Add QMetaMethod::fromSignal() function

Given a member function that's a signal, returns the corresponding
QMetaMethod. Inspired by the implementation of the template-based
QObject::connect().

The primary use case for this function is to have an effective and
exact (not subject to shadowing) way of checking whether a known
signal was connected to in reimplementations of
QObject::connectNotify(QMetaMethod), avoiding string comparisons.
Example:

void MyObject::connectNotify(const QMetaMethod &signal)
{
    if (signal == QMetaMethod::fromSignal(&MyObject::mySignal)) {
        // Someone connected to mySignal ...
    }
}

Change-Id: I5e4de434275fe543c004d569dcaa9ceda3442f03
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/src/corelib/kernel/qmetaobject.cpp b/src/corelib/kernel/qmetaobject.cpp
index 4414393d37..45667ae378 100644
--- a/src/corelib/kernel/qmetaobject.cpp
+++ b/src/corelib/kernel/qmetaobject.cpp
@@ -1857,6 +1857,41 @@ QMetaMethod::MethodType QMetaMethod::methodType() const
 }
 
 /*!
+    \fn QMetaMethod QMetaMethod::fromSignal(PointerToMemberFunction signal)
+    \since 5.0
+
+    Returns the meta-method that corresponds to the given \a signal, or an
+    invalid QMetaMethod if \a signal is not a signal of the class.
+
+    Example:
+
+    \snippet code/src_corelib_kernel_qmetaobject.cpp 9
+*/
+
+/*! \internal
+
+    Implementation of the fromSignal() function.
+
+    \a metaObject is the class's meta-object
+    \a signal is a pointer to a pointer to a member signal of the class
+*/
+QMetaMethod QMetaMethod::fromSignalImpl(const QMetaObject *metaObject, void **signal)
+{
+    int i = -1;
+    void *args[] = { &i, signal };
+    QMetaMethod result;
+    for (const QMetaObject *m = metaObject; m; m = m->d.superdata) {
+        m->static_metacall(QMetaObject::IndexOfMethod, 0, args);
+        if (i >= 0) {
+            result.mobj = m;
+            result.handle = priv(m->d.data)->methodData + 5*i;
+            break;
+        }
+    }
+    return result;
+}
+
+/*!
     Invokes this method on the object \a object. Returns true if the member could be invoked.
     Returns false if there is no such member or the parameters did not match.