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diff --git a/dom/events/IMEContentObserver.h b/dom/events/IMEContentObserver.h
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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_IMEContentObserver_h
+#define mozilla_IMEContentObserver_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/EditorBase.h"
+#include "mozilla/dom/Selection.h"
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIDocShell.h"  // XXX Why does only this need to be included here?
+#include "nsIReflowObserver.h"
+#include "nsIScrollObserver.h"
+#include "nsIWidget.h"
+#include "nsStubDocumentObserver.h"
+#include "nsStubMutationObserver.h"
+#include "nsThreadUtils.h"
+#include "nsWeakReference.h"
+
+class nsIContent;
+class nsINode;
+class nsPresContext;
+
+namespace mozilla {
+
+class EventStateManager;
+class TextComposition;
+
+namespace dom {
+class Selection;
+}  // namespace dom
+
+// IMEContentObserver notifies widget of any text and selection changes
+// in the currently focused editor
+class IMEContentObserver final : public nsStubMutationObserver,
+                                 public nsIReflowObserver,
+                                 public nsIScrollObserver,
+                                 public nsSupportsWeakReference {
+ public:
+  typedef widget::IMENotification::SelectionChangeData SelectionChangeData;
+  typedef widget::IMENotification::TextChangeData TextChangeData;
+  typedef widget::IMENotification::TextChangeDataBase TextChangeDataBase;
+  typedef widget::IMENotificationRequests IMENotificationRequests;
+  typedef widget::IMEMessage IMEMessage;
+
+  IMEContentObserver();
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(IMEContentObserver,
+                                           nsIReflowObserver)
+  NS_DECL_NSIMUTATIONOBSERVER_CHARACTERDATAWILLCHANGE
+  NS_DECL_NSIMUTATIONOBSERVER_CHARACTERDATACHANGED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTAPPENDED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
+  NS_DECL_NSIREFLOWOBSERVER
+
+  // nsIScrollObserver
+  virtual void ScrollPositionChanged() override;
+
+  /**
+   * OnSelectionChange() is called when selection is changed in the editor.
+   */
+  void OnSelectionChange(dom::Selection& aSelection);
+
+  MOZ_CAN_RUN_SCRIPT bool OnMouseButtonEvent(nsPresContext* aPresContext,
+                                             WidgetMouseEvent* aMouseEvent);
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  HandleQueryContentEvent(WidgetQueryContentEvent* aEvent);
+
+  /**
+   * Init() initializes the instance, i.e., retrieving necessary objects and
+   * starts to observe something.
+   * Be aware, callers of this method need to guarantee that the instance
+   * won't be released during calling this.
+   *
+   * @param aWidget         The widget which can access native IME.
+   * @param aPresContext    The PresContext which has aContent.
+   * @param aContent        An editable element or nullptr if this will observe
+   *                        design mode document.
+   * @param aEditorBase     The editor which is associated with aContent.
+   */
+  MOZ_CAN_RUN_SCRIPT void Init(nsIWidget& aWidget, nsPresContext& aPresContext,
+                               nsIContent* aContent, EditorBase& aEditorBase);
+
+  /**
+   * Destroy() finalizes the instance, i.e., stops observing contents and
+   * clearing the members.
+   * Be aware, callers of this method need to guarantee that the instance
+   * won't be released during calling this.
+   */
+  void Destroy();
+
+  /**
+   * Returns false if the instance refers some objects and observing them.
+   * Otherwise, true.
+   */
+  bool Destroyed() const;
+
+  /**
+   * IMEContentObserver is stored by EventStateManager during observing.
+   * DisconnectFromEventStateManager() is called when EventStateManager stops
+   * storing the instance.
+   */
+  void DisconnectFromEventStateManager();
+
+  /**
+   * MaybeReinitialize() tries to restart to observe the editor's root node.
+   * This is useful when the editor is reframed and all children are replaced
+   * with new node instances.
+   * Be aware, callers of this method need to guarantee that the instance
+   * won't be released during calling this.
+   *
+   * @return            Returns true if the instance is managing the content.
