Fri, 26 Aug 2011 02:41:00 GMT

�W�一�?Signal和Slot是同步的�q�是异步的？

　　我们知道Qt以他的signal和slot机制独步天下。但大家在用的时候有没有注意�q�，signal和slot之间是异步的�Q�还是同步的呢？为此我问�q�不��用Qt的道友。有��是同步的�Q�有��是异步的�Q�也有�h说要看当时你的�h品�?( #$%^&*

　　为此贫道�Q�特别做了以下几个测试：

First,在main()��d��数里�Q�设�|�两个基于QObject为父�cȝ��对象a和b�Q�a触发signal,b接受signal。请看具体案例：

 1 class MyTestA : public QObject
 2 {
 3     Q_OBJECT
 4 public:
 5     void emitSignal()
 6     {
 7         signalMyTestA();
 8     }
 9 
10 public slots:
11     void slotMyTestA()
12     {
13         qDebug()<<"slotMyTestA is called.";
14     }
15 signals:
16     void signalMyTestA();
17 };
18 
19 class MyTestB : public QObject
20 {
21     Q_OBJECT
22 public slots:
23     void slotMyTestB()
24     {
25         qDebug()<<"slotMyTestB is called.";
26     }
27 signals:
28     void signalMyTestB();
29 };
30 
31 int main(int argc, char *argv[])
32 {
33     QApplication app(argc, argv);
34 
35     MyTestA a;
36     MyTestB b;
37     QObject::connect(&a,SIGNAL(signalMyTestA()),&b,SLOT(slotMyTestB()));
38 
39     a.emitSignal();
40     
41     return app.exec();
42 }

在slotMyTestB的函数里打个断点�Q�看一下调用堆�?call stack)�?/p>

是同步调用的�Q�某些道友开始拈胡微�W�，实践出真知啊�?/p>

此时只见东方黑云滚滚�Q�电闪雷鸣，又有道友开始度劫了。突然一度劫道友横眉冷对�Q�拿��h��刷��L��改写了上面的代码。只见此道友把a对象挪到了一个新�U�程中（MyTestC创徏的）�Q�而b对象仍然在主�U�程中。然后a对象触发信号�?/p>

class MyTestA : public QObject
{
    Q_OBJECT
public:
    void emitSignal()
    {
        signalMyTestA();
    }
public slots:
    void slotMyTestA()
    {
        qDebug()<<"slotMyTestA is called.";
    }
signals:
    void signalMyTestA();
};
class MyTestB : public QObject
{
    Q_OBJECT
public slots:
    void slotMyTestB()
    {
        qDebug()<<"slotMyTestB is called.";
    }
signals:
    void signalMyTestB();
};
extern MyTestB *g_pMyTestB;
class MyTestC : public QThread
{
    Q_OBJECT
public:
    void run()
    {
        MyTestA a;
        connect(&a,SIGNAL(signalMyTestA()),g_pMyTestB,SLOT(slotMyTestB()));
        a.emitSignal();
        exec();
    }
public slots:
    void slotMyTestC()
    {
        qDebug()<<"slotMyTestC is called.";
    }
signals:
    void signalMyTestC();
};
class MyTest : public QDialog
{
    Q_OBJECT
public:
    MyTest(QWidget *parent = 0, Qt::WFlags flags = 0);
    ~MyTest();
private:
    Ui::MyTestClass ui;
};
////////////////////////////////////////////////
MyTestB *g_pMyTestB = NULL;
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MyTestB b;
    g_pMyTestB = &b;
    MyTestC c;
    c.start();
    return app.exec();
}

说时�q�，那时快。在一道��雷劈下之际，按下了F5。只见，此时的调用堆栈显�C?

