Qt MetaObject System详解之四：meta call

所谓meta call就是通过object的meta system的支持来动态调用object的方法，metacall也是signal&slot的机制的基石。本篇通过参考源代码来探究meta call的实现方法。

QMetaObject::invokeMethod():

bool ( QObject * obj , const char * member , Qt::ConnectionType type , QGenericReturnArgument ret , QGenericArgument val0 = QGenericArgument( 0 ), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument() )

QMetaObject这个静态方法可以动态地调用obj对象名字为member的方法，type参数表明该调用时同步的还是异步的。ret是一个 通用的用来存储返回值的类型，后面的9个参数是用来传递调用参数的，QGenericArgument()是一种通用的存储参数值的类型。（这里让人感觉 比较奇怪的是Qt为什么不将这个参数列表弄成某种动态的形式，而是最多九个）

所调用的方法必须是invocable的，也就是signal，slot或者是加了声明为Q_INVOCABLE的其他方法。

这个方法的实现如下：

if (!obj)
        return false;
    QVarLengthArray<char, 512> sig;
    int len = qstrlen(member);
    if (len <= 0)
        return false;
    sig.append(member, len);
    sig.append('(');
    const char *typeNames[] = {ret.name(), val0.name(), val1.name(), val2.name(), val3.name(),
                               val4.name(), val5.name(), val6.name(), val7.name(), val8.name(),
                               val9.name()};
    int paramCount;
    for (paramCount = 1; paramCount < MaximumParamCount; ++paramCount) {
        len = qstrlen(typeNames[paramCount]);
        if (len <= 0)
            break;
        sig.append(typeNames[paramCount], len);
        sig.append(',');
    }
    if (paramCount == 1)
        sig.append(')'); // no parameters
    else
        sig[sig.size() - 1] = ')';
    sig.append('\0');
    int idx = obj->metaObject()->indexOfMethod(sig.constData());
    if (idx < 0) {
        QByteArray norm = QMetaObject::normalizedSignature(sig.constData());
        idx = obj->metaObject()->indexOfMethod(norm.constData());
    }
    if (idx < 0 || idx >= obj->metaObject()->methodCount())
        return false;
    QMetaMethod method = obj->metaObject()->method(idx);
    return method.invoke(obj, type, ret,
                         val0, val1, val2, val3, val4, val5, val6, val7, val8, val9);
}

if (!obj) return false; QVarLengthArray<char, 512> sig; int len = qstrlen(member); if (len <= 0) return false; sig.append(member, len); sig.append('('); const char *typeNames[] = {ret.name(), val0.name(), val1.name(), val2.name(), val3.name(), val4.name(), val5.name(), val6.name(), val7.name(), val8.name(), val9.name()}; int paramCount; for (paramCount = 1; paramCount < MaximumParamCount; ++paramCount) { len = qstrlen(typeNames[paramCount]); if (len <= 0) break; sig.append(typeNames[paramCount], len); sig.append(','); } if (paramCount == 1) sig.append(')'); // no parameters else sig[sig.size() - 1] = ')'; sig.append('\0'); int idx = obj->metaObject()->indexOfMethod(sig.constData()); if (idx < 0) { QByteArray norm = QMetaObject::normalizedSignature(sig.constData()); idx = obj->metaObject()->indexOfMethod(norm.constData()); } if (idx < 0 || idx >= obj->metaObject()->methodCount()) return false; QMetaMethod method = obj->metaObject()->method(idx); return method.invoke(obj, type, ret, val0, val1, val2, val3, val4, val5, val6, val7, val8, val9); }

先依据传递的方法名称和参数，构造完整的函数签名（存储在局部变量sig）。参数的类型名就是调用时传递时的参数静态类型，这里可不会有什么类型转换，这是运行时的行为，参数类型转换是编译时的行为。

