self = responder;
  sync_state_context.state = state;
  sync_state_context.timestamp = timestamp;

  // Update pressing states.
  g_hash_table_foreach(responder->modifier_bit_to_checked_keys,
                       synchronize_pressed_states_loop_body,
                       &sync_state_context);
}

GHashTable* fl_key_embedder_responder_get_pressed_state(
    FlKeyEmbedderResponder* self) {
  return self->pressing_records;
}