QSignalBlocker: (new) RAII class for QObject::blockSignals()

I don't think I ever worked on a project of non-trivial
size that didn't at some point add a QSignalBlocker.

This commit adds code, tests and documentation. Later
commits will convert naked blockSignals() calls to use
QSignalBlocker.

The implementation is purely inline to avoid the heavy
overhead of cross-dll function calls for this miniscule
task. This should not be a problem because QSignalBlocker
only uses public API and a pattern that we anyway need
to keep working until Qt 6, at least, so even changing
the implementation later will be no problem as the old
implementation lurking in non-recompiled code will be
acceptable, too.

This implementation is an evolution from KDTools'
KDSignalBlocker, with the following changes:
- Implements unblock() and reblock()
- Uses the return value of blockSignals() instead of a
  separate signalsBlocked() call.

Change-Id: I1933dfd72a0f5190324be377cfca3c54cf3d6828
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

1 files changed, 46 insertions, 0 deletions

diff --git a/src/corelib/kernel/qobject.h b/src/corelib/kernel/qobject.h
index e2000afc82..f9239bf575 100644
--- a/src/corelib/kernel/qobject.h
+++ b/src/corelib/kernel/qobject.h
@@ -549,6 +549,52 @@ template <class T> inline const char * qobject_interface_iid()
 Q_CORE_EXPORT QDebug operator<<(QDebug, const QObject *);
 #endif
 
+class Q_CORE_EXPORT QSignalBlocker
+{
+public:
+    inline explicit QSignalBlocker(QObject *o);
+    inline explicit QSignalBlocker(QObject &o);
+    inline ~QSignalBlocker();
+
+    inline void reblock();
+    inline void unblock();
+private:
+    Q_DISABLE_COPY(QSignalBlocker)
+    QObject * const m_o;
+    bool m_blocked;
+    bool m_inhibited;
+};
+
+QSignalBlocker::QSignalBlocker(QObject *o)
+    : m_o(o),
+      m_blocked(o && o->blockSignals(true)),
+      m_inhibited(false)
+{}
+
+QSignalBlocker::QSignalBlocker(QObject &o)
+    : m_o(&o),
+      m_blocked(o.blockSignals(true)),
+      m_inhibited(false)
+{}
+
+QSignalBlocker::~QSignalBlocker()
+{
+    if (m_o && !m_inhibited)
+        m_o->blockSignals(m_blocked);
+}
+
+void QSignalBlocker::reblock()
+{
+    if (m_o) m_o->blockSignals(true);
+    m_inhibited = false;
+}
+
+void QSignalBlocker::unblock()
+{
+    if (m_o) m_o->blockSignals(m_blocked);
+    m_inhibited = true;
+}
+
 namespace QtPrivate {
     inline QObject & deref_for_methodcall(QObject &o) { return  o; }
     inline QObject & deref_for_methodcall(QObject *o) { return *o; }