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diff --git a/include/VBox/com/NativeEventQueue.h b/include/VBox/com/NativeEventQueue.h
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+/** @file
+ * MS COM / XPCOM Abstraction Layer - Event and EventQueue class declaration.
+ */
+
+/*
+ * Copyright (C) 2006-2022 Oracle and/or its affiliates.
+ *
+ * This file is part of VirtualBox base platform packages, as
+ * available from https://www.virtualbox.org.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, in version 3 of the
+ * License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <https://www.gnu.org/licenses>.
+ *
+ * The contents of this file may alternatively be used under the terms
+ * of the Common Development and Distribution License Version 1.0
+ * (CDDL), a copy of it is provided in the "COPYING.CDDL" file included
+ * in the VirtualBox distribution, in which case the provisions of the
+ * CDDL are applicable instead of those of the GPL.
+ *
+ * You may elect to license modified versions of this file under the
+ * terms and conditions of either the GPL or the CDDL or both.
+ *
+ * SPDX-License-Identifier: GPL-3.0-only OR CDDL-1.0
+ */
+
+#ifndef VBOX_INCLUDED_com_NativeEventQueue_h
+#define VBOX_INCLUDED_com_NativeEventQueue_h
+#ifndef RT_WITHOUT_PRAGMA_ONCE
+# pragma once
+#endif
+
+#ifndef VBOX_WITH_XPCOM
+# include <iprt/win/windows.h>
+#else
+# include <nsEventQueueUtils.h>
+#endif
+
+#include <VBox/com/defs.h>
+#include <VBox/com/assert.h>
+
+
+/** @defgroup grp_com_evt   Event and EventQueue Classes
+ * @ingroup grp_com
+ * @{
+ */
+
+namespace com
+{
+
+class MainEventQueue;
+
+/**
+ *  Base class for all events. Intended to be subclassed to introduce new
+ *  events and handlers for them.
+ *
+ *  Subclasses usually reimplement virtual #handler() (that does nothing by
+ *  default) and add new data members describing the event.
+ */
+class NativeEvent
+{
+public:
+
+    NativeEvent() {}
+    virtual ~NativeEvent() {};
+
+protected:
+
+    /**
+     *  Event handler. Called in the context of the event queue's thread.
+     *  Always reimplemented by subclasses
+     *
+     *  @return reserved, should be NULL.
+     */
+    virtual void *handler() { return NULL; }
+
+    friend class NativeEventQueue;
+};
+
+/**
+ *  Simple event queue.
+ *
+ *  When using XPCOM, this will map onto the default XPCOM queue for the thread.
+ *  So, if a queue is created on the main thread, it automatically processes
+ *  XPCOM/IPC events while waiting.
+ *
+ *  When using Windows, Darwin and OS/2, this will map onto the native thread
+ *  queue/runloop.  So, windows messages and what not will be processed while
+ *  waiting for events.
+ *
+ *  @note It is intentional that there is no way to retrieve arbitrary
+ *  events and controlling their processing. There is no use case which
+ *  warrants introducing the complexity of platform independent events.
+ */
+class NativeEventQueue
+{
+public:
+
+    NativeEventQueue();
+    virtual ~NativeEventQueue();
+
+    BOOL postEvent(NativeEvent *event);
+    int processEventQueue(RTMSINTERVAL cMsTimeout);
+    int interruptEventQueueProcessing();
+    int getSelectFD();
+    static int init();
+    static int uninit();
+    static NativeEventQueue *getMainEventQueue();
+
+#ifdef VBOX_WITH_XPCOM
+    already_AddRefed<nsIEventQueue> getIEventQueue()
+    {
+        return mEventQ.get();
+    }
+#else
+    static int dispatchMessageOnWindows(MSG const *pMsg, int rc);
+#endif
+
+private:
+    static NativeEventQueue *sMainQueue;
+
+#ifndef VBOX_WITH_XPCOM
+
+    /** The thread which the queue belongs to. */
+    DWORD mThreadId;
+    /** Duplicated thread handle for MsgWaitForMultipleObjects. */
+    HANDLE mhThread;
+
+#else // VBOX_WITH_XPCOM
+
+    /** Whether it was created (and thus needs destroying) or if a queue already
+     *  associated with the thread was used. */
+    bool mEQCreated;
+
+    /** Whether event processing should be interrupted. */
+    bool mInterrupted;
+
+    nsCOMPtr <nsIEventQueue> mEventQ;
+    nsCOMPtr <nsIEventQueueService> mEventQService;
+
+    static void *PR_CALLBACK plEventHandler(PLEvent *self);
+    static void PR_CALLBACK plEventDestructor(PLEvent *self);
+
+#endif // VBOX_WITH_XPCOM
+};
+
+} /* namespace com */
+
+/** @} */
+
+#endif /* !VBOX_INCLUDED_com_NativeEventQueue_h */
+