diff --git a/src/blackmisc/blackmisc.h b/src/blackmisc/blackmisc.h
index e47c80df1..ceddc4ab4 100644
--- a/src/blackmisc/blackmisc.h
+++ b/src/blackmisc/blackmisc.h
@@ -27,6 +27,11 @@
  * \brief Classes related to configuration settings.
  */
 
+/*!
+ * \namespace BlackMisc::SharedState
+ * \brief Utilities for sharing state between multiple objects.
+ */
+
 /*!
  * \namespace BlackMisc::Predicates
  * \brief Functor classes for evaluating predicate calculus expressions.
diff --git a/src/blackmisc/blackmisc.pro b/src/blackmisc/blackmisc.pro
index cb1c0399c..475ab64d0 100644
--- a/src/blackmisc/blackmisc.pro
+++ b/src/blackmisc/blackmisc.pro
@@ -38,6 +38,7 @@ HEADERS +=  *.h \
             $$files($$PWD/network/settings/*.h) \
             $$files($$PWD/network/external/*.h) \
             $$files($$PWD/pq/*.h) \
+            $$files($$PWD/sharedstate/*.h) \
             $$files($$PWD/simulation/*.h) \
             $$files($$PWD/simulation/data/*.h) \
             $$files($$PWD/simulation/settings/*.h) \
@@ -61,6 +62,7 @@ SOURCES +=  *.cpp \
             $$files($$PWD/network/settings/*.cpp) \
             $$files($$PWD/network/external/*.cpp) \
             $$files($$PWD/pq/*.cpp) \
+            $$files($$PWD/sharedstate/*.cpp) \
             $$files($$PWD/simulation/*.cpp) \
             $$files($$PWD/simulation/data/*.cpp) \
             $$files($$PWD/simulation/settings/*.cpp) \
diff --git a/src/blackmisc/registermetadata.cpp b/src/blackmisc/registermetadata.cpp
index a9893a3c8..75f544e60 100644
--- a/src/blackmisc/registermetadata.cpp
+++ b/src/blackmisc/registermetadata.cpp
@@ -17,6 +17,7 @@
 #include "blackmisc/geo/registermetadatageo.h"
 #include "blackmisc/pq/registermetadatapq.h"
 
+#include "blackmisc/sharedstate/passiveobserver.h"
 #include "blackmisc/applicationinfolist.h"
 #include "blackmisc/countrylist.h"
 #include "blackmisc/crashsettings.h"
@@ -100,6 +101,8 @@ namespace BlackMisc
         Settings::CCrashSettings::registerMetadata();
         Weather::registerMetadata();
 
