Qt 源码之元对象(QMetaObject)

1、 简述：QMetaObject是用来保存Qt元对象的元信息，当继承QObject类并使用宏Q_OBJECT时，创建的类产生一个静态QMetaObject实例staticMetaObject，这个实例会保存类名、信号名称及索引、槽的名字及索引等等在对象操作时需要的基本信息，而这也是信号与槽的根本所在：

#define Q_OBJECT \

public: \

Q_OBJECT_CHECK \

static const QMetaObject staticMetaObject; \

virtual const QMetaObject *metaObject() const; \

virtual void *qt_metacast(const char *); \

QT_TR_FUNCTIONS \

virtual int qt_metacall(QMetaObject::Call, int, void **); \

private: \

Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); \

struct QPrivateSignal {};

2、signals、slots、SIGNAL、SLOT、emit

(1)、Qt基于C++，那么就必须遵循C++的语法规则，否则编译出错；但在信号与槽的类声明中，如此使用：

{

... ...

signals:

void sig(...);

private slots:

void on_slots(....);

... ...

}

通过以上例子看出， signals、slots好像不是C++里的关键字，但在Qt中又是合法的修饰符，到底是什么原因呢？查看Qt源代码：

ifndef QT_NO_SIGNALS_SLOTS_KEYWORDS

# define slots

# define signals public

# endif

现在看来，他们都是地道的宏定义，只是包装了C++控制修饰符：public、private、protected，但是细看会发现信号void sig(...)未实现，而我们使用的语句 emit sig(...);何解？ 再看：

# ifndef QT_NO_EMIT

# define emit

# endif

完美的一个空宏，也造就了完美的普通函数调用，关于这个普通的函数没有具体实现的原因，得从moc(meta object compiler)说起，通过查看moc_BasePage.cpp如下：

// SIGNAL 0

void BasePage::readyRead(quint16 _t1, char * _t2, quint16 _t3)

{

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)) };

QMetaObject::activate(this, &staticMetaObject, 0, _a);

}

由此得知，“信号函数”不需要开发人员实现，moc会自动实现“信号函数”。

(2)、连接信号与槽时：connect(sender, SIGNAL(...), receiver(...), SLOT(...))

