Qt信号槽机制源码学习

简述

这里并不全面介绍Qt的信号槽机制的实现，仅以qt-opensource-windows-x86-msvc2015_64-5.6.0的源码为原料，以一个简单的信号槽例子为点，打通一条线。下面的源代码大部分是经过删减和修改的，为了使源码更简单并且增加可读性。

简单的信号槽例子

在vs2015中新建qt控制台项目，然后添加一个头文件和一个源文件。
mian.h内容

#pragma once

#include <QObject>

class Counter :public QObject
{
    Q_OBJECT

public:
    Counter() :m_value(0) {}

    void count()
    {
        ++m_value;
        emit valueChanged(m_value);
    }

    public slots:
    void printValue();

signals:
    void valueChanged(int value);
    void valueReset();

private:
    int m_value;
};

main.cpp内容

#include <QCoreApplication>
#include <iostream>
#include "main.h"

void Counter::printValue()
{
    std::cout << m_value << std::endl;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Counter c;
    QObject::connect(&c, &Counter::valueChanged, &c, &Counter::printValue);
    c.count();

    return a.exec();
}

编译之后qt自动生成moc_main.cpp

#include "../../main.h"
#include <QtCore/qbytearray.h>
#include <QtCore/qmetatype.h>
#if !defined(Q_MOC_OUTPUT_REVISION)
#error "The header file 'main.h' doesn't include <QObject>."
#elif Q_MOC_OUTPUT_REVISION != 67
#error "This file was generated using the moc from 5.6.0. It"
#error "cannot be used with the include files from this version of Qt."
#error "(The moc has changed too much.)"
#endif

QT_BEGIN_MOC_NAMESPACE
struct qt_meta_stringdata_Counter_t {
    QByteArrayData data[6];
    char stringdata0[50];
};
#define QT_MOC_LITERAL(idx, ofs, len) \
    Q_STATIC_BYTE_ARRAY_DATA_HEADER_INITIALIZER_WITH_OFFSET(len, \
    qptrdiff(offsetof(qt_meta_stringdata_Counter_t, stringdata0) + ofs \
        - idx * sizeof(QByteArrayData)) \
    )
static const qt_meta_stringdata_Counter_t qt_meta_stringdata_Counter = {
    {
QT_MOC_LITERAL(0, 0, 7), // "Counter"
QT_MOC_LITERAL(1, 8, 12), // "valueChanged"
QT_MOC_LITERAL(2, 21, 0), // ""
QT_MOC_LITERAL(3, 22, 5), // "value"
QT_MOC_LITERAL(4, 28, 10), // "valueReset"
QT_MOC_LITERAL(5, 39, 10) // "printValue"

    },
    "Counter\0valueChanged\0\0value\0valueReset\0"
    "printValue"
};
#undef QT_MOC_LITERAL

static const uint qt_meta_data_Counter[] = {

 // content:
       7,       // revision
       0,       // classname
       0,    0, // classinfo
       3,   14, // methods
       0,    0, // properties
       0,    0, // enums/sets
       0,    0, // constructors
       0,       // flags
       2,       // signalCount

 // signals: name, argc, parameters, tag, flags
       1,    1,   29,    2, 0x06 /* Public */,
       4,    0,   32,    2, 0x06 /* Public */,

 // slots: name, argc, parameters, tag, flags
       5,    0,   33,    2, 0x0a /* Public */,

 // signals: parameters
    QMetaType::Void, QMetaType::Int,    3,
    QMetaType::Void,

 // slots: parameters
    QMetaType::Void,

       0        // eod
};

void Counter::qt_static_metacall(QObject *_o, QMetaObject::Call _c, int _id, void **_a)
{
    if (_c == QMetaObject::InvokeMetaMethod) {
        Counter *_t = static_cast<Counter *>(_o);
        Q_UNUSED(_t)
        switch (_id) {
        case 0: _t->valueChanged((*reinterpret_cast< int(*)>(_a[1]))); break;
        case 1: _t->valueReset(); break;
        case 2: _t->printValue(); break;
        default: ;
        }
    } else if (_c == QMetaObject::IndexOfMethod) {
        int *result = reinterpret_cast<int *>(_a[0]);
        void **func = reinterpret_cast<void **>(_a[1]);
        {
            typedef void (Counter::*_t)(int );
            if (*reinterpret_cast<_t *>(func) == static_cast<_t>(&Counter::valueChanged)) {
                *result = 0;
                return;
            }
        }
        {
            typedef void (Counter::*_t)();
            if (*reinterpret_cast<_t *>(func) == static_cast<_t>(&Counter::valueReset)) {
                *result = 1;
                return;
            }
        }
    }
}