然后通过这个sig签名在obj中去查找该方法，查询的结果就是一个QMetaMethod值，再将调用委托给QMetaMethod：：invoke方法。

bool QMetaMethod::invoke(QObject *object,
                         Qt::ConnectionType connectionType,
                         QGenericReturnArgument returnValue,
                         QGenericArgument val0,
                         QGenericArgument val1,
                         QGenericArgument val2,
                         QGenericArgument val3,
                         QGenericArgument val4,
                         QGenericArgument val5,
                         QGenericArgument val6,
                         QGenericArgument val7,
                         QGenericArgument val8,
                         QGenericArgument val9) const
{
    if (!object || !mobj)
        return false;
    // check return type
    if (returnValue.data()) {
        const char *retType = typeName();
        if (qstrcmp(returnValue.name(), retType) != 0) {
            // normalize the return value as well
            // the trick here is to make a function signature out of the return type
            // so that we can call normalizedSignature() and avoid duplicating code
            QByteArray unnormalized;
            int len = qstrlen(returnValue.name());
            unnormalized.reserve(len + 3);
            unnormalized = "_(";        // the function is called "_"
            unnormalized.append(returnValue.name());
            unnormalized.append(')');
            QByteArray normalized = QMetaObject::normalizedSignature(unnormalized.constData());
            normalized.truncate(normalized.length() - 1); // drop the ending ')'
            if (qstrcmp(normalized.constData() + 2, retType) != 0)
                return false;
        }
    }
    // check argument count (we don't allow invoking a method if given too few arguments)
    const char *typeNames[] = {
        returnValue.name(),
        val0.name(),
        val1.name(),
        val2.name(),
        val3.name(),
        val4.name(),
        val5.name(),
        val6.name(),
        val7.name(),
        val8.name(),
        val9.name()
    };
    int paramCount;
    for (paramCount = 1; paramCount < MaximumParamCount; ++paramCount) {
        if (qstrlen(typeNames[paramCount]) <= 0)
            break;
    }
    int metaMethodArgumentCount = 0;
    {
        // based on QMetaObject::parameterNames()
        const char *names = mobj->d.stringdata + mobj->d.data[handle + 1];
        if (*names == 0) {
            // do we have one or zero arguments?
            const char *signature = mobj->d.stringdata + mobj->d.data[handle];
            while (*signature && *signature != '(')
                ++signature;
            if (*++signature != ')')
                ++metaMethodArgumentCount;
        } else {
            --names;
            do {
                ++names;
                while (*names && *names != ',')
                    ++names;
                ++metaMethodArgumentCount;
            } while (*names);
        }
    }
    if (paramCount <= metaMethodArgumentCount)
        return false;
    // check connection type
    QThread *currentThread = QThread::currentThread();
    QThread *objectThread = object->thread();
    if (connectionType == Qt::AutoConnection) {
        connectionType = currentThread == objectThread
                         ? Qt::DirectConnection
                         : Qt::QueuedConnection;
    }
    // invoke!
    void *param[] = {
        returnValue.data(),
        val0.data(),
        val1.data(),
        val2.data(),
        val3.data(),
        val4.data(),
        val5.data(),
        val6.data(),
        val7.data(),
        val8.data(),
        val9.data()
    };
    // recompute the methodIndex by reversing the arithmetic in QMetaObject::property()
    int methodIndex = ((handle - priv(mobj->d.data)->methodData) / 5) + mobj->methodOffset();
    if (connectionType == Qt::DirectConnection) {
        return QMetaObject::metacall(object, QMetaObject::InvokeMetaMethod, methodIndex, param) < 0;
    } else {
        if (returnValue.data()) {
            qWarning("QMetaMethod::invoke: Unable to invoke methods with return values in "
                     "queued connections");
            return false;
        }
        int nargs = 1; // include return type
        void **args = (void **) qMalloc(paramCount * sizeof(void *));
        Q_CHECK_PTR(args);
        int *types = (int *) qMalloc(paramCount * sizeof(int));
        Q_CHECK_PTR(types);
        types[0] = 0; // return type
        args[0] = 0;
        for (int i = 1; i < paramCount; ++i) {
            types[i] = QMetaType::type(typeNames[i]);
            if (types[i]) {
                args[i] = QMetaType::construct(types[i], param[i]);
                ++nargs;
            } else if (param[i]) {
                qWarning("QMetaMethod::invoke: Unable to handle unregistered datatype '%s'",
                         typeNames[i]);
                for (int x = 1; x < i; ++x) {
                    if (types[x] && args[x])
                        QMetaType::destroy(types[x], args[x]);
                }
                qFree(types);
                qFree(args);
                return false;
            }
        }
        if (connectionType == Qt::QueuedConnection) {
            QCoreApplication::postEvent(object, new QMetaCallEvent(methodIndex,
                                                                   0,
                                                                   -1,
                                                                   nargs,
                                                                   types,
                                                                   args));
        } else {
            if (currentThread == objectThread) {
                qWarning("QMetaMethod::invoke: Dead lock detected in "
                         "BlockingQueuedConnection: Receiver is %s(%p)",
                         mobj->className(), object);
            }
            // blocking queued connection
#ifdef QT_NO_THREAD
            QCoreApplication::postEvent(object, new QMetaCallEvent(methodIndex,
                                                                   0,
                                                                   -1,
                                                                   nargs,
                                                                   types,
                                                                   args));
#else
            QSemaphore semaphore;
            QCoreApplication::postEvent(object, new QMetaCallEvent(methodIndex,
                                                                   0,
                                                                   -1,
                                                                   nargs,
                                                                   types,
                                                                   args,
                                                                   &semaphore));
            semaphore.acquire();
#endif // QT_NO_THREAD
        }
    }
    return true;
}

