【Qt源码】窥视信号槽实现机制

为了便于通过调试进源码探究下Qt信号槽实现原理，这里简单写一段代码如下所示。

1.自定义信号槽连接

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow) {
    ui->setupUi(this);
    QObject::connect(ui->pushButton,&QPushButton::clicked,this,&MainWindow::on_pushButton_clicked);
}
MainWindow::~MainWindow() { delete ui;}
void MainWindow::on_pushButton_clicked() { qDebug() << "this is slot function"; }

2.调试进入源码观察槽函数调用流程

为了观察QPushButton::clicked()信号发射后如何自动调用on_pushButton_clicked槽函数，首先在源码里打上断点，通过如下流程最后实现由信号发射到槽函数调用。

// step1.这里打上断点
void QAbstractButton::mouseReleaseEvent(QMouseEvent *e)
{
    Q_D(QAbstractButton);

    if (e->button() != Qt::LeftButton) {
        e->ignore();
        return;
    }

    d->pressed = false;

    if (!d->down) {
        // refresh is required by QMacStyle to resume the default button animation
        d->refresh();
        e->ignore();
        return;
    }

    if (hitButton(e->pos())) {
        d->repeatTimer.stop();
        d->click(); // 这里发射click信号
        e->accept();
    } else {
        setDown(false);
        e->ignore();
    }
}
// step2. 进入click函数
void QAbstractButtonPrivate::click()
{
    Q_Q(QAbstractButton);

    down = false;
    blockRefresh = true;
    bool changeState = true;
    if (checked && queryCheckedButton() == q) {
        // the checked button of an exclusive or autoexclusive group cannot be unchecked
#if QT_CONFIG(buttongroup)
        if (group ? group->d_func()->exclusive : autoExclusive)
#else
        if (autoExclusive)
#endif
            changeState = false;
    }

    QPointer<QAbstractButton> guard(q);
    if (changeState) {
        q->nextCheckState();
        if (!guard)
            return;
    }
    blockRefresh = false;
    refresh();
    q->repaint();
    if (guard)
        emitReleased();
    if (guard)
        emitClicked();
}
// step3. 发射clciked
void QAbstractButtonPrivate::emitClicked()
{
    Q_Q(QAbstractButton);
    QPointer<QAbstractButton> guard(q);
    // 这里F11进入activate函数，该函数在moc_abstractbutton.cpp文件中，该文件不可见
    emit q->clicked(checked); 
#if QT_CONFIG(buttongroup)
    if (guard && group) {
        emit group->buttonClicked(group->id(q));
        if (guard && group)
            emit group->buttonClicked(q);
    }
#endif
}
// step4. 元对象自动生成的activate函数
void QMetaObject::activate(QObject *sender, const QMetaObject *m, int local_signal_index,
                           void **argv)
{
    activate(sender, QMetaObjectPrivate::signalOffset(m), local_signal_index, argv);
}
// step5 进入activate函数
void QMetaObject::activate(QObject *sender, int signalOffset, int local_signal_index, void **argv)
{
    int signal_index = signalOffset + local_signal_index;

    if (sender->d_func()->blockSig)
        return;

    Q_TRACE_SCOPE(QMetaObject_activate, sender, signal_index);

    if (sender->d_func()->isDeclarativeSignalConnected(signal_index)
            && QAbstractDeclarativeData::signalEmitted) {
        Q_TRACE_SCOPE(QMetaObject_activate_declarative_signal, sender, signal_index);
        QAbstractDeclarativeData::signalEmitted(sender->d_func()->declarativeData, sender,
                                                signal_index, argv);
    }

    if (!sender->d_func()->isSignalConnected(signal_index, /*checkDeclarative =*/ false)
        && !qt_signal_spy_callback_set.signal_begin_callback
        && !qt_signal_spy_callback_set.signal_end_callback) {
        // The possible declarative connection is done, and nothing else is connected, so:
        return;
    }

    void *empty_argv[] = { 0 };
    if (qt_signal_spy_callback_set.signal_begin_callback != 0) {
        qt_signal_spy_callback_set.signal_begin_callback(sender, signal_index,
                                                         argv ? argv : empty_argv);
    }

    {
    QMutexLocker locker(signalSlotLock(sender));
    struct ConnectionListsRef {
        QObjectConnectionListVector *connectionLists;
        ConnectionListsRef(QObjectConnectionListVector *connectionLists) : connectionLists(connectionLists)
        {
            if (connectionLists)
                ++connectionLists->inUse;
        }
        ~ConnectionListsRef()
        {
            if (!connectionLists)
                return;

            --connectionLists->inUse;
            Q_ASSERT(connectionLists->inUse >= 0);
            if (connectionLists->orphaned) {
                if (!connectionLists->inUse)
                    delete connectionLists;
            }
        }

