QT（2）：信号槽机制

信号槽

信号槽是观察者模式的一种实现，订阅-发布。
一个信号就是一个能够被观察的事件，一个槽就是一个观察者，被观察者-多观察者。
信号发出者调用所有注册的槽函数

信号槽本质上是两个对象的函数地址映射
单线程下，相当于函数指针调用
多线程下，发送者将槽函数的调用转化为一次调用事件放入事件循环队列，接收者线程执行到下一次事件处理时，调用相应的函数

效率比直接调用慢十倍

1.标准connect链接

a.将信号槽的函数名称按照相应规则组合成字符串，通过connect解析字符串，分别获取信号槽的绝对索引，然后将信号槽的链接关系添加到信号槽容器中
b.所以信号槽的检查都在运行时解析字符串的方式进行，即使拼写错误，也可以编译成功，只会在运行时报错
c.支持重载
d.不能调用普通函数
e.默认autoConnection，不可手动设置

QMetaObject::Connection QObject::connect(const QObject *sender, const char *signal,
                                     const QObject *receiver, const char *method,
                                     Qt::ConnectionType type)
{//检查四个参数是否有空
    if (sender == nullptr || receiver == nullptr || signal == nullptr || method == nullptr) {
        qWarning("QObject::connect: Cannot connect %s::%s to %s::%s",
                 sender ? sender->metaObject()->className() : "(nullptr)",
                 (signal && *signal) ? signal+1 : "(nullptr)",
                 receiver ? receiver->metaObject()->className() : "(nullptr)",
                 (method && *method) ? method+1 : "(nullptr)");
        return QMetaObject::Connection(0);
    }
    QByteArray tmp_signal_name;
//验证信号宏指令（SIGNAL）的正确性
    if (!check_signal_macro(sender, signal, "connect", "bind"))
        return QMetaObject::Connection(0);
    const QMetaObject *smeta = sender->metaObject();
    const char *signal_arg = signal;
    ++signal; //skip code
    QArgumentTypeArray signalTypes;
    Q_ASSERT(QMetaObjectPrivate::get(smeta)->revision >= 7);
    QByteArray signalName = QMetaObjectPrivate::decodeMethodSignature(signal, signalTypes);
//获取信号相对索引
    int signal_index = QMetaObjectPrivate::indexOfSignalRelative(
            &smeta, signalName, signalTypes.size(), signalTypes.constData());
    if (signal_index < 0) {
        // check for normalized signatures
        tmp_signal_name = QMetaObject::normalizedSignature(signal - 1);
        signal = tmp_signal_name.constData() + 1;

        signalTypes.clear();
        signalName = QMetaObjectPrivate::decodeMethodSignature(signal, signalTypes);
        smeta = sender->metaObject();
        signal_index = QMetaObjectPrivate::indexOfSignalRelative(
                &smeta, signalName, signalTypes.size(), signalTypes.constData());
    }
    if (signal_index < 0) {
        err_method_notfound(sender, signal_arg, "connect");
        err_info_about_objects("connect", sender, receiver);
        return QMetaObject::Connection(0);
    }
    signal_index = QMetaObjectPrivate::originalClone(smeta, signal_index);
//加上偏移量得到信号全局索引值（绝对索引）
    signal_index += QMetaObjectPrivate::signalOffset(smeta);

    QByteArray tmp_method_name;
    int membcode = extract_code(method);
//检查槽函数代码是否有效
    if (!check_method_code(membcode, receiver, method, "connect"))
        return QMetaObject::Connection(0);
    const char *method_arg = method;
    ++method; // skip code

