qt中的信号与槽

连接形式

connect有好几种形式

static QMetaObject::Connection connect(const QObject *sender, const char *signal, const QObject *receiver, const char *member, Qt::ConnectionType = Qt::AutoConnection);

static QMetaObject::Connection connect(const QObject *sender, const QMetaMethod &signal, const QObject *receiver, const QMetaMethod &method, Qt::ConnectionType type = Qt::AutoConnection);

inline QMetaObject::Connection connect(const QObject *sender, const char *signal, const char *member, Qt::ConnectionType type = Qt::AutoConnection) const;

连接方式有

连接类型	说明
Qt::AutoConnection	如果发送者和接收者在同一线程，使用Qt::DirectConnection,否则使用Qt::QueuedConnection
Qt::DirectConnection	信号被发射时立即执行，信号和槽函数在同一线程
Qt::QueuedConnection	在事件循环回到接收者线程后执行槽函数，信号和槽函数不在同一线程
Qt::BlockingQueuedConnection	信号线程会阻塞直到槽函数执行完毕，当信号与槽函数在同一线程时，不能使用这种方式
Qt::UniqueConnection	表示不能重复连接信号和槽

涉及到的宏

# define SLOT(a)     "1"#a
# define SIGNAL(a)   "2"#a

connect底层到底做了什么呢？
其实际上添加发送者对应信号索引的监听器列表中

QObjectPrivate::Connection *QMetaObjectPrivate::connect(const QObject *sender,
                                 int signal_index, const QMetaObject *smeta,
                                 const QObject *receiver, int method_index,
                                 const QMetaObject *rmeta, int type, int *types)
{
    QObject *s = const_cast<QObject *>(sender);
    QObject *r = const_cast<QObject *>(receiver);

    int method_offset = rmeta ? rmeta->methodOffset() : 0;
    Q_ASSERT(!rmeta || QMetaObjectPrivate::get(rmeta)->revision >= 6);
    QObjectPrivate::StaticMetaCallFunction callFunction = rmeta ? rmeta->d.static_metacall : nullptr;

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

    QObjectPrivate::ConnectionData *scd  = QObjectPrivate::get(s)->connections.loadRelaxed();
    if (type & Qt::UniqueConnection && scd) {
        if (scd->signalVectorCount() > signal_index) {
            const QObjectPrivate::Connection *c2 = scd->signalVector.loadRelaxed()->at(signal_index).first.loadRelaxed();

            int method_index_absolute = method_index + method_offset;

            while (c2) {
                if (!c2->isSlotObject && c2->receiver.loadRelaxed() == receiver && c2->method() == method_index_absolute)
                    return nullptr;
                c2 = c2->nextConnectionList.loadRelaxed();
            }
        }
        type &= Qt::UniqueConnection - 1;
    }

    std::unique_ptr<QObjectPrivate::Connection> c{new QObjectPrivate::Connection};
    c->sender = s;
    c->signal_index = signal_index;
    c->receiver.storeRelaxed(r);
    QThreadData *td = r->d_func()->threadData;
    td->ref();
    c->receiverThreadData.storeRelaxed(td);
    c->method_relative = method_index;
    c->method_offset = method_offset;
    c->connectionType = type;
    c->isSlotObject = false;
    c->argumentTypes.storeRelaxed(types);
    c->callFunction = callFunction;

    QObjectPrivate::get(s)->addConnection(signal_index, c.get());

    locker.unlock();
    QMetaMethod smethod = QMetaObjectPrivate::signal(smeta, signal_index);
    if (smethod.isValid())
        s->connectNotify(smethod);

    return c.release();
}

void QObjectPrivate::addConnection(int signal, Connection *c)
{
    Q_ASSERT(c->sender == q_ptr);
    ensureConnectionData();
    ConnectionData *cd = connections.loadRelaxed();
    cd->resizeSignalVector(signal + 1);