bool QMetaMethod::invoke(QObject *object, Qt::ConnectionType connectionType, QGenericReturnArgument returnValue, QGenericArgument val0, QGenericArgument val1, QGenericArgument val2, QGenericArgument val3, QGenericArgument val4, QGenericArgument val5, QGenericArgument val6, QGenericArgument val7, QGenericArgument val8, QGenericArgument val9) const { if (!object || !mobj) return false; // check return type if (returnValue.data()) { const char *retType = typeName(); if (qstrcmp(returnValue.name(), retType) != 0) { // normalize the return value as well // the trick here is to make a function signature out of the return type // so that we can call normalizedSignature() and avoid duplicating code QByteArray unnormalized; int len = qstrlen(returnValue.name()); unnormalized.reserve(len + 3); unnormalized = "_("; // the function is called "_" unnormalized.append(returnValue.name()); unnormalized.append(')'); QByteArray normalized = QMetaObject::normalizedSignature(unnormalized.constData()); normalized.truncate(normalized.length() - 1); // drop the ending ')' if (qstrcmp(normalized.constData() + 2, retType) != 0) return false; } } // check argument count (we don't allow invoking a method if given too few arguments) const char *typeNames[] = { returnValue.name(), val0.name(), val1.name(), val2.name(), val3.name(), val4.name(), val5.name(), val6.name(), val7.name(), val8.name(), val9.name() }; int paramCount; for (paramCount = 1; paramCount < MaximumParamCount; ++paramCount) { if (qstrlen(typeNames[paramCount]) <= 0) break; } int metaMethodArgumentCount = 0; { // based on QMetaObject::parameterNames() const char *names = mobj->d.stringdata + mobj->d.data[handle + 1]; if (*names == 0) { // do we have one or zero arguments? const char *signature = mobj->d.stringdata + mobj->d.data[handle]; while (*signature && *signature != '(') ++signature; if (*++signature != ')') ++metaMethodArgumentCount; } else { --names; do { ++names; while (*names && *names != ',') ++names; ++metaMethodArgumentCount; } while (*names); } } if (paramCount <= metaMethodArgumentCount) return false; // check connection type QThread *currentThread = QThread::currentThread(); QThread *objectThread = object->thread(); if (connectionType == Qt::AutoConnection) { connectionType = currentThread == objectThread ? Qt::DirectConnection : Qt::QueuedConnection; } // invoke! void *param[] = { returnValue.data(), val0.data(), val1.data(), val2.data(), val3.data(), val4.data(), val5.data(), val6.data(), val7.data(), val8.data(), val9.data() }; // recompute the methodIndex by reversing the arithmetic in QMetaObject::property() int methodIndex = ((handle - priv(mobj->d.data)->methodData) / 5) + mobj->methodOffset(); if (connectionType == Qt::DirectConnection) { return QMetaObject::metacall(object, QMetaObject::InvokeMetaMethod, methodIndex, param) < 0; } else { if (returnValue.data()) { qWarning("QMetaMethod::invoke: Unable to invoke methods with return values in " "queued connections"); return false; } int nargs = 1; // include return type void **args = (void **) qMalloc(paramCount * sizeof(void *)); Q_CHECK_PTR(args); int *types = (int *) qMalloc(paramCount * sizeof(int)); Q_CHECK_PTR(types); types[0] = 0; // return type args[0] = 0; for (int i = 1; i < paramCount; ++i) { types[i] = QMetaType::type(typeNames[i]); if (types[i]) { args[i] = QMetaType::construct(types[i], param[i]); ++nargs; } else if (param[i]) { qWarning("QMetaMethod::invoke: Unable to handle unregistered datatype '%s'", typeNames[i]); for (int x = 1; x < i; ++x) { if (types[x] && args[x]) QMetaType::destroy(types[x], args[x]); } qFree(types); qFree(args); return false; } } if (connectionType == Qt::QueuedConnection) { QCoreApplication::postEvent(object, new QMetaCallEvent(methodIndex, 0, -1, nargs, types, args)); } else { if (currentThread == objectThread) { qWarning("QMetaMethod::invoke: Dead lock detected in " "BlockingQueuedConnection: Receiver is %s(%p)", mobj->className(), object); } // blocking queued connection #ifdef QT_NO_THREAD QCoreApplication::postEvent(object, new QMetaCallEvent(methodIndex, 0, -1, nargs, types, args)); #else QSemaphore semaphore; QCoreApplication::postEvent(object, new QMetaCallEvent(methodIndex, 0, -1, nargs, types, args, &semaphore)); semaphore.acquire(); #endif // QT_NO_THREAD } } return true; }