    QArgumentTypeArray methodTypes;
    QByteArray methodName = QMetaObjectPrivate::decodeMethodSignature(method, methodTypes);
    const QMetaObject *rmeta = receiver->metaObject();
    int method_index_relative = -1;
    Q_ASSERT(QMetaObjectPrivate::get(rmeta)->revision >= 7);
//获取槽函数（或信号，信号连接信号时）相对索引
    switch (membcode) {
    case QSLOT_CODE:
        method_index_relative = QMetaObjectPrivate::indexOfSlotRelative(
                &rmeta, methodName, methodTypes.size(), methodTypes.constData());
        break;
    case QSIGNAL_CODE:
        method_index_relative = QMetaObjectPrivate::indexOfSignalRelative(
                &rmeta, methodName, methodTypes.size(), methodTypes.constData());
        break;
    }
    if (method_index_relative < 0) {
        // check for normalized methods
        tmp_method_name = QMetaObject::normalizedSignature(method);
        method = tmp_method_name.constData();

        methodTypes.clear();
        methodName = QMetaObjectPrivate::decodeMethodSignature(method, methodTypes);
        // rmeta may have been modified above
        rmeta = receiver->metaObject();
        switch (membcode) {
        case QSLOT_CODE:
            method_index_relative = QMetaObjectPrivate::indexOfSlotRelative(
                    &rmeta, methodName, methodTypes.size(), methodTypes.constData());
            break;
        case QSIGNAL_CODE:
            method_index_relative = QMetaObjectPrivate::indexOfSignalRelative(
                    &rmeta, methodName, methodTypes.size(), methodTypes.constData());
            break;
        }
    }

    if (method_index_relative < 0) {
        err_method_notfound(receiver, method_arg, "connect");
        err_info_about_objects("connect", sender, receiver);
        return QMetaObject::Connection(0);
    }
//检查连接信号与槽时的参数是否匹配。
    if (!QMetaObjectPrivate::checkConnectArgs(signalTypes.size(), signalTypes.constData(),
                                              methodTypes.size(), methodTypes.constData())) {
        qWarning("QObject::connect: Incompatible sender/receiver arguments"
                 "\n        %s::%s --> %s::%s",
                 sender->metaObject()->className(), signal,
                 receiver->metaObject()->className(), method);
        return QMetaObject::Connection(0);
    }

    int *types = 0;
//检查连接方式
    if ((type == Qt::QueuedConnection)
            && !(types = queuedConnectionTypes(signalTypes.constData(), signalTypes.size()))) {
        return QMetaObject::Connection(0);
    }

#ifndef QT_NO_DEBUG
    QMetaMethod smethod = QMetaObjectPrivate::signal(smeta, signal_index);
    QMetaMethod rmethod = rmeta->method(method_index_relative + rmeta->methodOffset());
    check_and_warn_compat(smeta, smethod, rmeta, rmethod);
#endif
//调用QMetaObjectPrivate::connect
    QMetaObject::Connection handle = QMetaObject::Connection(QMetaObjectPrivate::connect(
        sender, signal_index, smeta, receiver, method_index_relative, rmeta ,type, types));
    return handle;
}

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)
{
//Const_cast 变成可写
    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;
//确保在多线程环境下，信号和槽的连接操作是线程安全的
    QOrderedMutexLocker locker(signalSlotLock(sender),
                               signalSlotLock(receiver));
//type为Qt::UniqueConnection方式时排重
    QObjectPrivate::ConnectionData *scd  = QObjectPrivate::get(s)->connections.loadRelaxed();
    if (type & Qt::UniqueConnection && scd) {
        if (scd->signalVectorCount() > signal_index) {
            //一个信号对应一个signalVector中的元素，是connectionlist类，所以一个信号对应一个connectionlist
            const QObjectPrivate::Connection *c2 = scd->signalVector.loadRelaxed()->at(signal_index).first.loadRelaxed();

            int method_index_absolute = method_index + method_offset;

            while (c2) {
                //已经存在就不建立连接
                if (!c2->isSlotObject && c2->receiver.loadRelaxed() == receiver && c2->method() == method_index_absolute)
                    return nullptr;
                c2 = c2->nextConnectionList.loadRelaxed();
            }
        }
        type &= Qt::UniqueConnection - 1;
    }
//创建Connection类c，填充数据，一个connection对象对应一个connect连接
    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;
//将创建的connection存储到connectionLists（多个connection的qvector），get返回发送者的私有类
    QObjectPrivate::get(s)->addConnection(signal_index, c.get());