        QObjectConnectionListVector *operator->() const { return connectionLists; }
    };
    // 1.获取sender里所有connection列表
    ConnectionListsRef connectionLists = sender->d_func()->connectionLists;
    if (!connectionLists.connectionLists) {
        locker.unlock();
        if (qt_signal_spy_callback_set.signal_end_callback != 0)
            qt_signal_spy_callback_set.signal_end_callback(sender, signal_index);
        return;
    }

    const QObjectPrivate::ConnectionList *list;
    if (signal_index < connectionLists->count())
        list = &connectionLists->at(signal_index);
    else
        list = &connectionLists->allsignals;

    Qt::HANDLE currentThreadId = QThread::currentThreadId();

    // 2.在sender的connection列表中遍历
    do {
        QObjectPrivate::Connection *c = list->first;
        if (!c) continue;
        // We need to check against last here to ensure that signals added
        // during the signal emission are not emitted in this emission.
        QObjectPrivate::Connection *last = list->last;

        do {
            if (!c->receiver)
                continue;

            QObject * const receiver = c->receiver;
            const bool receiverInSameThread = currentThreadId == receiver->d_func()->threadData->threadId.load();

            // determine if this connection should be sent immediately or
            // put into the event queue
            if ((c->connectionType == Qt::AutoConnection && !receiverInSameThread)
                || (c->connectionType == Qt::QueuedConnection)) {
                queued_activate(sender, signal_index, c, argv ? argv : empty_argv, locker);
                continue;
#if QT_CONFIG(thread)
            } else if (c->connectionType == Qt::BlockingQueuedConnection) {
                if (receiverInSameThread) {
                    qWarning("Qt: Dead lock detected while activating a BlockingQueuedConnection: "
                    "Sender is %s(%p), receiver is %s(%p)",
                    sender->metaObject()->className(), sender,
                    receiver->metaObject()->className(), receiver);
                }
                QSemaphore semaphore;
                QMetaCallEvent *ev = c->isSlotObject ?
                    new QMetaCallEvent(c->slotObj, sender, signal_index, 0, 0, argv ? argv : empty_argv, &semaphore) :
                    new QMetaCallEvent(c->method_offset, c->method_relative, c->callFunction, sender, signal_index, 0, 0, argv ? argv : empty_argv, &semaphore);
                QCoreApplication::postEvent(receiver, ev);
                locker.unlock();
                semaphore.acquire();
                locker.relock();
                continue;
#endif
            }

            QConnectionSenderSwitcher sw;

            if (receiverInSameThread) {
                sw.switchSender(receiver, sender, signal_index);
            }
            if (c->isSlotObject) {
                c->slotObj->ref();
                QScopedPointer<QtPrivate::QSlotObjectBase, QSlotObjectBaseDeleter> obj(c->slotObj);
                locker.unlock();

                {
                    Q_TRACE_SCOPE(QMetaObject_activate_slot_functor, obj.data());
                    obj->call(receiver, argv ? argv : empty_argv);
                }

                // Make sure the slot object gets destroyed before the mutex is locked again, as the
                // destructor of the slot object might also lock a mutex from the signalSlotLock() mutex pool,
                // and that would deadlock if the pool happens to return the same mutex.
                obj.reset();

                locker.relock();
            } else if (c->callFunction && c->method_offset <= receiver->metaObject()->methodOffset()) {
                //we compare the vtable to make sure we are not in the destructor of the object.
                const int methodIndex = c->method();
                const int method_relative = c->method_relative;
                const auto callFunction = c->callFunction;
                locker.unlock();
                if (qt_signal_spy_callback_set.slot_begin_callback != 0)
                    qt_signal_spy_callback_set.slot_begin_callback(receiver, methodIndex, argv ? argv : empty_argv);

                {
                    Q_TRACE_SCOPE(QMetaObject_activate_slot, receiver, methodIndex);
                    callFunction(receiver, QMetaObject::InvokeMetaMethod, method_relative, argv ? argv : empty_argv);
                }

                if (qt_signal_spy_callback_set.slot_end_callback != 0)
                    qt_signal_spy_callback_set.slot_end_callback(receiver, methodIndex);
                locker.relock();
            } else {
                const int method = c->method_relative + c->method_offset;
                locker.unlock();

                if (qt_signal_spy_callback_set.slot_begin_callback != 0) {
                    qt_signal_spy_callback_set.slot_begin_callback(receiver,
                                                                method,
                                                                argv ? argv : empty_argv);
                }

