Add QMetaMethod::isValid() function

This function provides a proper way of determining whether a function
returned by QMetaObject::method() is valid. (Checking whether
signature() returns a 0 pointer, which e.g. testlib does, is not an
ideal API -- especially given that signature() will soon be removed
and replaced by a function that returns a QByteArray.)

Change-Id: I644f476b09904925f2042945f5d0ad744482b682
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/corelib/kernel/qmetaobject.cpp

@@ -1248,6 +1248,14 @@ bool QMetaObject::invokeMethod(QObject *obj,
     \value Scriptable
 */
 
+/*!
+    \fn bool QMetaMethod::isValid() const
+    \since 5.0
+
+    Returns true if this method is valid (can be introspected and
+    invoked), otherwise returns false.
+*/
+
 /*!
     \fn const QMetaObject *QMetaMethod::enclosingMetaObject() const
     \internal

src/corelib/kernel/qmetaobject.h

@@ -134,6 +134,8 @@ public:
                       val0, val1, val2, val3, val4, val5, val6, val7, val8, val9);
     }
 
+    inline bool isValid() const { return mobj != 0; }
+
 private:
     const QMetaObject *mobj;
     uint handle;

tests/auto/corelib/kernel/qmetamethod/tst_qmetamethod.cpp

@@ -52,6 +52,8 @@ class tst_QMetaMethod : public QObject
 private slots:
     void method_data();
     void method();
+
+    void invalidMethod();
 };
 
 struct CustomType { };
@@ -597,6 +599,7 @@ void tst_QMetaMethod::method()
     QVERIFY(index != -1);
     QMetaMethod method = (methodType == QMetaMethod::Constructor)
                          ? mo->constructor(index) : mo->method(index);
+    QVERIFY(method.isValid());
     QCOMPARE(method.methodType(), methodType);
     QCOMPARE(method.access(), access);
 
@@ -611,5 +614,17 @@ void tst_QMetaMethod::method()
     QCOMPARE(method.parameterNames(), parameterNames);
 }
 
+void tst_QMetaMethod::invalidMethod()
+{
+    QMetaMethod method;
+    QVERIFY(!method.isValid());
+
+    QMetaMethod method2 = staticMetaObject.method(staticMetaObject.methodCount());
+    QVERIFY(!method2.isValid());
+
+    QMetaMethod method3 = staticMetaObject.method(-1);
+    QVERIFY(!method3.isValid());
+}
+
 QTEST_MAIN(tst_QMetaMethod)
 #include "tst_qmetamethod.moc"