粗略一看，大体已经知道了SIGNAL、SLOT是宏定义，其源代码为：

# ifndef QT_NO_KEYWORDS

#  define METHOD(a)   "0"#a

# endif

# define SLOT(a)     "1"#a

# define SIGNAL(a)   "2"#a

#endif

what it means？主要是使用了C语言的字节拼接符"#"把信号名称转换成了以2开头的字符串，Qt发送信号时才能在元数据中查找到与之对应的信号。

3、MOC(Meta-Object Compiler)，以下是ContactsPage类编译后生成的moc源代码：#include "../../untitled/metadatatest.h"#include <QtCore/qbytearray.h>#include <QtCore/qmetatype.h>#if !defined(Q_MOC_OUTPUT_REVISION)#error "The header file 'metadatatest.h' doesn't include <QObject>."#elif Q_MOC_OUTPUT_REVISION != 67#error "This file was generated using the moc from 5.3.2. It"#error "cannot be used with the include files from this version of Qt."#error "(The moc has changed too much.)"#endifQT_BEGIN_MOC_NAMESPACEstruct qt_meta_stringdata_MetaDataTest_t {QByteArrayData data[6];char stringdata[51];};//存储信号、槽、类名称及相关索引、偏移的结构体#define QT_MOC_LITERAL(idx, ofs, len) \Q_STATIC_BYTE_ARRAY_DATA_HEADER_INITIALIZER_WITH_OFFSET(len, \qptrdiff(offsetof(qt_meta_stringdata_MetaDataTest_t, stringdata) + ofs \- idx * sizeof(QByteArrayData)) \)static const qt_meta_stringdata_MetaDataTest_t qt_meta_stringdata_MetaDataTest = {{QT_MOC_LITERAL(0, 0, 12),QT_MOC_LITERAL(1, 13, 8),QT_MOC_LITERAL(2, 22, 0),QT_MOC_LITERAL(3, 23, 7),QT_MOC_LITERAL(4, 31, 8),QT_MOC_LITERAL(5, 40, 10)},"MetaDataTest\0msgPrint\0\0sendMsg\0printMsg\0""receiveMsg"}; //moc查看头文件，根据signals、slots提取信号与槽的名称与索引保存到qt_meta_stringdata_MetaDataTest 中#undef QT_MOC_LITERALstatic const uint qt_meta_data_MetaDataTest[] = {// content:7, // revision0, // classname0, 0, // classinfo2, 14, // methods0, 0, // properties0, 0, // enums/sets0, 0, // constructors0, // flags1, // signalCount// signals: name, argc, parameters, tag, flags1, 1, 24, 2, 0x06 /* Public */,// slots: name, argc, parameters, tag, flags4, 1, 27, 2, 0x08 /* Private */,// signals: parametersQMetaType::Void, QMetaType::QString, 3,// slots: parametersQMetaType::Void, QMetaType::QString, 5,0 // eod}; //建立类构造、方法、属性等的元数据，保存到类全局数组qt_meta_data_MetaDataTest中void MetaDataTest::qt_static_metacall(QObject *_o, QMetaObject::Call _c, int _id, void **_a){if (_c == QMetaObject::InvokeMetaMethod) {MetaDataTest *_t = static_cast<MetaDataTest *>(_o);switch (_id) {case 0: _t->msgPrint((*reinterpret_cast< QString(*)>(_a[1]))); break;case 1: _t->printMsg((*reinterpret_cast< QString(*)>(_a[1]))); break;default: ;}} else if (_c == QMetaObject::IndexOfMethod) {int *result = reinterpret_cast<int *>(_a[0]);void **func = reinterpret_cast<void **>(_a[1]);{typedef void (MetaDataTest::*_t)(QString );if (*reinterpret_cast<_t *>(func) == static_cast<_t>(&MetaDataTest::msgPrint)) {*result = 0;}}}} //Param1:MetaDataTest实例指针；Param2:操作类型(枚举)；Param3：信号或槽索引；Param4:不知道干嘛const QMetaObject MetaDataTest::staticMetaObject = {{ &QObject::staticMetaObject, qt_meta_stringdata_MetaDataTest.data,qt_meta_data_MetaDataTest, qt_static_metacall, 0, 0}}; //关联qt_meta_stringdata_MetaDataTest、qt_meta_data_MetaDataTest元数据到staticMetaObjectconst QMetaObject *MetaDataTest::metaObject() const{return QObject::d_ptr->metaObject ? QObject::d_ptr->dynamicMetaObject() : &staticMetaObject;}//返回当前对象的元对象，可以根据此元对象获取索引、类名等信息，在基类QObject中调用void *MetaDataTest::qt_metacast(const char *_clname){if (!_clname) return 0;if (!strcmp(_clname, qt_meta_stringdata_MetaDataTest.stringdata))return static_cast<void*>(const_cast< MetaDataTest*>(this));return QObject::qt_metacast(_clname);}//类、信号、槽名称比较int MetaDataTest::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) {if (_id < 2)qt_static_metacall(this, _c, _id, _a);_id -= 2;} else if (_c == QMetaObject::RegisterMethodArgumentMetaType) {if (_id < 2)*reinterpret_cast<int*>(_a[0]) = -1;_id -= 2;}return _id;}//其中id就是信号或槽的索引void MetaDataTest::msgPrint(QString _t1){void *_a[] = { 0, const_cast<void*>(reinterpret_cast<const void*>(&_t1)) };QMetaObject::activate(this, &staticMetaObject, 0, _a);} // SIGNAL 0 （信号索引）QT_END_MOC_NAMESPACE4、void QMetaObject::activate(QObject sender, int signalOffset, int local_signal_index);

(1)、void QMetaObject::activate(QObject *sender, const QMetaObject *m, int local_signal_index,  void **argv){

//当前信号通过m查找到信号的索引和偏移量并触发相应的信号或槽动作

activate(sender, QMetaObjectPrivate::signalOffset(m), local_signal_index, argv);//很长的函数

}//据我理解信号的触发从这里开始

(2)、static QMetaObject::Connection connect(const QObject *sender, const char *signal,

const QObject *receiver, const char *member, Qt::ConnectionType = Qt::AutoConnection);//也是很长的函数

QObject::connect(...)函数会根据# define SLOT(a) "1"#a、 # define SIGNAL(a) "2"#a的第一位判断是信号还是槽，然后调用QMetaObjectPrivate::indexOfSignalRelative()或QMetaObjectPrivate::indexOfSlotRelative(),最后调用模板函数indexOfMethodRelative().

总之, connect函数的作用就是关联相同对象、不同对象之间信号与信号及信号与槽...