+        SharedState::CAnyMatch::registerMetadata();
+
         // needed by XSwiftBus proxy class
         qDBusRegisterMetaType<CSequence<double>>();
         qRegisterMetaTypeStreamOperators<CSequence<double>>();
diff --git a/src/blackmisc/sharedstate/passivemutator.cpp b/src/blackmisc/sharedstate/passivemutator.cpp
new file mode 100644
index 000000000..d94ea8169
--- /dev/null
+++ b/src/blackmisc/sharedstate/passivemutator.cpp
@@ -0,0 +1,22 @@
+/* Copyright (C) 2017
+ * swift Project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution. No part of swift project, including this file, may be copied, modified, propagated,
+ * or distributed except according to the terms contained in the LICENSE file.
+ */
+
+//! \file
+
+#include "blackmisc/sharedstate/passivemutator.h"
+
+namespace BlackMisc
+{
+    namespace SharedState
+    {
+        void CPassiveMutator::postEvent(const CVariant &param)
+        {
+            emit eventPosted(param);
+        }
+    }
+}
diff --git a/src/blackmisc/sharedstate/passivemutator.h b/src/blackmisc/sharedstate/passivemutator.h
new file mode 100644
index 000000000..f3af23bc3
--- /dev/null
+++ b/src/blackmisc/sharedstate/passivemutator.h
@@ -0,0 +1,53 @@
+/* Copyright (C) 2017
+ * swift Project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution. No part of swift project, including this file, may be copied, modified, propagated,
+ * or distributed except according to the terms contained in the LICENSE file.
+ */
+
+//! \file
+
+#ifndef BLACKMISC_SHAREDSTATE_PASSIVEMUTATOR_H
+#define BLACKMISC_SHAREDSTATE_PASSIVEMUTATOR_H
+
+#include "blackmisc/variant.h"
+#include "blackmisc/blackmiscexport.h"
+#include <QObject>
+#include <QSharedPointer>
+
+namespace BlackMisc
+{
+    namespace SharedState
+    {
+        class CActiveMutator;
+
+        /*!
+         * Endpoint which can emit events to subscribers.
+         */
+        class BLACKMISC_EXPORT CPassiveMutator : public QObject, public QEnableSharedFromThis<CPassiveMutator>
+        {
+            Q_OBJECT
+            friend CActiveMutator;
+            friend QSharedPointer<CPassiveMutator>;
+
+            CPassiveMutator(QObject* parent) : QObject(parent) {}
+
+        public:
+            //! Factory method.
+            static auto create(QObject *parent) { return QSharedPointer<CPassiveMutator>::create(parent); }
+
+            //! Emit an event.
+            void postEvent(const CVariant &param);
+
+            //! Get a QWeakPointer pointing to this object.
+            QWeakPointer<const CPassiveMutator> weakRef() const { return sharedFromThis(); }
+
+        signals:
+            //! Emitted by postEvent.
+            void eventPosted(const BlackMisc::CVariant &param);
+        };
+    }
+}
+
+#endif
diff --git a/src/blackmisc/sharedstate/passiveobserver.cpp b/src/blackmisc/sharedstate/passiveobserver.cpp
new file mode 100644
index 000000000..186766865
--- /dev/null
+++ b/src/blackmisc/sharedstate/passiveobserver.cpp
@@ -0,0 +1,36 @@
+/* Copyright (C) 2017
+ * swift Project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution. No part of swift project, including this file, may be copied, modified, propagated,
+ * or distributed except according to the terms contained in the LICENSE file.
+ */
+
+//! \file
+
+#include "blackmisc/sharedstate/passiveobserver.h"
+
+namespace BlackMisc
+{
+    namespace SharedState
+    {
+        void CPassiveObserver::setEventSubscription(const CVariant &param)
+        {
+            QMutexLocker lock(&m_eventSubscriptionMutex);
+            m_eventSubscription = param;
+            lock.unlock();
+            emit eventSubscriptionChanged(param);
+        }
+
+        CVariant CPassiveObserver::eventSubscription() const
+        {
+            QMutexLocker lock(&m_eventSubscriptionMutex);
+            return m_eventSubscription;
+        }
+
+        void CPassiveObserver::handleEvent(const CVariant& param) const
+        {
+            m_eventHandler(param);
+        }
+    }
+}
diff --git a/src/blackmisc/sharedstate/passiveobserver.h b/src/blackmisc/sharedstate/passiveobserver.h
new file mode 100644
index 000000000..f81691c3e
--- /dev/null
+++ b/src/blackmisc/sharedstate/passiveobserver.h
@@ -0,0 +1,92 @@
+/* Copyright (C) 2017
+ * swift Project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution. No part of swift project, including this file, may be copied, modified, propagated,
+ * or distributed except according to the terms contained in the LICENSE file.
+ */
+
+//! \file
+
+#ifndef BLACKMISC_SHAREDSTATE_PASSIVEOBSERVER_H
+#define BLACKMISC_SHAREDSTATE_PASSIVEOBSERVER_H
+
+#include "blackmisc/variant.h"
+#include "blackmisc/valueobject.h"
+#include "blackmisc/blackmiscexport.h"
+#include <QObject>
+#include <QSharedPointer>
+#include <QMutex>
+
+namespace BlackMisc
+{
+    namespace SharedState
+    {
+        class CActiveObserver;
+
+        /*!
+         * Endpoint which can subscribe to events emitted by CPassiveMutator.
+         */
+        class BLACKMISC_EXPORT CPassiveObserver : public QObject, public QEnableSharedFromThis<CPassiveObserver>
+        {
+            Q_OBJECT
+            friend CActiveObserver;
+            friend QSharedPointer<CPassiveObserver>;
+
+            template <typename T, typename F>
+            CPassiveObserver(T *parent, F eventHandler) :
+                QObject(parent),
+                m_eventHandler([ = ](const CVariant &param) { Private::invokeMethod(parent, eventHandler, param); })
+            {}
+
+        public:
+            //! Factory method.
+            template <typename T, typename F>
+            static auto create(T *parent, F eventHandler) { return QSharedPointer<CPassiveObserver>::create(parent, eventHandler); }
+
+            //! Set the object that determines which events are subscribed to.
+            void setEventSubscription(const CVariant &param);
+
+            //! Get the object that determines which events are subscribed to.
+            CVariant eventSubscription() const;
+
+            //! Called when a subscribed event is emitted.
+            void handleEvent(const CVariant& param) const;
+
+            //! Get a QWeakPointer pointing to this object.
+            QWeakPointer<const CPassiveObserver> weakRef() const { return sharedFromThis(); }
+
+        signals:
+            //! Emitted by setEventSubscription.
+            void eventSubscriptionChanged(const BlackMisc::CVariant &param);
+
+        private:
+            const std::function<void(const CVariant &param)> m_eventHandler;
+
+            mutable QMutex m_eventSubscriptionMutex;
+            CVariant m_eventSubscription;
+        };
+
+        /*!
+         * Dummy value class that matches any event.
+         */
+        class BLACKMISC_EXPORT CAnyMatch : public CValueObject<CAnyMatch>
+        {
+        public:
+            //! Returns true.
+            bool matches(const CVariant &) const { return true; }
+
+            //! To string.
+            QString convertToQString(bool = false) const { return QStringLiteral("any"); }
+
+        private:
+            int m_dummy = 0;
+
+            BLACK_METACLASS(CAnyMatch, BLACK_METAMEMBER(dummy));
+        };
+    }
+}
+
+Q_DECLARE_METATYPE(BlackMisc::SharedState::CAnyMatch)
+
+#endif