QT中关于信号与槽机制的实现原理

QT中关于信号与槽机制的实现原理，需要用到的类，在本文中一一展现，代码较多，内容如下：

一、每个对象都有一个相应的纪录该对象的元对象

关于元对象的类：

QMetaObject类：

/*******************生成元对象需要的输入参数*****************/
//类名
const char * const class_name,
//父类名
QMetaObject *superclass,
//记录slot 信息
const QMetaData * const slot_data,
//记录槽的个数
int n_slots,
//记录signal 信息
const QMetaData * const signal_data,
//记录信号的个数
int n_signals
/******************* 元对象类提供的方法**************************/
int   numSlots( bool super = FALSE ) const;//返回槽的个数
int   numSignals( bool super = FALSE ) const;//返回信号的个数
int   findSlot( const char *, bool super = FALSE ) const;//查找槽
int   findSignal( const char *, bool super = FALSE ) const;//查找信号
//返回指定位置的槽
const QMetaData *slot( int index, bool super = FALSE ) const;
//返回指定位置的信号
const QMetaData *signal( int index, bool super = FALSE ) const;
//所有槽名字的列表
QStrList  slotNames( bool super = FALSE ) const;
//所有信号名字的列表
QStrList  signalNames( bool super = FALSE ) const;
//槽的起始索引
int   slotOffset() const;
//信号的起始索引
int   signalOffset() const;
/***********************两个获取类的元对象的方法*****************/
static QMetaObject *metaObject( const char *class_name );
static bool hasMetaObject( const char *class_name );

QMetaData类：

//记录元对象数据for 信号与槽
struct QMetaData
{
const char *name;  //名称
const QUMethod* method; //详细描述信息
enum Access { Private, Protected, Public };
Access access; //访问权限
};

二、QObject类实现了信号与槽机制

它利用元对象纪录的信息，实现了信号与槽机制

（1）信号与槽建立连接的实现

接口函数：

//连接
//参数（发送对象，信号，接收对象，处理信号的信号/槽）
static bool  connect( const QObject *sender, const char *signal,
const QObject *receiver, const char *member );
bool connect(const QObject *sender, const char *signal,
const char *member ) const;
static bool  disconnect( const QObject *sender, const char *signal,
const QObject *receiver, const char *member );
bool disconnect(const char *signal=0,
const QObject *receiver=0, const char *member=0 );
bool disconnect( const QObject *receiver, const char *member=0 );
//连接的内部实现
//（发送对象，信号的索引，接收对象，处理信号的类型，处理信号信号/槽的索引）
static void connectInternal(const QObject *sender, int signal_index,
const QObject *receiver, int membcode, int member_index );
static bool disconnectInternal(const QObject *sender, int signal_index,
const QObject *receiver, int membcode, int member_index );

信号与槽连接的实现原理：

①阶段
bool QObject::connect( const QObject *sender,//发送对象
const char *signal,//信号
const QObject *receiver, //接收对象
const char *member //槽
)
{
//检查发送对象，信号，接收对象，槽不为null
if ( sender == 0 || receiver == 0 || signal == 0 || member == 0 ) {
return FALSE;
}
//获取发送对象的元对象
QMetaObject *smeta = sender->metaObject();
//检查信号
if ( !check_signal_macro( sender, signal, "connect", "bind" ) )
return FALSE;
//获取信号的索引
int signal_index = smeta->findSignal( signal, TRUE );
if ( signal_index < 0 ) {                // normalize and retry
nw_signal = qt_rmWS( signal-1 ); // remove whitespace
signal = nw_signal.data()+1;         // skip member type code
signal_index = smeta->findSignal( signal, TRUE );
}
//如果信号不存在，则退出
if ( signal_index < 0  ) {                    // no such signal
return FALSE;
}
//获取信号的元数据对象
const QMetaData *sm = smeta->signal( signal_index, TRUE );
//获取信号名字
signal = sm->name;
//获取处理信号的类型（是信号/槽）
int membcode = member[0] - '0';        // get member code
//发送信号对象
QObject *s = (QObject *)sender;        // we need to change them
//接收信号对象
QObject *r = (QObject *)receiver;             //   internally
//获取接收对象的元对象
QMetaObject *rrmeta = r->metaObject();
int member_index = -1;
switch ( membcode ) {                // get receiver member
case QSLOT_CODE://如果是槽
//获取槽索引
member_index = rmeta->findSlot( member, TRUE );
if ( member_index < 0 ) {            // normalize and retry
nw_member = qt_rmWS(member);     // remove whitespace
member = nw_member;
member_index = rmeta->findSlot( member, TRUE );
}
break;
case QSIGNAL_CODE://如果是信号
//获取信号索引
member_index = rmeta->findSignal( member, TRUE );
if ( member_index < 0 ) {           // normalize and retry
nw_member = qt_rmWS(member);     // remove whitespace
member = nw_member;
member_index = rmeta->findSignal( member, TRUE );
}
break;
}
/如果接收对象不存在相应的信号或槽，则退出
if ( member_index < 0  ) {
return FALSE;
}
//检查连接的参数(发送的信号，接收对象，处理信号的槽或信号)
if ( !s->checkConnectArgs(signal,receiver,member) ) {
return FALSE;
} else {
//获取处理信号的元数据对象
const QMetaData *rm = membcode == QSLOT_CODE ?
rmeta->slot( member_index, TRUE ) :
rmeta->signal( member_index, TRUE );
if ( rm ) {
//建立连接
//(发送信号的对象，信号的索引，接收信号的对象，
处理信号的类型，处理信号的索引)
connectInternal( sender, signal_index, receiver, membcode, member_index );
}
}
return TRUE;
}
②阶段
//建立连接
//(发送信号的对象，信号的索引，接收信号的对象，处理信号的类型，处理信号的索引)
void QObject::connectInternal( const QObject *sender, int signal_index,
const QObject *receiver, int membcode, int member_index )
{
//发送信号的对象
QObject *s = (QObject*)sender;
//接收信号的对象
QObject *r = (QObject*)receiver;
//如果发送对象的连接查询表为null，则建立
if ( !s->connections ) {                // create connections lookup table
s->connections = new QSignalVec( signal_index+1 );
Q_CHECK_PTR( s->connections );
s->connections->setAutoDelete( TRUE );
}
//获取发送对象的相应信号的连接列表