+   *                    Otherwise, false.
+   */
+  MOZ_CAN_RUN_SCRIPT bool MaybeReinitialize(nsIWidget& aWidget,
+                                            nsPresContext& aPresContext,
+                                            nsIContent* aContent,
+                                            EditorBase& aEditorBase);
+
+  bool IsManaging(nsPresContext* aPresContext, nsIContent* aContent) const;
+  bool IsManaging(const TextComposition* aTextComposition) const;
+  bool WasInitializedWith(const EditorBase& aEditorBase) const {
+    return mEditorBase == &aEditorBase;
+  }
+  bool IsEditorHandlingEventForComposition() const;
+  bool KeepAliveDuringDeactive() const {
+    return mIMENotificationRequests &&
+           mIMENotificationRequests->WantDuringDeactive();
+  }
+  nsIWidget* GetWidget() const { return mWidget; }
+  void SuppressNotifyingIME();
+  void UnsuppressNotifyingIME();
+  nsPresContext* GetPresContext() const;
+  nsresult GetSelectionAndRoot(dom::Selection** aSelection,
+                               nsIContent** aRoot) const;
+
+  /**
+   * TryToFlushPendingNotifications() should be called when pending events
+   * should be flushed.  This tries to run the queued IMENotificationSender.
+   * Doesn't do anything in child processes where flushing happens
+   * asynchronously unless aAllowAsync is false.
+   */
+  void TryToFlushPendingNotifications(bool aAllowAsync);
+
+  /**
+   * MaybeNotifyCompositionEventHandled() posts composition event handled
+   * notification into the pseudo queue.
+   */
+  void MaybeNotifyCompositionEventHandled();
+
+  /**
+   * Following methods are called when the editor:
+   *   - an edit action handled.
+   *   - before handling an edit action.
+   *   - canceled handling an edit action after calling BeforeEditAction().
+   */
+  void OnEditActionHandled();
+  void BeforeEditAction();
+  void CancelEditAction();
+
+ private:
+  ~IMEContentObserver() = default;
+
+  enum State {
+    eState_NotObserving,
+    eState_Initializing,
+    eState_StoppedObserving,
+    eState_Observing
+  };
+  State GetState() const;
+  MOZ_CAN_RUN_SCRIPT bool InitWithEditor(nsPresContext& aPresContext,
+                                         nsIContent* aContent,
+                                         EditorBase& aEditorBase);
+  void OnIMEReceivedFocus();
+  void Clear();
+  bool IsObservingContent(nsPresContext* aPresContext,
+                          nsIContent* aContent) const;
+  bool IsReflowLocked() const;
+  bool IsSafeToNotifyIME() const;
+  bool IsEditorComposing() const;
+
+  // Following methods are called by DocumentObserver when
+  // beginning to update the contents and ending updating the contents.
+  void BeginDocumentUpdate();
+  void EndDocumentUpdate();
+
+  // Following methods manages added nodes during a document change.
+
+  /**
+   * MaybeNotifyIMEOfAddedTextDuringDocumentChange() may send text change
+   * notification caused by the nodes added between mFirstAddedContent in
+   * mFirstAddedContainer and mLastAddedContent in
+   * mLastAddedContainer and forgets the range.
+   */
+  void MaybeNotifyIMEOfAddedTextDuringDocumentChange();
+
+  /**
+   * IsInDocumentChange() returns true while the DOM tree is being modified
+   * with mozAutoDocUpdate.  E.g., it's being modified by setting innerHTML or
+   * insertAdjacentHTML().  This returns false when user types something in
+   * the focused editor editor.
+   */
+  bool IsInDocumentChange() const {
+    return mDocumentObserver && mDocumentObserver->IsUpdating();
+  }
+
+  /**
+   * Forget the range of added nodes during a document change.
+   */
+  void ClearAddedNodesDuringDocumentChange();
+
+  /**
+   * HasAddedNodesDuringDocumentChange() returns true when this stores range
+   * of nodes which were added into the DOM tree during a document change but
+   * have not been sent to IME.  Note that this should always return false when
+   * IsInDocumentChange() returns false.
