Add QTimer::connectTo(), a shorthand way of connecting to timeout()

There are a couple of Qt classes where you almost always use the
same signal, for example QTimer::timeout, QPushButton::clicked,
and QAction::triggered.

Simply doing timer.connectTo([]{}) is much more convenient, less
tedious and even fun.

Not overloading connect() as it would be confusing to see the
receiver as first argument.

And not naming it onTimeout, as that's a popular way of doing it in
other frameworks. People would assume you could use on* with any signal.
If we ever have on* it should be all or nothing.

[ChangeLog][QtCore] Added QTimer::connectTo(), a shorthand way of
connecting to the timeout() signal.

Change-Id: Ida57e5442b13d50972ed585c3ea7be07e3d8e8d2
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/kernel/qtimer.cpp

@@ -570,6 +570,48 @@ void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiv
     \sa start()
 */
 
+/*!
+    \fn template<typename Functor> QMetaObject::Connection connectTo(Functor functor, Qt::ConnectionType connectionType = Qt::AutoConnection)
+    \since 5.12
+    \overload
+
+    Creates a connection from the timeout() signal to \a functor, and returns a
+    handle to the connection.
+
+    This method is provided for convenience.
+    It's equivalent to calling \c {QObject::connect(timer, &QTimer::timeout, timer, functor, connectionType)}.
+
+    \sa QObject::connect(), timeout()
+*/
+
+/*!
+    \fn template<typename Functor> QMetaObject::Connection connectTo(QObject *context, Functor functor, Qt::ConnectionType connectionType = Qt::AutoConnection)
+    \since 5.12
+    \overload connectTo()
+
+    Creates a connection from the timeout() signal to \a functor to be placed in a specific
+    event loop of \a context, and returns a handle to the connection.
+
+    This method is provided for convenience. It's equivalent to calling
+    \c {QObject::connect(timer, &QTimer::timeout, context, functor, connectionType)}.
+
+    \sa QObject::connect(), timeout()
+*/
+
+/*!
+    \fn template<typename PointerToMemberFunction> QMetaObject::Connection connectTo(QObject *receiver, PointerToMemberFunction method, Qt::ConnectionType connectionType = Qt::AutoConnection)
+    \since 5.12
+    \overload connectTo()
+
+    Creates a connection from the timeout() signal to the \a method in the \a receiver object. Returns
+    a handle to the connection.
+
+    This method is provided for convenience. It's equivalent to calling
+    \c {QObject::connect(timer, &QTimer::timeout, receiver, method, connectionType)}.
+
+    \sa QObject::connect(), timeout()
+*/
+
 /*!
     \fn void QTimer::start(std::chrono::milliseconds msec)
     \since 5.8

src/corelib/kernel/qtimer.h

@@ -96,6 +96,12 @@ public:
     static void singleShot(int msec, const QObject *context, Functor functor);
     template<typename Functor, int>
     static void singleShot(int msec, Qt::TimerType timerType, const QObject *context, Functor functor);
+    template <typename Functor>
+    QMetaObject::Connection connectTo(Functor slot, Qt::ConnectionType connectionType = Qt::AutoConnection);
+    template <typename Functor>
+    QMetaObject::Connection connectTo(const QObject *context, Functor slot, Qt::ConnectionType connectionType = Qt::AutoConnection);
+    template <typename PointerToMemberFunction>
+    QMetaObject::Connection connectTo(const QObject *receiver, PointerToMemberFunction slot, Qt::ConnectionType connectionType = Qt::AutoConnection);
 #else
     // singleShot to a QObject slot
     template <typename Duration, typename Func1>
@@ -152,6 +158,13 @@ public:
                        new QtPrivate::QFunctorSlotObject<Func1, 0,
                             typename QtPrivate::List_Left<void, 0>::Value, void>(std::move(slot)));
     }
+
+    template <typename ... Args>
+    QMetaObject::Connection connectTo(Args && ...args)
+    {
+        return QObject::connect(this, &QTimer::timeout, std::forward<Args>(args)... );
+    }
+
 #endif
 
 public Q_SLOTS:

tests/auto/corelib/kernel/qtimer/tst_qtimer.cpp

@@ -75,6 +75,7 @@ private slots:
 
     void dontBlockEvents();
     void postedEventsShouldNotStarveTimers();
+    void connectTo();
 };
 
 class TimerHelper : public QObject
@@ -1020,5 +1021,31 @@ void tst_QTimer::crossThreadSingleShotToFunctor()
     delete o;
 }
 
+void tst_QTimer::connectTo()
+{
+    QTimer timer;
+    TimerHelper timerHelper;
+    timer.setInterval(0);
+    timer.start();
+
+    auto context = new QObject();
+
+    int count = 0;
+    timer.connectTo([&count] { count++; });
+    QMetaObject::Connection connection = timer.connectTo(context, [&count] { count++; });
+    timer.connectTo(&timerHelper, &TimerHelper::timeout);
+    timer.connectTo(&timer, &QTimer::stop);
+
+
+    QTest::qWait(100);
+    QCOMPARE(count, 2);
+    QCOMPARE(timerHelper.count, 1);
+
+    // Test that connection is bound to context lifetime
+    QVERIFY(connection);
+    delete context;
+    QVERIFY(!connection);
+}
+
 QTEST_MAIN(tst_QTimer)
 #include "tst_qtimer.moc"