                {
                    Q_TRACE_SCOPE(QMetaObject_activate_slot, receiver, method);
                    // 这里元调用
                    metacall(receiver, QMetaObject::InvokeMetaMethod, method, argv ? argv : empty_argv); 
                }

                if (qt_signal_spy_callback_set.slot_end_callback != 0)
                    qt_signal_spy_callback_set.slot_end_callback(receiver, method);

                locker.relock();
            }

            if (connectionLists->orphaned)
                break;
        } while (c != last && (c = c->nextConnectionList) != 0);

        if (connectionLists->orphaned)
            break;
    } while (list != &connectionLists->allsignals &&
        //start over for all signals;
        ((list = &connectionLists->allsignals), true));

    }

    if (qt_signal_spy_callback_set.signal_end_callback != 0)
        qt_signal_spy_callback_set.signal_end_callback(sender, signal_index);
}
// step6. 进入metalcall
int QMetaObject::metacall(QObject *object, Call cl, int idx, void **argv)
{
    if (object->d_ptr->metaObject)
        return object->d_ptr->metaObject->metaCall(object, cl, idx, argv);
    else
        return object->qt_metacall(cl, idx, argv);
}
// step7. Q_OBJECT扩展，MOC.exe自动生成
int MainWindow::qt_metacall(QMetaObject::Call _c, int _id, void **_a)
{
    _id = QMainWindow::qt_metacall(_c, _id, _a);
    if (_id < 0)
        return _id;
    if (_c == QMetaObject::InvokeMetaMethod) {
        if (_id < 1)
            // 这里调用静态元调用
            qt_static_metacall(this, _c, _id, _a); 
        _id -= 1;
    } else if (_c == QMetaObject::RegisterMethodArgumentMetaType) {
        if (_id < 1)
            *reinterpret_cast<int*>(_a[0]) = -1;
        _id -= 1;
    }
    return _id;
}
// step8 Q_OBJECT扩展，MOC.exe自动生成
void MainWindow::qt_static_metacall(QObject *_o, QMetaObject::Call _c, int _id, void **_a)
{
    if (_c == QMetaObject::InvokeMetaMethod) {
        auto *_t = static_cast<MainWindow *>(_o);
        Q_UNUSED(_t)
        switch (_id) {
        // 这里最终调用槽函数
        case 0: _t->on_pushButton_clicked(); break; 
        default: ;
        }
    }
    Q_UNUSED(_a);
}

从代码里可以看出Qt源码中大量使用d指针，从上面可以看出Qt元对象系统内维护了sender的连接链表，接下来看下connect函数是如何将信号和槽函数加入链表中去的。

3.观察connect函数实现

打开QObject::connect函数，进入源码。

// step 1 进入connect源码中实现
    template <typename Func1, typename Func2>
    static inline typename std::enable_if<QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1, QMetaObject::Connection>::type
            connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
                    Qt::ConnectionType type = Qt::AutoConnection)
    {
        typedef QtPrivate::FunctionPointer<Func1> SignalType;
        const int FunctorArgumentCount = QtPrivate::ComputeFunctorArgumentCount<Func2 , typename SignalType::Arguments>::Value;

        Q_STATIC_ASSERT_X((FunctorArgumentCount >= 0),
                          "Signal and slot arguments are not compatible.");
        const int SlotArgumentCount = (FunctorArgumentCount >= 0) ? FunctorArgumentCount : 0;
        typedef typename QtPrivate::FunctorReturnType<Func2, typename QtPrivate::List_Left<typename SignalType::Arguments, SlotArgumentCount>::Value>::Value SlotReturnType;

        Q_STATIC_ASSERT_X((QtPrivate::AreArgumentsCompatible<SlotReturnType, typename SignalType::ReturnType>::value),
                          "Return type of the slot is not compatible with the return type of the signal.");

        Q_STATIC_ASSERT_X(QtPrivate::HasQ_OBJECT_Macro<typename SignalType::Object>::Value,
                          "No Q_OBJECT in the class with the signal");

        const int *types = nullptr;
        if (type == Qt::QueuedConnection || type == Qt::BlockingQueuedConnection)
            types = QtPrivate::ConnectionTypes<typename SignalType::Arguments>::types();

        return connectImpl(sender, reinterpret_cast<void **>(&signal), context, nullptr,
                           new QtPrivate::QFunctorSlotObject<Func2, SlotArgumentCount,
                                typename QtPrivate::List_Left<typename SignalType::Arguments, SlotArgumentCount>::Value,
                                typename SignalType::ReturnType>(std::move(slot)),
                           type, types, &SignalType::Object::staticMetaObject);
    }
//  step2
QMetaObject::Connection QObject::connectImpl(const QObject *sender, void **signal,
                                             const QObject *receiver, void **slot,
                                             QtPrivate::QSlotObjectBase *slotObj, Qt::ConnectionType type,
                                             const int *types, const QMetaObject *senderMetaObject)
{
    if (!signal) {
        qWarning("QObject::connect: invalid null parameter");
        if (slotObj)
            slotObj->destroyIfLastRef();
        return QMetaObject::Connection();
    }