+   */
+  bool HasAddedNodesDuringDocumentChange() const {
+    return mFirstAddedContainer && mLastAddedContainer;
+  }
+
+  /**
+   * Returns true if the passed-in node in aParent is the next node of
+   * mLastAddedContent in pre-order tree traversal of the DOM.
+   */
+  bool IsNextNodeOfLastAddedNode(nsINode* aParent, nsIContent* aChild) const;
+
+  void PostFocusSetNotification();
+  void MaybeNotifyIMEOfFocusSet();
+  void PostTextChangeNotification();
+  void MaybeNotifyIMEOfTextChange(const TextChangeDataBase& aTextChangeData);
+  void CancelNotifyingIMEOfTextChange();
+  void PostSelectionChangeNotification();
+  void MaybeNotifyIMEOfSelectionChange(bool aCausedByComposition,
+                                       bool aCausedBySelectionEvent,
+                                       bool aOccurredDuringComposition);
+  void PostPositionChangeNotification();
+  void MaybeNotifyIMEOfPositionChange();
+  void CancelNotifyingIMEOfPositionChange();
+  void PostCompositionEventHandledNotification();
+
+  void NotifyContentAdded(nsINode* aContainer, nsIContent* aFirstContent,
+                          nsIContent* aLastContent);
+  void ObserveEditableNode();
+  /**
+   *  NotifyIMEOfBlur() notifies IME of blur.
+   */
+  void NotifyIMEOfBlur();
+  /**
+   *  UnregisterObservers() unregisters all listeners and observers.
+   */
+  void UnregisterObservers();
+  void FlushMergeableNotifications();
+  bool NeedsTextChangeNotification() const {
+    return mIMENotificationRequests &&
+           mIMENotificationRequests->WantTextChange();
+  }
+  bool NeedsPositionChangeNotification() const {
+    return mIMENotificationRequests &&
+           mIMENotificationRequests->WantPositionChanged();
+  }
+  void ClearPendingNotifications() {
+    mNeedsToNotifyIMEOfFocusSet = false;
+    mNeedsToNotifyIMEOfTextChange = false;
+    mNeedsToNotifyIMEOfSelectionChange = false;
+    mNeedsToNotifyIMEOfPositionChange = false;
+    mNeedsToNotifyIMEOfCompositionEventHandled = false;
+    mTextChangeData.Clear();
+  }
+  bool NeedsToNotifyIMEOfSomething() const {
+    return mNeedsToNotifyIMEOfFocusSet || mNeedsToNotifyIMEOfTextChange ||
+           mNeedsToNotifyIMEOfSelectionChange ||
+           mNeedsToNotifyIMEOfPositionChange ||
+           mNeedsToNotifyIMEOfCompositionEventHandled;
+  }
+
+  /**
+   * UpdateSelectionCache() updates mSelectionData with the latest selection.
+   * This should be called only when IsSafeToNotifyIME() returns true.
+   */
+  MOZ_CAN_RUN_SCRIPT bool UpdateSelectionCache(bool aRequireFlush = true);
+
+  nsCOMPtr<nsIWidget> mWidget;
+  // mFocusedWidget has the editor observed by the instance.  E.g., if the
+  // focused editor is in XUL panel, this should be the widget of the panel.
+  // On the other hand, mWidget is its parent which handles IME.
+  nsCOMPtr<nsIWidget> mFocusedWidget;
+  RefPtr<dom::Selection> mSelection;
+  nsCOMPtr<nsIContent> mRootContent;
+  nsCOMPtr<nsINode> mEditableNode;
+  nsCOMPtr<nsIDocShell> mDocShell;
+  RefPtr<EditorBase> mEditorBase;
+
+  /**
+   * Helper classes to notify IME.
