Qt MetaObject System详解之五：signal&slot

本篇探析signal slot的连接和调用是如何实现的。

宏SLOT，SIGNAL

在qobjectdefs.h中有这样的定义：

# define METHOD(a)   "0"#a
# define SLOT(a)     "1"#a
# define SIGNAL(a)   "2"#a

不过是在方法签名之前加了一个数字标记。因为我们既可以将signal连接到slot，也可以将signal连接到signal，所有必须要有某种方法区分一下。

QObject::connect()

bool QObject::connect(const QObject *sender, const char *signal,
                      const QObject *receiver, const char *method,
                      Qt::ConnectionType type)
{
    {
        const void *cbdata[] = { sender, signal, receiver, method, &type };
        if (QInternal::activateCallbacks(QInternal::ConnectCallback, (void **) cbdata))
            return true;
    }
#ifndef QT_NO_DEBUG
    bool warnCompat = true;
#endif
    if (type == Qt::AutoCompatConnection) {
        type = Qt::AutoConnection;
#ifndef QT_NO_DEBUG
        warnCompat = false;
#endif
    }
    if (sender == 0 || receiver == 0 || signal == 0 || method == 0) {
        qWarning("QObject::connect: Cannot connect %s::%s to %s::%s",
                 sender ? sender->metaObject()->className() : "(null)",
                 (signal && *signal) ? signal+1 : "(null)",
                 receiver ? receiver->metaObject()->className() : "(null)",
                 (method && *method) ? method+1 : "(null)");
        return false;
    }
    QByteArray tmp_signal_name;
    if (!check_signal_macro(sender, signal, "connect", "bind"))
        return false;
    const QMetaObject *smeta = sender->metaObject();
    const char *signal_arg = signal;
    ++signal; //skip code
    int signal_index = QMetaObjectPrivate::indexOfSignalRelative(&smeta, signal);
    if (signal_index < 0) {
        // check for normalized signatures
        tmp_signal_name = QMetaObject::normalizedSignature(signal - 1);
        signal = tmp_signal_name.constData() + 1;
        smeta = sender->metaObject();
        signal_index = QMetaObjectPrivate::indexOfSignalRelative(&smeta, signal);
        if (signal_index < 0) {
            err_method_notfound(sender, signal_arg, "connect");
            err_info_about_objects("connect", sender, receiver);
            return false;
        }
    }
    signal_index = QMetaObjectPrivate::originalClone(smeta, signal_index);
    int signalOffset, methodOffset;
    computeOffsets(smeta, &signalOffset, &methodOffset);
    int signal_absolute_index = signal_index + methodOffset;
    signal_index += signalOffset;
    QByteArray tmp_method_name;
    int membcode = extract_code(method);
    if (!check_method_code(membcode, receiver, method, "connect"))
        return false;
    const char *method_arg = method;
    ++method; // skip code
    const QMetaObject *rmeta = receiver->metaObject();
    int method_index = -1;
    switch (membcode) {
    case QSLOT_CODE:
        method_index = rmeta->indexOfSlot(method);
        break;
    case QSIGNAL_CODE:
        method_index = rmeta->indexOfSignal(method);
        break;
    }
    if (method_index < 0) {
        // check for normalized methods
        tmp_method_name = QMetaObject::normalizedSignature(method);
        method = tmp_method_name.constData();
        switch (membcode) {
        case QSLOT_CODE:
            method_index = rmeta->indexOfSlot(method);
            break;
        case QSIGNAL_CODE:
            method_index = rmeta->indexOfSignal(method);
            break;
        }
    }
    if (method_index < 0) {
        err_method_notfound(receiver, method_arg, "connect");
        err_info_about_objects("connect", sender, receiver);
        return false;
    }
    if (!QMetaObject::checkConnectArgs(signal, method)) {
        qWarning("QObject::connect: Incompatible sender/receiver arguments"
                 "\n        %s::%s --> %s::%s",
                 sender->metaObject()->className(), signal,
                 receiver->metaObject()->className(), method);
        return false;
    }
    int *types = 0;
    if ((type == Qt::QueuedConnection || type == Qt::BlockingQueuedConnection)
            && !(types = queuedConnectionTypes(smeta->method(signal_absolute_index).parameterTypes())))
        return false;
#ifndef QT_NO_DEBUG
    {
        QMetaMethod smethod = smeta->method(signal_absolute_index);
        QMetaMethod rmethod = rmeta->method(method_index);
        if (warnCompat) {
            if(smethod.attributes() & QMetaMethod::Compatibility) {
                if (!(rmethod.attributes() & QMetaMethod::Compatibility))
                    qWarning("QObject::connect: Connecting from COMPAT signal (%s::%s)", smeta->className(), signal);
            } else if(rmethod.attributes() & QMetaMethod::Compatibility && membcode != QSIGNAL_CODE) {
                qWarning("QObject::connect: Connecting from %s::%s to COMPAT slot (%s::%s)",
                         smeta->className(), signal, rmeta->className(), method);
            }
        }
    }
#endif
    if (!QMetaObjectPrivate::connect(sender, signal_index, receiver, method_index, type, types))
        return false;
    const_cast(sender)->connectNotify(signal - 1);
    return true;
}

