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, 73 insertions, 0 deletions

diff --git a/src/corelib/kernel/qobject.cpp b/src/corelib/kernel/qobject.cpp
index a52fd25e08..72ce941b6e 100644
--- a/src/corelib/kernel/qobject.cpp
+++ b/src/corelib/kernel/qobject.cpp
@@ -483,6 +483,79 @@ void QMetaCallEvent::placeMetaCall(QObject *object)
 }
 
 /*!
+    \class QSignalBlocker
+    \brief Exception-safe wrapper around QObject::blockSignals()
+    \since 5.3
+    \ingroup objectmodel
+
+    \reentrant
+
+    QSignalBlocker can be used whereever you would otherwise use a
+    pair of calls to blockSignals(). It blocks signals in its
+    constructor and in the destructor it resets the state to what
+    it was before the constructor ran.
+
+    \code
+    {
+    const QSignalBlocker blocker(someQObject);
+    // no signals here
+    }
+    \endcode
+    is thus equivalent to
+    \code
+    const bool wasBlocked = someQObject->blockSignals(true);
+    // no signals here
+    someQObject->blockSignals(false);
+    \endcode
+
+    except the code using QSignalBlocker is safe in the face of
+    exceptions.
+
+    \sa QMutexLocker, QEventLoopLocker
+*/
+
+/*!
+    \fn QSignalBlocker::QSignalBlocker(QObject *object)
+
+    Constructor. Calls \a{object}->blockSignals(true).
+*/
+
+/*!
+    \fn QSignalBlocker::QSignalBlocker(QObject &object)
+    \overload
+
+    Calls \a{object}.blockSignals(true).
+*/
+
+/*!
+    \fn QSignalBlocker::~QSignalBlocker()
+
+    Destructor. Restores the QObject::signalsBlocked() state to what it
+    was before the constructor ran, unless unblock() has been called
+    without a following reblock(), in which case it does nothing.
+*/
+
+/*!
+    \fn void QSignalBlocker::reblock()
+
+    Re-blocks signals after a previous unblock().
+
+    The numbers of reblock() and unblock() calls are not counted, so
+    every reblock() undoes any number of unblock() calls.
+*/
+
+/*!
+    \fn void QSignalBlocker::unblock()
+
+    Temporarily restores the QObject::signalsBlocked() state to what
+    it was before this QSignaBlocker's constructor ran. To undo, use
+    reblock().
+
+    The numbers of reblock() and unblock() calls are not counted, so
+    every unblock() undoes any number of reblock() calls.
+*/
+
+/*!
     \class QObject
     \inmodule QtCore
     \brief The QObject class is the base class of all Qt objects.