+   */
+
+  class AChangeEvent : public Runnable {
+   protected:
+    enum ChangeEventType {
+      eChangeEventType_Focus,
+      eChangeEventType_Selection,
+      eChangeEventType_Text,
+      eChangeEventType_Position,
+      eChangeEventType_CompositionEventHandled
+    };
+
+    explicit AChangeEvent(const char* aName,
+                          IMEContentObserver* aIMEContentObserver)
+        : Runnable(aName),
+          mIMEContentObserver(do_GetWeakReference(
+              static_cast<nsIReflowObserver*>(aIMEContentObserver))) {
+      MOZ_ASSERT(aIMEContentObserver);
+    }
+
+    already_AddRefed<IMEContentObserver> GetObserver() const {
+      nsCOMPtr<nsIReflowObserver> observer =
+          do_QueryReferent(mIMEContentObserver);
+      return observer.forget().downcast<IMEContentObserver>();
+    }
+
+    nsWeakPtr mIMEContentObserver;
+
+    /**
+     * CanNotifyIME() checks if mIMEContentObserver can and should notify IME.
+     */
+    bool CanNotifyIME(ChangeEventType aChangeEventType) const;
+
+    /**
+     * IsSafeToNotifyIME() checks if it's safe to noitify IME.
+     */
+    bool IsSafeToNotifyIME(ChangeEventType aChangeEventType) const;
+  };
+
+  class IMENotificationSender : public AChangeEvent {
+   public:
+    explicit IMENotificationSender(IMEContentObserver* aIMEContentObserver)
+        : AChangeEvent("IMENotificationSender", aIMEContentObserver),
+          mIsRunning(false) {}
+    MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHOD Run() override;
+
+    void Dispatch(nsIDocShell* aDocShell);
+
+   private:
+    MOZ_CAN_RUN_SCRIPT void SendFocusSet();
+    MOZ_CAN_RUN_SCRIPT void SendSelectionChange();
+    void SendTextChange();
+    void SendPositionChange();
+    void SendCompositionEventHandled();
+
+    bool mIsRunning;
+  };
+
+  // mQueuedSender is, it was put into the event queue but not run yet.
+  RefPtr<IMENotificationSender> mQueuedSender;
+
+  /**
+   * IMEContentObserver is a mutation observer of mRootContent.  However,
+   * it needs to know the beginning of content changes and end of it too for
+   * reducing redundant computation of text offset with ContentEventHandler.
+   * Therefore, it needs helper class to listen only them since if
+   * both mutations were observed by IMEContentObserver directly, each
+   * methods need to check if the changing node is in mRootContent but it's
+   * too expensive.
+   */
+  class DocumentObserver final : public nsStubDocumentObserver {
+   public:
+    explicit DocumentObserver(IMEContentObserver& aIMEContentObserver)
+        : mIMEContentObserver(&aIMEContentObserver), mDocumentUpdating(0) {}
+
+    NS_DECL_CYCLE_COLLECTION_CLASS(DocumentObserver)
+    NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+    NS_DECL_NSIDOCUMENTOBSERVER_BEGINUPDATE
+    NS_DECL_NSIDOCUMENTOBSERVER_ENDUPDATE
+
+    void Observe(dom::Document*);
+    void StopObserving();
+    void Destroy();
+
+    bool Destroyed() const { return !mIMEContentObserver; }
+    bool IsObserving() const { return mDocument != nullptr; }
+    bool IsUpdating() const { return mDocumentUpdating != 0; }
+
+   private:
+    DocumentObserver() = delete;
+    virtual ~DocumentObserver() { Destroy(); }
+
+    RefPtr<IMEContentObserver> mIMEContentObserver;
+    RefPtr<dom::Document> mDocument;
+    uint32_t mDocumentUpdating;
+  };
+  RefPtr<DocumentObserver> mDocumentObserver;
+
+  /**
+   * FlatTextCache stores flat text length from start of the content to
+   * mNodeOffset of mContainerNode.
+   */
+  struct FlatTextCache {
+    // mContainerNode and mNode represent a point in DOM tree.  E.g.,
+    // if mContainerNode is a div element, mNode is a child.
+    nsCOMPtr<nsINode> mContainerNode;
+    // mNode points to the last child which participates in the current
+    // mFlatTextLength. If mNode is null, then that means that the end point for
+    // mFlatTextLength is immediately before the first child of mContainerNode.