代码首先检查返回值的类型是否正确；再检查参数的个数是否匹配，看懂这段代码需要参考该系列之二对moc文件的解析；再依据当前线程和被调对象所属 线程来调整connnection type；如果是directconnection，直接调用 QMetaObject::metacall(object, QMetaObject::InvokeMetaMethod, methodIndex, param)，param是将所有参数值指针排列组成的指针数组。如果不是directconnection，也即异步调用，就通过一个post一个 QMetaCallEvent到obj，此时须将所有的参数复制一份存入event对象。

QMetaObject::metacall的实现如下：

/*!
    \internal
*/
int QMetaObject::metacall(QObject *object, Call cl, int idx, void **argv)
{
    if (QMetaObject *mo = object->d_ptr->metaObject)
        return static_cast(mo)->metaCall(cl, idx, argv);
    else
        return object->qt_metacall(cl, idx, argv);
}

/*! \internal */ int QMetaObject::metacall(QObject *object, Call cl, int idx, void **argv) { if (QMetaObject *mo = object->d_ptr->metaObject) return static_cast<QAbstractDynamicMetaObject*>(mo)->metaCall(cl, idx, argv); else return object->qt_metacall(cl, idx, argv); } 

如果object->d_ptr->metaObject（QMetaObjectPrivate）存在，通过该metaobject 来调用，这里要参考该系列之三对QMetaObjectPrivate的介绍，这个条件实际上就是object就是QObject类型，而不是派生类型。 否则调用object：：qt_metacall。

对于异步调用,QObject的event函数里有如下代码：

    case QEvent::MetaCall:
        {
            d_func()->inEventHandler = false;
            QMetaCallEvent *mce = static_cast(e);
            QObjectPrivate::Sender currentSender;
            currentSender.sender = const_cast(mce->sender());
            currentSender.signal = mce->signalId();
            currentSender.ref = 1;
            QObjectPrivate::Sender * const previousSender =
                QObjectPrivate::setCurrentSender(this, ¤tSender);
#if defined(QT_NO_EXCEPTIONS)
            mce->placeMetaCall(this);
#else
            QT_TRY {
                mce->placeMetaCall(this);
            } QT_CATCH(...) {
                QObjectPrivate::resetCurrentSender(this, ¤tSender, previousSender);
                QT_RETHROW;
            }
#endif
            QObjectPrivate::resetCurrentSender(this, ¤tSender, previousSender);
            break;
        }

case QEvent::MetaCall: { d_func()->inEventHandler = false; QMetaCallEvent *mce = static_cast<QMetaCallEvent*>(e); QObjectPrivate::Sender currentSender; currentSender.sender = const_cast<QObject*>(mce->sender()); currentSender.signal = mce->signalId(); currentSender.ref = 1; QObjectPrivate::Sender * const previousSender = QObjectPrivate::setCurrentSender(this, ¤tSender); #if defined(QT_NO_EXCEPTIONS) mce->placeMetaCall(this); #else QT_TRY { mce->placeMetaCall(this); } QT_CATCH(...) { QObjectPrivate::resetCurrentSender(this, ¤tSender, previousSender); QT_RETHROW; } #endif QObjectPrivate::resetCurrentSender(this, ¤tSender, previousSender); break; }

QMetaCallEvent的代码很简单：

int QMetaCallEvent::placeMetaCall(QObject *object)
{    return QMetaObject::metacall(object, QMetaObject::InvokeMetaMethod, id_, args_);}