    ConnectionList &connectionList = cd->connectionsForSignal(signal);
    if (connectionList.last.loadRelaxed()) {
        Q_ASSERT(connectionList.last.loadRelaxed()->receiver.loadRelaxed());
        connectionList.last.loadRelaxed()->nextConnectionList.storeRelaxed(c);
    } else {
        connectionList.first.storeRelaxed(c);
    }
    c->id = ++cd->currentConnectionId;
    c->prevConnectionList = connectionList.last.loadRelaxed();
    connectionList.last.storeRelaxed(c);

    QObjectPrivate *rd = QObjectPrivate::get(c->receiver.loadRelaxed());
    rd->ensureConnectionData();

    c->prev = &(rd->connections.loadRelaxed()->senders);
    c->next = *c->prev;
    *c->prev = c;
    if (c->next)
        c->next->prev = &c->next;
}

在添加连接设置连接对象时，将isSlotObject设置为false，callFunction设置为接收对象元对象的static_metacall

QObjectPrivate::StaticMetaCallFunction callFunction = rmeta ? rmeta->d.static_metacall : nullptr;

在doActivate调用时就会走callFunction分支

什么时候会触发接收者的调用

在emit 触发信号时。
moc（元对象编译器）在处理定义有SIGNALS的类时，会针对定义的信号生成对应的处理函数，在内部会调用QMetaObject::active方法，其定义为

void QMetaObject::activate(QObject *sender, const QMetaObject *m, int local_signal_index,
                           void **argv)
{
    int signal_index = local_signal_index + QMetaObjectPrivate::signalOffset(m);

    if (Q_UNLIKELY(qt_signal_spy_callback_set.loadRelaxed()))
        doActivate<true>(sender, signal_index, argv);
    else
        doActivate<false>(sender, signal_index, argv);
}

doActivate会从发送者的信号索引取出连接列表，调用处理函数
对于 Qt::AutoConnection，会判断发送线程和接收处理线程是否在同一线程，如果在同一线程，QMetaObject::metacall作同步调用，否则生成QMetaCallEvent发送qt的事件处理线程中
对于 Qt::DirectConnection， QMetaObject::metacall作同步调用
对于Qt::QueuedConnection，直接生成QMetaCallEvent发送qt的事件处理线程中
对于Qt::BlockingQueuedConnection，如果发现发送线程和接收处理线程是在同一线程，输出告警日志，会使用信号量QSemaphore，生成QMetaCallEvent发送qt的事件处理线程中等待处理结果

template <bool callbacks_enabled>
void doActivate(QObject *sender, int signal_index, void **argv)
{
    QObjectPrivate *sp = QObjectPrivate::get(sender);

    if (sp->blockSig)
        return;

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

    const QSignalSpyCallbackSet *signal_spy_set = callbacks_enabled ? qt_signal_spy_callback_set.loadAcquire() : nullptr;

    void *empty_argv[] = { nullptr };
    if (!argv)
        argv = empty_argv;

    if (!sp->maybeSignalConnected(signal_index)) {
        // The possible declarative connection is done, and nothing else is connected
        if (callbacks_enabled && signal_spy_set->signal_begin_callback != nullptr)
            signal_spy_set->signal_begin_callback(sender, signal_index, argv);
        if (callbacks_enabled && signal_spy_set->signal_end_callback != nullptr)
            signal_spy_set->signal_end_callback(sender, signal_index);
        return;
    }

    if (callbacks_enabled && signal_spy_set->signal_begin_callback != nullptr)
        signal_spy_set->signal_begin_callback(sender, signal_index, argv);

    bool senderDeleted = false;
    {
    Q_ASSERT(sp->connections.loadAcquire());
    QObjectPrivate::ConnectionDataPointer connections(sp->connections.loadRelaxed());
    QObjectPrivate::SignalVector *signalVector = connections->signalVector.loadRelaxed();

    const QObjectPrivate::ConnectionList *list;
    if (signal_index < signalVector->count())
        list = &signalVector->at(signal_index);
    else
        list = &signalVector->at(-1);