const QMetaObject Counter::staticMetaObject = {
    { &QObject::staticMetaObject, qt_meta_stringdata_Counter.data,
      qt_meta_data_Counter,  qt_static_metacall, Q_NULLPTR, Q_NULLPTR}
};


const QMetaObject *Counter::metaObject() const
{
    return QObject::d_ptr->metaObject ? QObject::d_ptr->dynamicMetaObject() : &staticMetaObject;
}

void *Counter::qt_metacast(const char *_clname)
{
    if (!_clname) return Q_NULLPTR;
    if (!strcmp(_clname, qt_meta_stringdata_Counter.stringdata0))
        return static_cast<void*>(const_cast< Counter*>(this));
    return QObject::qt_metacast(_clname);
}

int Counter::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 < 3)
            qt_static_metacall(this, _c, _id, _a);
        _id -= 3;
    } else if (_c == QMetaObject::RegisterMethodArgumentMetaType) {
        if (_id < 3)
            *reinterpret_cast<int*>(_a[0]) = -1;
        _id -= 3;
    }
    return _id;
}

// SIGNAL 0
void Counter::valueChanged(int _t1)
{
    void *_a[] = { Q_NULLPTR, const_cast<void*>(reinterpret_cast<const void*>(&_t1)) };
    QMetaObject::activate(this, &staticMetaObject, 0, _a);
}

// SIGNAL 1
void Counter::valueReset()
{
    QMetaObject::activate(this, &staticMetaObject, 1, Q_NULLPTR);
}
QT_END_MOC_NAMESPACE

上面moc_main.cpp的代码有些地方不好理解，并且在本例中不需要用到，把这些内容删除，简化后如下

static const uint qt_meta_data_Counter[] = {
 // content:
       7,       // revision
       0,       // classname
       0,    0, // classinfo
       3,   14, // methods
       0,    0, // properties
       0,    0, // enums/sets
       0,    0, // constructors
       0,       // flags
       2,       // signalCount
};

void Counter::qt_static_metacall(QObject *_o, QMetaObject::Call _c, int _id, void **_a)
{
    if (_c == QMetaObject::InvokeMetaMethod)
    {   // 根据_id调用信号或者槽函数
        Counter *_t = static_cast<Counter *>(_o);
        switch (_id) {
        case 0: _t->valueChanged((*reinterpret_cast< int(*)>(_a[1]))); break;
        case 1: _t->valueReset(); break;
        case 2: _t->printValue(); break;
        default:;
        }
    }
    else if (_c == QMetaObject::IndexOfMethod)
    {   // 根据信号函数指针获取其_id
        int *result = reinterpret_cast<int *>(_a[0]);
        void **func = reinterpret_cast<void **>(_a[1]);
        {
            typedef void (Counter::*_t)(int);
            if (*reinterpret_cast<_t *>(func) == static_cast<_t>(&Counter::valueChanged))
            {
                *result = 0;
                return;
            }
        }
        {
            typedef void (Counter::*_t)();
            if (*reinterpret_cast<_t *>(func) == static_cast<_t>(&Counter::valueReset)) {
                *result = 1;
                return;
            }
        }
    }
}

// 每个QObject或者QObject子类型都包含一个静态QMetaObject对象
// QMetaObject对象初始化
const QMetaObject Counter::staticMetaObject = {
{
    // 父类元对象指针
        &QObject::staticMetaObject, nullptr,
        qt_meta_data_Counter,
        qt_static_metacall/*d.static_metacall*/, nullptr, nullptr
    }
};

const QMetaObject *Counter::metaObject() const
{
    return &staticMetaObject;
}

// SIGNAL 0
void Counter::valueChanged(int _t1)
{
    // _a[0]是返回值存放地址，这里返回类型为void，所以是nullptr
    void *_a[] = { nullptr, &_t1 };
    QMetaObject::activate(this, &staticMetaObject, 0, _a);
}

// SIGNAL 1
void Counter::valueReset()
{
    // 既没有参数也没有返回值，所以最后一个参数是nullptr
    QMetaObject::activate(this, &staticMetaObject, 1, nullptr);
}

QMetaObject介绍

在这不全面介绍QMetaObject，可以参考Qt手册，仅仅介绍本例相关的一些QMetaObject实现细节。

Qt手册上说QMetaObject类包含了Qt对象的元数据，例如类名、父类信息、方法属性等等，每个QObject类或子类都对应一个QMetaObject对象。在信号槽机制中，信号和槽的连接关系的保存，信号触发时执行槽函数这些操作也需要通过QMetaObject才能进行。

下面是Qt手册对元对象的介绍：

The meta-object syste