bool QObject::connect(const QObject *sender, const char *signal, const QObject *receiver, const char *method, Qt::ConnectionType type) { { const void *cbdata[] = { sender, signal, receiver, method, &type }; if (QInternal::activateCallbacks(QInternal::ConnectCallback, (void **) cbdata)) return true; } #ifndef QT_NO_DEBUG bool warnCompat = true; #endif if (type == Qt::AutoCompatConnection) { type = Qt::AutoConnection; #ifndef QT_NO_DEBUG warnCompat = false; #endif } if (sender == 0 || receiver == 0 || signal == 0 || method == 0) { qWarning("QObject::connect: Cannot connect %s::%s to %s::%s", sender ? sender->metaObject()->className() : "(null)", (signal && *signal) ? signal+1 : "(null)", receiver ? receiver->metaObject()->className() : "(null)", (method && *method) ? method+1 : "(null)"); return false; } QByteArray tmp_signal_name; if (!check_signal_macro(sender, signal, "connect", "bind")) return false; const QMetaObject *smeta = sender->metaObject(); const char *signal_arg = signal; ++signal; //skip code int signal_index = QMetaObjectPrivate::indexOfSignalRelative(&smeta, signal); if (signal_index < 0) { // check for normalized signatures tmp_signal_name = QMetaObject::normalizedSignature(signal - 1); signal = tmp_signal_name.constData() + 1; smeta = sender->metaObject(); signal_index = QMetaObjectPrivate::indexOfSignalRelative(&smeta, signal); if (signal_index < 0) { err_method_notfound(sender, signal_arg, "connect"); err_info_about_objects("connect", sender, receiver); return false; } } signal_index = QMetaObjectPrivate::originalClone(smeta, signal_index); int signalOffset, methodOffset; computeOffsets(smeta, &signalOffset, &methodOffset); int signal_absolute_index = signal_index + methodOffset; signal_index += signalOffset; QByteArray tmp_method_name; int membcode = extract_code(method); if (!check_method_code(membcode, receiver, method, "connect")) return false; const char *method_arg = method; ++method; // skip code const QMetaObject *rmeta = receiver->metaObject(); int method_index = -1; switch (membcode) { case QSLOT_CODE: method_index = rmeta->indexOfSlot(method); break; case QSIGNAL_CODE: method_index = rmeta->indexOfSignal(method); break; } if (method_index < 0) { // check for normalized methods tmp_method_name = QMetaObject::normalizedSignature(method); method = tmp_method_name.constData(); switch (membcode) { case QSLOT_CODE: method_index = rmeta->indexOfSlot(method); break; case QSIGNAL_CODE: method_index = rmeta->indexOfSignal(method); break; } } if (method_index < 0) { err_method_notfound(receiver, method_arg, "connect"); err_info_about_objects("connect", sender, receiver); return false; } if (!QMetaObject::checkConnectArgs(signal, method)) { qWarning("QObject::connect: Incompatible sender/receiver arguments" "\n %s::%s --> %s::%s", sender->metaObject()->className(), signal, receiver->metaObject()->className(), method); return false; } int *types = 0; if ((type == Qt::QueuedConnection || type == Qt::BlockingQueuedConnection) && !(types = queuedConnectionTypes(smeta->method(signal_absolute_index).parameterTypes()))) return false; #ifndef QT_NO_DEBUG { QMetaMethod smethod = smeta->method(signal_absolute_index); QMetaMethod rmethod = rmeta->method(method_index); if (warnCompat) { if(smethod.attributes() & QMetaMethod::Compatibility) { if (!(rmethod.attributes() & QMetaMethod::Compatibility)) qWarning("QObject::connect: Connecting from COMPAT signal (%s::%s)", smeta->className(), signal); } else if(rmethod.attributes() & QMetaMethod::Compatibility && membcode != QSIGNAL_CODE) { qWarning("QObject::connect: Connecting from %s::%s to COMPAT slot (%s::%s)", smeta->className(), signal, rmeta->className(), method); } } } #endif if (!QMetaObjectPrivate::connect(sender, signal_index, receiver, method_index, type, types)) return false; const_cast<QObject*>(sender)->connectNotify(signal - 1); return true; }