+    nsCOMPtr<nsINode> mNode;
+    // Length of flat text generated from contents between the start of content
+    // and a child node whose index is mNodeOffset of mContainerNode.
+    uint32_t mFlatTextLength;
+
+    FlatTextCache() : mFlatTextLength(0) {}
+
+    void Clear() {
+      mContainerNode = nullptr;
+      mNode = nullptr;
+      mFlatTextLength = 0;
+    }
+
+    void Cache(nsINode* aContainer, nsINode* aNode, uint32_t aFlatTextLength) {
+      MOZ_ASSERT(aContainer, "aContainer must not be null");
+      MOZ_ASSERT(!aNode || aNode->GetParentNode() == aContainer,
+                 "aNode must be either null or a child of aContainer");
+      mContainerNode = aContainer;
+      mNode = aNode;
+      mFlatTextLength = aFlatTextLength;
+    }
+
+    bool Match(nsINode* aContainer, nsINode* aNode) const {
+      return aContainer == mContainerNode && aNode == mNode;
+    }
+  };
+  // mEndOfAddedTextCache caches text length from the start of content to
+  // the end of the last added content only while an edit action is being
+  // handled by the editor and no other mutation (e.g., removing node)
+  // occur.
+  FlatTextCache mEndOfAddedTextCache;
+  // mStartOfRemovingTextRangeCache caches text length from the start of content
+  // to the start of the last removed content only while an edit action is being
+  // handled by the editor and no other mutation (e.g., adding node) occur.
+  FlatTextCache mStartOfRemovingTextRangeCache;
+
+  // mFirstAddedContainer is parent node of first added node in current
+  // document change.  So, this is not nullptr only when a node was added
+  // during a document change and the change has not been included into
+  // mTextChangeData yet.
+  // Note that this shouldn't be in cycle collection since this is not nullptr
+  // only during a document change.
+  nsCOMPtr<nsINode> mFirstAddedContainer;
+  // mLastAddedContainer is parent node of last added node in current
+  // document change.  So, this is not nullptr only when a node was added
+  // during a document change and the change has not been included into
+  // mTextChangeData yet.
+  // Note that this shouldn't be in cycle collection since this is not nullptr
+  // only during a document change.
+  nsCOMPtr<nsINode> mLastAddedContainer;
+
+  // mFirstAddedContent is the first node added in mFirstAddedContainer.
+  nsCOMPtr<nsIContent> mFirstAddedContent;
+  // mLastAddedContent is the last node added in mLastAddedContainer;
+  nsCOMPtr<nsIContent> mLastAddedContent;
+
+  TextChangeData mTextChangeData;
+
+  // mSelectionData is the last selection data which was notified.  The
+  // selection information is modified by UpdateSelectionCache().  The reason
+  // of the selection change is modified by MaybeNotifyIMEOfSelectionChange().
+  SelectionChangeData mSelectionData;
+
+  EventStateManager* mESM;
+
+  const IMENotificationRequests* mIMENotificationRequests;
+  uint32_t mSuppressNotifications;
+  int64_t mPreCharacterDataChangeLength;
+
+  // mSendingNotification is a notification which is now sending from
+  // IMENotificationSender.  When the value is NOTIFY_IME_OF_NOTHING, it's
+  // not sending any notification.
+  IMEMessage mSendingNotification;
+
+  bool mIsObserving;
+  bool mIMEHasFocus;
+  bool mNeedsToNotifyIMEOfFocusSet;
+  bool mNeedsToNotifyIMEOfTextChange;
+  bool mNeedsToNotifyIMEOfSelectionChange;
+  bool mNeedsToNotifyIMEOfPositionChange;
+  bool mNeedsToNotifyIMEOfCompositionEventHandled;
+  // mIsHandlingQueryContentEvent is true when IMEContentObserver is handling
+  // WidgetQueryContentEvent with ContentEventHandler.
+  bool mIsHandlingQueryContentEvent;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_IMEContentObserver_h