QConnectionList *clist = s->connections->at( signal_index );

if ( !clist ) {                         // create receiver list
clist = new QConnectionList;
Q_CHECK_PTR( clist );
clist->setAutoDelete( TRUE );
s->connections->insert( signal_index, clist );
}
QMetaObject *rrmeta = r->metaObject();
const QMetaData *rm = 0;
switch ( membcode ) {                // get receiver member
case QSLOT_CODE:
rm = rmeta->slot( member_index, TRUE );
break;
case QSIGNAL_CODE:
rm = rmeta->signal( member_index, TRUE );
break;
}
//建立连接
QConnection *c = new QConnection( r, member_index, rm ? rm->name :
"qt_invoke", membcode );
Q_CHECK_PTR( c );
//把连接添加到发送对象的连接列表中
clist->append( c );
//判断接收对象的发送对象列表是否为null
if ( !r->senderObjects )               // create list of senders
{
//建立接收对象的发送对象列表
r->senderObjects = new QSenderObjectList;
}
//把发送对象添加到发送对象列表中
r->senderObjects->append( s );           // add sender to list
}

（2）信号发生时激活的操作函数。 激活slot的方法

接口：
void QObject::activate_signal( int signal )
{
#ifndef QT_NO_PRELIMINARY_SIGNAL_SPY
if ( qt_preliminary_signal_spy ) {
//信号没有被阻塞
//信号>=0
//连接列表不为空，或者信号对应的连接存在
if ( !signalsBlocked() && signal >= 0 &&
( !connections || !connections->at( signal ) ) ) {
//
QUObject o[1];
qt_spy_signal( this, signal, o );
return;
}
}
#endif
if ( !connections || signalsBlocked() || signal < 0 )
return;
//获取信号对应的连接列表
QConnectionList *clist = connections->at( signal );
if ( !clist )
return;
QUObject o[1];
//
activate_signal( clist, o );
}

void QObject::activate_signal( QConnectionList *clist, QUObject *o )
{
if ( !clist )
return;
#ifndef QT_NO_PRELIMINARY_SIGNAL_SPY
if ( qt_preliminary_signal_spy )
qt_spy_signal( this, connections->findRef( clist), o );
#endif
QObject *object;
//发送对象列表
QSenderObjectList* sol;
//旧的发送对象
QObject* oldSender = 0;
//连接
QConnection *c;
if ( clist->count() == 1 ) { // save iterator
//获取连接
c = clist->first();
//
object = c->object();
//获取发送对象列表
sol = object->senderObjects;
if ( sol ) {
//获取旧的发送对象
oldSender = sol->currentSender;
//
sol->ref();
//设置新的发送对象
sol->currentSender = this;
}
if ( c->memberType() == QSIGNAL_CODE )//如果是信号，则发送出去
object->qt_emit( c->member(), o );
else
object->qt_invoke( c->member(), o );//如果是槽，则执行
//
if ( sol ) {
//设置恢复为旧的发送对象
sol->currentSender = oldSender;
if ( sol->deref() )
delete sol;
}
} else {
QConnection *cd = 0;
QConnectionListIt it(*clist);
while ( (c=it.current()) ) {
++it;
if ( c == cd )
continue;
ccd = c;
object = c->object();
//操作前设置当前发送对象
sol = object->senderObjects;
if ( sol ) {
oldSender = sol->currentSender;
sol->ref();
sol->currentSender = this;
}
//如果是信号，则发送出去
if ( c->memberType() == QSIGNAL_CODE ){
object->qt_emit( c->member(), o );
}
//如果是槽，则执行
else{
object->qt_invoke( c->member(), o );
}
//操作后恢复当前发送对象
if (sol ) {
sol->currentSender = oldSender;
if ( sol->deref() )
delete sol;
}
}
}
}