忽略细节，只关注主要的流程，这段代码的做的事情就是将signal在sender的meta system中的signal索引找出，以及接受者方法（signal或slot）在receiver的meta system中的索引找出来。在委托QMetaObjectPrivate::connect（）执行连接。

bool QMetaObjectPrivate::connect(const QObject *sender, int signal_index,
                                 const QObject *receiver, int method_index, int type, int *types)
{
    QObject *s = const_cast(sender);
    QObject *r = const_cast(receiver);
    QOrderedMutexLocker locker(signalSlotLock(sender),
                               signalSlotLock(receiver));
    if (type & Qt::UniqueConnection) {
        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->method == method_index)
                    return false;
                c2 = c2->nextConnectionList;
            }
        }
        type &= Qt::UniqueConnection - 1;
    }
    QObjectPrivate::Connection *c = new QObjectPrivate::Connection;
    c->sender = s;
    c->receiver = r;
    c->method = method_index;
    c->connectionType = type;
    c->argumentTypes = types;
    c->nextConnectionList = 0;
    QT_TRY {
        QObjectPrivate::get(s)->addConnection(signal_index, c);
    } QT_CATCH(...) {
        delete c;
        QT_RETHROW;
    }
    c->prev = &(QObjectPrivate::get(r)->senders);
    c->next = *c->prev;
    *c->prev = c;
    if (c->next)
        c->next->prev = &c->next;
    QObjectPrivate *const sender_d = QObjectPrivate::get(s);
    if (signal_index < 0) {
        sender_d->connectedSignals[0] = sender_d->connectedSignals[1] = ~0;
    } else if (signal_index < (int)sizeof(sender_d->connectedSignals) * 8) {
        sender_d->connectedSignals[signal_index >> 5] |= (1 << (signal_index & 0x1f));
    }
    return true;
}

bool QMetaObjectPrivate::connect(const QObject *sender, int signal_index, const QObject *receiver, int method_index, int type, int *types) { QObject *s = const_cast<QObject *>(sender); QObject *r = const_cast<QObject *>(receiver); QOrderedMutexLocker locker(signalSlotLock(sender), signalSlotLock(receiver)); if (type & Qt::UniqueConnection) { 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->method == method_index) return false; c2 = c2->nextConnectionList; } } type &= Qt::UniqueConnection - 1; } QObjectPrivate::Connection *c = new QObjectPrivate::Connection; c->sender = s; c->receiver = r; c->method = method_index; c->connectionType = type; c->argumentTypes = types; c->nextConnectionList = 0; QT_TRY { QObjectPrivate::get(s)->addConnection(signal_index, c); } QT_CATCH(...) { delete c; QT_RETHROW; } c->prev = &(QObjectPrivate::get(r)->senders); c->next = *c->prev; *c->prev = c; if (c->next) c->next->prev = &c->next; QObjectPrivate *const sender_d = QObjectPrivate::get(s); if (signal_index < 0) { sender_d->connectedSignals[0] = sender_d->connectedSignals[1] = ~0; } else if (signal_index < (int)sizeof(sender_d->connectedSignals) * 8) { sender_d->connectedSignals[signal_index >> 5] |= (1 << (signal_index & 0x1f)); } return true; }

同样忽略细节，这段代码首先在connecttype要求UniqueConnection的时候检查一下是不是有重复的连接。然后创建一个 QObjectPrivate::Connection结构，这个结构包含了sender，receiver，接受方法的method_index，然后 加入到某个连接存储表中；连接存储表可能是一种hash结构，signal_index就是key。可以推测，当signal方法被调用时，一定会到这个 结构中查找所有连接到该signal的connection，connection保存了receiver和method index，由上一篇可知，可以很容易调用到receiver的对应method。