    int signal_index = -1;
    void *args[] = { &signal_index, signal };
    for (; senderMetaObject && signal_index < 0; senderMetaObject = senderMetaObject->superClass()) {
        senderMetaObject->static_metacall(QMetaObject::IndexOfMethod, 0, args);
        if (signal_index >= 0 && signal_index < QMetaObjectPrivate::get(senderMetaObject)->signalCount)
            break;
    }
    if (!senderMetaObject) {
        qWarning("QObject::connect: signal not found in %s", sender->metaObject()->className());
        slotObj->destroyIfLastRef();
        return QMetaObject::Connection(0);
    }
    signal_index += QMetaObjectPrivate::signalOffset(senderMetaObject);
    return QObjectPrivate::connectImpl(sender, signal_index, receiver, slot, slotObj, type, types, senderMetaObject);
}
// step2. 进入connectImpl函数
QMetaObject::Connection QObjectPrivate::connectImpl(const QObject *sender, int signal_index,
                                             const QObject *receiver, void **slot,
                                             QtPrivate::QSlotObjectBase *slotObj, Qt::ConnectionType type,
                                             const int *types, const QMetaObject *senderMetaObject)
{
    if (!sender || !receiver || !slotObj || !senderMetaObject) {
        const char *senderString = sender ? sender->metaObject()->className()
                                          : senderMetaObject ? senderMetaObject->className()
                                          : "Unknown";
        const char *receiverString = receiver ? receiver->metaObject()->className()
                                              : "Unknown";
        qWarning("QObject::connect(%s, %s): invalid null parameter", senderString, receiverString);
        if (slotObj)
            slotObj->destroyIfLastRef();
        return QMetaObject::Connection();
    }

    QObject *s = const_cast<QObject *>(sender);
    QObject *r = const_cast<QObject *>(receiver);

    QOrderedMutexLocker locker(signalSlotLock(sender),
                               signalSlotLock(receiver));

    if (type & Qt::UniqueConnection && slot) {
        QObjectConnectionListVector *connectionLists = QObjectPrivate::get(s)->connectionLists;
        if (connectionLists && connectionLists->count() > signal_index) {
            const QObjectPrivate::Connection *c2 =
                (*connectionLists)[signal_index].first;

            while (c2) {
                if (c2->receiver == receiver && c2->isSlotObject && c2->slotObj->compare(slot)) {
                    slotObj->destroyIfLastRef();
                    return QMetaObject::Connection();
                }
                c2 = c2->nextConnectionList;
            }
        }
        type = static_cast<Qt::ConnectionType>(type ^ Qt::UniqueConnection);
    }

    QScopedPointer<QObjectPrivate::Connection> c(new QObjectPrivate::Connection);
    c->sender = s;
    c->signal_index = signal_index;
    c->receiver = r;
    c->slotObj = slotObj;
    c->connectionType = type;
    c->isSlotObject = true;
    if (types) {
        c->argumentTypes.store(types);
        c->ownArgumentTypes = false;
    }
    // 这里获取sender的d指针并调用addConnect函数，将信号索引和connection数据加入链表中
    QObjectPrivate::get(s)->addConnection(signal_index, c.data());
    QMetaObject::Connection ret(c.take());
    locker.unlock();

    QMetaMethod method = QMetaObjectPrivate::signal(senderMetaObject, signal_index);
    Q_ASSERT(method.isValid());
    s->connectNotify(method);

    return ret;
}
// step3 维护链表实现
void QObjectPrivate::addConnection(int signal, Connection *c)
{
    Q_ASSERT(c->sender == q_ptr);
    if (!connectionLists)
        connectionLists = new QObjectConnectionListVector();
    if (signal >= connectionLists->count())
        connectionLists->resize(signal + 1);

    ConnectionList &connectionList = (*connectionLists)[signal];
    if (connectionList.last) {
        connectionList.last->nextConnectionList = c;
    } else {
        connectionList.first = c;
    }
    connectionList.last = c;

    cleanConnectionLists();

    c->prev = &(QObjectPrivate::get(c->receiver)->senders);
    c->next = *c->prev;
    *c->prev = c;
    if (c->next)
        c->next->prev = &c->next;

    if (signal < 0) {
        connectedSignals[0].store(~0);
        connectedSignals[1].store(~0);
    } else if (signal < (int)sizeof(connectedSignals) * 8) {
        connectedSignals[signal >> 5].store(connectedSignals[signal >> 5].load() | (1 << (signal & 0x1f)));
    }
}

4.总结

上述两段代码摘自Qt5.12.12的源码，粗略查看下信号槽实现机制，对于很多细节完全没有关注，后续有时间应该要更加细致的拜读下。

5.参考资料

d指针、q指针以及Qt元对象系统相关资料可自行bing查询