QObject: add a single shot connection flag

If one needed to listen to a signal just once, one had to
store the QMetaObject::Connection object returned by connect()
and use it to disconnect the slot after the first signal
activation.

This has led to a proliferation of using wrappers (and enough
TMP); they usually look like this:

1) create a shared_ptr<QMO::Connection>, allocating its payload;
2) create a lambda, capturing the shared_ptr by value;
3) in the lambda, disconnect the connection (through the shared_ptr),
   and call the actual slot;
4) connect the signal to the lambda, storing the returned
   QMO::Connection into the shared_ptr.

This is expensive, error prone for newcomers, and tricky to
support as a general facility inside one's projects.
We can do better, just support single shot connections right
in QObject.

[ChangeLog][QtCore][QObject] Added the Qt::SingleShotConnection
flag. When a connection is established with this flag set,
the slot is going to be activated at most once; when the signal
is emitted, the connection gets automatically broken by Qt.

Change-Id: I5f5feeae7f76c9c3d6323d841efba81c8f98ce7e
Fixes: QTBUG-44219
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index 4a60d37191..1f13274945 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -153,9 +153,10 @@ public:
         ushort method_offset;
         ushort method_relative;
         signed int signal_index : 27; // In signal range (see QObjectPrivate::signalIndex())
-        ushort connectionType : 3; // 0 == auto, 1 == direct, 2 == queued, 4 == blocking
+        ushort connectionType : 2; // 0 == auto, 1 == direct, 2 == queued, 3 == blocking
         ushort isSlotObject : 1;
         ushort ownArgumentTypes : 1;
+        ushort isSingleShot : 1;
         Connection() : ref_(2), ownArgumentTypes(true) {
             //ref_ is 2 for the use in the internal lists, and for the use in QMetaObject::Connection
         }
@@ -348,10 +349,11 @@ public:
 
     static QMetaObject::Connection connectImpl(const QObject *sender, int signal_index,
                                                const QObject *receiver, void **slot,
-                                               QtPrivate::QSlotObjectBase *slotObj, Qt::ConnectionType type,
+                                               QtPrivate::QSlotObjectBase *slotObj, int type,
                                                const int *types, const QMetaObject *senderMetaObject);
     static QMetaObject::Connection connect(const QObject *sender, int signal_index, QtPrivate::QSlotObjectBase *slotObj, Qt::ConnectionType type);
     static bool disconnect(const QObject *sender, int signal_index, void **slot);
+    static bool disconnect(Connection *c);
 
     void ensureConnectionData()
     {