    locker.unlock();
//通知发送者，它的信号已经被连接到一个槽函数上
    QMetaMethod smethod = QMetaObjectPrivate::signal(smeta, signal_index);
    if (smethod.isValid())
        s->connectNotify(smethod);

    return c.release();
}

ConnectionList与信号一 一对应
ConnectionList的每一个节点Conntion与connect()一 一对应
ConnectList是Qt中用于管理信号和槽连接的数据结构，它跟踪连接信息并在信号发射时找到对应的槽函数进行调用。
ConnectList中的每个条目Conntion都表示一个连接，包含信号发送者、信号索引、槽接收者和槽索引等信息。

2. 函数指针链接(qt5后)

a.编译时检查，拼写错误编译不通过
b.使用函数指针作为槽函数参数
c.不支持重载函数
d.可以调用普通函数
e.默认autoConnection，不可手动设置

 //connect to a function pointer  (not a member)
    template <typename Func1, typename Func2>
    static inline typename std::enable_if<int(QtPrivate::FunctionPointer<Func2>::ArgumentCount) >= 0 &&
                                          !QtPrivate::FunctionPointer<Func2>::IsPointerToMemberFunction, 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;
        typedef QtPrivate::FunctionPointer<Func2> SlotType;
//"检查函数数据-没有Q_OBJECT宏，信号参数多余槽参数，参数不兼容，返回类型不兼容时编译报错"
        Q_STATIC_ASSERT_X(QtPrivate::HasQ_OBJECT_Macro<typename SignalType::Object>::Value,
                          "No Q_OBJECT in the class with the signal");

        //compilation error if the arguments does not match.
        Q_STATIC_ASSERT_X(int(SignalType::ArgumentCount) >= int(SlotType::ArgumentCount),
                          "The slot requires more arguments than the signal provides.");
        Q_STATIC_ASSERT_X((QtPrivate::CheckCompatibleArguments<typename SignalType::Arguments, typename SlotType::Arguments>::value),
                          "Signal and slot arguments are not compatible.");
        Q_STATIC_ASSERT_X((QtPrivate::AreArgumentsCompatible<typename SlotType::ReturnType, typename SignalType::ReturnType>::value),
                          "Return type of the slot is not compatible with the return type of the signal.");
//是否是队列链接或阻塞队列链接
        const int *types = nullptr;
        if (type == Qt::QueuedConnection || type == Qt::BlockingQueuedConnection)
            types = QtPrivate::ConnectionTypes<typename SignalType::Arguments>::types();
//调用QObject::connnectlmpl()
        return connectImpl(sender, reinterpret_cast<void **>(&signal), context, nullptr,
                           new QtPrivate::QStaticSlotObject<Func2,
                                                 typename QtPrivate::List_Left<typename SignalType::Arguments, SlotType::ArgumentCount>::Value,
                                                 typename SignalType::ReturnType>(slot),
                           type, types, &SignalType::Object::staticMetaObject);
    }

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)
{
//检查信号和槽的参数列表
    if (!signal) {
        qWarning("QObject::connect: invalid nullptr parameter");
        if (slotObj)
            slotObj->destroyIfLastRef();
        return QMetaObject::Connection();
    }
//检查信号索引有效性
    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;
    }
//检查发送者元对象
    if (!senderMetaObject) {
        qWarning("QObject::connect: signal not found in %s", sender->metaObject()->className());
        slotObj->destroyIfLastRef();
        return QMetaObject::Connection(0);
    }
//信号绝对索引
    signal_index += QMetaObjectPrivate::signalOffset(senderMetaObject);
//调用QObjectPrivate::connectLmpl（）
    return QObjectPrivate::connectImpl(sender, signal_index, receiver, slot, slotObj, type, types, senderMetaObject);
}

QObjectPrivate::connectLmpl和QMetaObjectPrivate::connect类似，创建connection对象，加入到connectionlist中

