Issue #15 Created CPromise, a way to set the result of QFuture objects

This enables several new features:
* singleShot can return a QFuture.
* a version of invokeMethod that returns a QFuture.
* CGenericDBusInterface::callDBusFuture, which returns a QFuture.

src/blackmisc/genericdbusinterface.h

@@ -12,12 +12,14 @@
 #define BLACKMISC_GENERICDBUSINTERFACE_H
 
 #include "logmessage.h"
+#include "blackmisc/promise.h"
 #include <QDBusAbstractInterface>
 #include <QDBusPendingCall>
 #include <QDBusPendingReply>
 #include <QDBusError>
 #include <QObject>
 #include <QMetaMethod>
+#include <QSharedPointer>
 
 #ifndef Q_MOC_RUN
 /*!
@@ -102,6 +104,15 @@ namespace BlackMisc
             return pcw;
         }
 
+        //! Call DBus with asynchronous return as a future
+        template <typename Ret, typename... Args>
+        QFuture<Ret> callDBusFuture(QLatin1String method, Args&&... args)
+        {
+            auto sharedPromise = QSharedPointer<CPromise<Ret>>::create();
+            this->callDBusAsync(method, [ = ](auto pcw) { sharedPromise->setResult(QDBusPendingReply<Ret>(*pcw)); }, std::forward<Args>(args)...);
+            return sharedPromise->future();
+        }
+
         //! Cancel all asynchronous DBus calls which are currently pending
         //! \warning Don't call this from inside an async callback!
         void cancelAllPendingAsyncCalls()