    Qt::HANDLE currentThreadId = QThread::currentThreadId();
    bool inSenderThread = currentThreadId == QObjectPrivate::get(sender)->threadData->threadId.loadRelaxed();

    // We need to check against the highest connection id to ensure that signals added
    // during the signal emission are not emitted in this emission.
    uint highestConnectionId = connections->currentConnectionId.loadRelaxed();
    do {
        QObjectPrivate::Connection *c = list->first.loadRelaxed();
        if (!c)
            continue;

        do {
            QObject * const receiver = c->receiver.loadRelaxed();
            if (!receiver)
                continue;

            QThreadData *td = c->receiverThreadData.loadRelaxed();
            if (!td)
                continue;

            bool receiverInSameThread;
            if (inSenderThread) {
                receiverInSameThread = currentThreadId == td->threadId.loadRelaxed();
            } else {
                // need to lock before reading the threadId, because moveToThread() could interfere
                QMutexLocker lock(signalSlotLock(receiver));
                receiverInSameThread = currentThreadId == td->threadId.loadRelaxed();
            }


            // 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);
                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;
                {
                    QBasicMutexLocker locker(signalSlotLock(sender));
                    if (!c->receiver.loadAcquire())
                        continue;
                    QMetaCallEvent *ev = c->isSlotObject ?
                        new QMetaCallEvent(c->slotObj, sender, signal_index, argv, &semaphore) :
                        new QMetaCallEvent(c->method_offset, c->method_relative, c->callFunction,
                                           sender, signal_index, argv, &semaphore);
                    QCoreApplication::postEvent(receiver, ev);
                }
                semaphore.acquire();
                continue;
#endif
            }

            QObjectPrivate::Sender senderData(receiverInSameThread ? receiver : nullptr, sender, signal_index);

            if (c->isSlotObject) {
                c->slotObj->ref();

                struct Deleter {
                    void operator()(QtPrivate::QSlotObjectBase *slot) const {
                        if (slot) slot->destroyIfLastRef();
                    }
                };
                const std::unique_ptr<QtPrivate::QSlotObjectBase, Deleter> obj{c->slotObj};

                {
                    Q_TRACE_SCOPE(QMetaObject_activate_slot_functor, obj.get());
                    obj->call(receiver, argv);
                }
            } 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 method_relative = c->method_relative;
                const auto callFunction = c->callFunction;
                const int methodIndex = (Q_HAS_TRACEPOINTS || callbacks_enabled) ? c->method() : 0;
                if (callbacks_enabled && signal_spy_set->slot_begin_callback != nullptr)
                    signal_spy_set->slot_begin_callback(receiver, methodIndex, argv);

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

                if (callbacks_enabled && signal_spy_set->slot_end_callback != nullptr)
                    signal_spy_set->slot_end_callback(receiver, methodIndex);
            } else {
                const int method = c->method_relative + c->method_offset;

                if (callbacks_enabled && signal_spy_set->slot_begin_callback != nullptr) {
                    signal_spy_set->slot_begin_callback(receiver, method, argv);
                }

                {
                    Q_TRACE_SCOPE(QMetaObject_activate_slot, receiver, method);
                    QMetaObject::metacall(receiver, QMetaObject::InvokeMetaMethod, method, argv);
                }

                if (callbacks_enabled && signal_spy_set->slot_end_callback != nullptr)
                    signal_spy_set->slot_end_callback(receiver, method);
            }
        } while ((c = c->nextConnectionList.loadRelaxed()) != nullptr && c->id <= highestConnectionId);

    } while (list != &signalVector->at(-1) &&
        //start over for all signals;
        ((list = &signalVector->at(-1)), true));

        if (connections->currentConnectionId.loadRelaxed() == 0)
            senderDeleted = true;
    }
    if (!senderDeleted) {
        sp->connections.loadRelaxed()->cleanOrphanedConnections(sender);

        if (callbacks_enabled && signal_spy_set->signal_end_callback != nullptr)
            signal_spy_set->signal_end_callback(sender, signal_index);
    }
}