3. Lamba表达式

a.编译时检查
b.不支持信号重载，支持槽函数重载
c.前二者的融合体
d.可以调用普通函数
e.默认directConnection，不可手动设置

QObject::connect()的最后一个参数将指定连接类型：
Qt::DirectConnection：直接连接意味着槽函数将在信号发出的线程直接调用
Qt::QueuedConnection：队列连接意味着向接受者所在线程发送一个事件，该线程的事件循环将获得这个事件，然后之后的某个时刻调用槽函数
Qt::BlockingQueuedConnection：阻塞的队列连接就像队列连接，但是发送者线程将会阻塞，直到接受者所在线程的事件循环获得这个事件，槽函数被调用之后，函数才会返回
Qt::AutoConnection：自动连接（默认）意味着如果接受者所在线程就是当前线程，则使用直接连接；否则将使用队列连接

四、信号槽触发：

在定义信号后，MOC编译器会在moc文件中定义该信号的信号函数。

// SIGNAL 0
void QDBusAdaptorConnector::relaySignal(QObject * _t1, const QMetaObject * _t2, int _t3, const QVariantList & _t4)
{
//----------_a是返回值和参数--------------------
    void *_a[] = { 0, const_cast<void*>(reinterpret_cast<const void*>(&_t1)), const_cast<void*>(reinterpret_cast<const void*>(&_t2)), const_cast<void*>(reinterpret_cast<const void*>(&_t3)), const_cast<void*>(reinterpret_cast<const void*>(&_t4)) };
    QMetaObject::activate(this, &staticMetaObject, 0, _a);
}

当在程序中发出一个信号，实际上是通过调用这个信号函数来触发的。在这个函数中调用了activate()

void QMetaObject::activate(QObject *sender, const QMetaObject *m, int local_signal_index,
                           void **argv)
{
	//-------------由相对索引+偏移量得到绝对索引-------------------------
    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);
}

在得到绝对索引后，调用doActivate().

template <bool callbacks_enabled>
void doActivate(QObject *sender, int signal_index, void **argv)
{
    QObjectPrivate *sp = QObjectPrivate::get(sender);
//------------判断是否为阻塞状态--------------------------
    if (sp->blockSig)
        return;

    Q_TRACE_SCOPE(QMetaObject_activate, sender, signal_index);
//----------------检查是否存在信号槽链接--------------------------------
    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);

//-------获取信号所在对象中该信号对应的connection容器connectList------------------
    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);
//-----------------------------------获取线程id-------------------------------
    Qt::HANDLE currentThreadId = QThread::currentThreadId();
    //获取当前线程，并判断是否是发送者的线程
    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.
//-------------遍历connectionlist，依次触发该信号所有链接的槽函数-----------------
    uint highestConnectionId = connections->currentConnectionId.loadRelaxed();
    do {
        QObjectPrivate::Connection *c = list->first.loadRelaxed();
        if (!c)
            continue;

        do {
        	//获取接收者线程数据
            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
                //信号所在线程和发送者不在一个线程，在获取connection中的接收者线程时上锁
                QMutexLocker lock(signalSlotLock(receiver));
                receiverInSameThread = currentThreadId == td->threadId.loadRelaxed();
            }

//--------------如果是多线程连接，即队列和阻塞队列连接-------------------------
            // determine if this connection should be sent immediately or
            // put into the event queue
            //如果是自动连接而且发送线程和接收者线程不一样，或者是队列链接，使用queued_activated调用槽函数
            if ((c->connectionType == Qt::AutoConnection && !receiverInSameThread)
                || (c->connectionType == Qt::QueuedConnection)) {
                queued_activate(sender, signal_index, c, argv);
                continue;
#if QT_CONFIG(thread)
			//如果是阻塞队列连接
            } else if (c->connectionType == Qt::BlockingQueuedConnection) {
            	//当发送的线程和接收者在同一个线程，警告：当激活一个阻塞队列连接时检测到死锁
                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);
                }
                //postEvent发送一个qmetacallevent事件
                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
            }
//-----------------------如果是单线程连接-------------------------------------
            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);
    }
}