殊途同归。

最后来看一下object->qt_metacall是如何实现的，这又回到了该系统之二所提供的示例moc文件中去了。该文件提供了该方法的实现：

# int TestObject::qt_metacall(QMetaObject::Call _c, int _id, void **_a)
# {
#     _id = QObject::qt_metacall(_c, _id, _a);
#     if (_id < 0)
#         return _id;
#     if (_c == QMetaObject::InvokeMetaMethod) {
#         switch (_id) {
#         case 0: clicked(); break;
#         case 1: pressed(); break;
#         case 2: onEventA((*reinterpret_cast< const QString(*)>(_a[1]))); break;
#         case 3: onEventB((*reinterpret_cast< int(*)>(_a[1]))); break;
#         default: ;
#         }
#         _id -= 4;
#     }
# #ifndef QT_NO_PROPERTIES
#       else if (_c == QMetaObject::ReadProperty) {
#         void *_v = _a[0];
#         switch (_id) {
#         case 0: *reinterpret_cast< QString*>(_v) = getPropertyA(); break;
#         case 1: *reinterpret_cast< QString*>(_v) = getPropertyB(); break;
#         }
#         _id -= 2;
#     } else if (_c == QMetaObject::WriteProperty) {
#         void *_v = _a[0];
#         switch (_id) {
#         case 0: getPropertyA(*reinterpret_cast< QString*>(_v)); break;
#         case 1: getPropertyB(*reinterpret_cast< QString*>(_v)); break;
#         }
#         _id -= 2;
#     } else if (_c == QMetaObject::ResetProperty) {
#         switch (_id) {
#         case 0: resetPropertyA(); break;
#         case 1: resetPropertyB(); break;
#         }
#         _id -= 2;
#     } else if (_c == QMetaObject::QueryPropertyDesignable) {
#         _id -= 2;
#     } else if (_c == QMetaObject::QueryPropertyScriptable) {
#         _id -= 2;
#     } else if (_c == QMetaObject::QueryPropertyStored) {
#         _id -= 2;
#     } else if (_c == QMetaObject::QueryPropertyEditable) {
#         _id -= 2;
#     } else if (_c == QMetaObject::QueryPropertyUser) {
#         _id -= 2;
#     }
# #endif // QT_NO_PROPERTIES
#     return _id;
# }

# int TestObject::qt_metacall(QMetaObject::Call _c, int _id, void **_a) # { # _id = QObject::qt_metacall(_c, _id, _a); # if (_id < 0) # return _id; # if (_c == QMetaObject::InvokeMetaMethod) { # switch (_id) { # case 0: clicked(); break; # case 1: pressed(); break; # case 2: onEventA((*reinterpret_cast< const QString(*)>(_a[1]))); break; # case 3: onEventB((*reinterpret_cast< int(*)>(_a[1]))); break; # default: ; # } # _id -= 4; # } # #ifndef QT_NO_PROPERTIES # else if (_c == QMetaObject::ReadProperty) { # void *_v = _a[0]; # switch (_id) { # case 0: *reinterpret_cast< QString*>(_v) = getPropertyA(); break; # case 1: *reinterpret_cast< QString*>(_v) = getPropertyB(); break; # } # _id -= 2; # } else if (_c == QMetaObject::WriteProperty) { # void *_v = _a[0]; # switch (_id) { # case 0: getPropertyA(*reinterpret_cast< QString*>(_v)); break; # case 1: getPropertyB(*reinterpret_cast< QString*>(_v)); break; # } # _id -= 2; # } else if (_c == QMetaObject::ResetProperty) { # switch (_id) { # case 0: resetPropertyA(); break; # case 1: resetPropertyB(); break; # } # _id -= 2; # } else if (_c == QMetaObject::QueryPropertyDesignable) { # _id -= 2; # } else if (_c == QMetaObject::QueryPropertyScriptable) { # _id -= 2; # } else if (_c == QMetaObject::QueryPropertyStored) { # _id -= 2; # } else if (_c == QMetaObject::QueryPropertyEditable) { # _id -= 2; # } else if (_c == QMetaObject::QueryPropertyUser) { # _id -= 2; # } # #endif // QT_NO_PROPERTIES # return _id; # }

这段代码将调用最终转到我们自己的实现的函数中来。这个函数不经提供了metamethod的动态调用，而且也提供了property的动态操作方法。可想而知，property的动态调用的实现方式一定和invocalbe method是一致的。