奇迹出现了，居然变成异步调用了。只见东方天�I�Z��道金光射下，在阵阵仙乐声中，传来朗朗之声:"贫道��事已了�Q�再无牵�?�?/p>

��N��Qt真的是靠人品的，或者Qt莫不是也是修仙道友，不日也将飞升�?/p>

在吾�{�众��拜加疑惑之时�Q�只见飞升前辈，留下一条偈语。内事不决问癑ֺ��Q�外事不决问��h��?/p>

各��众�h立刻搜寻�Q�恍然大物�?/p>

原来signal和slot是异步调用还是同步调用，取决于对connect的设定。其实connect�q�有一个参�?Qt::ConnectionType),是它军_��了是同步�q�是异步�?/span>以下是ConnectionType的定�?/p>

只不�q�，�q�_��它有一个默认值Qt::AutoConnection,我们忽略了它。这时有道友问道�Q��ؓ何在AutoConnection模式下，有时是同步，有时是异步，莫非Auto��是人品代名词�?/p>

非也�Q�其实Auto是这栯��定的�Q?/p>

当sender和receiver在同一�U�程�Ӟ��是同步模式�Q�而在不同�U�程�Ӟ��则是异步模式�?/span>

众�h皆曰善�?/span>

��在众�h弹冠相庆之时�Q�突然一道类似眼镜发出的寒光闪过�Q�一个黑影渐渐清��C��h��?/span>

他居然就�?.................

青春气R��Q�十二年如一日的柯南君，他招牌式的磁性声音给众道友一晴天霚w��Q?#8220;�怽�以�ؓ�q�就是全部的真相吗？”

接着他刷��L��又改写了代码�Q�在�ȝ��E�中生成a,b两个对象�Q�而a对象在新�U�程�Q�MyTestC创徏的）中触发信受��?/span>

class MyTestA : public QObject
{
    Q_OBJECT
public:
    void emitSignal()
    {
        signalMyTestA();
    }
public slots:
    void slotMyTestA()
    {
        qDebug()<<"slotMyTestA is called.";
    }
signals:
    void signalMyTestA();
};
class MyTestB : public QObject
{
    Q_OBJECT
public slots:
    void slotMyTestB()
    {
        qDebug()<<"slotMyTestB is called.";
    }
signals:
    void signalMyTestB();
};
extern MyTestB *g_pMyTestB;
extern MyTestA *g_pMyTestA;
class MyTestC : public QThread
{
    Q_OBJECT
public:
    void run()
    {
        g_pMyTestA->emitSignal();
        exec();
    }
public slots:
    void slotMyTestC()
    {
        qDebug()<<"slotMyTestC is called.";
    }
signals:
    void signalMyTestC();
};
/////////////////////////////////////////////
MyTestB *g_pMyTestB = NULL;
MyTestA *g_pMyTestA = NULL;
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MyTestA a;
    g_pMyTestA = &a;
    MyTestB b;
    g_pMyTestB = &b;
    QObject::connect(&a,SIGNAL(signalMyTestA()),&b,SLOT(slotMyTestB()));
    MyTestC c;
    c.start();
    return app.exec();
}

在众人疑惑的眼光中，此君淡定的按下了F5。只见调用堆�?call stack)昄��

众�h皆惊��|��“Impossible”。a和b明明是属于一个线�E�的�Q��ؓ何会异步调用。此时我们熟悉的语录�Q�又在而��回响,�?我相信真相只有一个！�Q�！"�q�句话吗�Q�No�Q�只见柯南君�Q�优雅地挥了挥手指，"Nothing impossible",从口中缓�~�滑出�?/p>

。众人皆扑街�Q?#8220;有屁快放”�?/p>

此时柯南君缓�~�从口袋中，摸出一张纸�Q�抛向空中，然后转��n��d��。只见随风飘落的�U�R��上面摘录了这么一�D�Qt源代码，在Auto模式下，如果要同步调用，不仅要求sender和receiver是同一�U�程�Q�而且sender触发的时候，所在的�U�程也要和receiver一致�?/p>

 // determine if this connection should be sent immediately or
            // put into the event queue
            if ((c->connectionType == Qt::AutoConnection
                 && (currentThreadData != sender->d_func()->threadData
                     || receiver->d_func()->threadData != sender->d_func()->threadData))
                || (c->connectionType == Qt::QueuedConnection)) {
                queued_activate(sender, signal_absolute_index, c, argv ? argv : empty_argv);
                continue;
            } else if (c->connectionType == Qt::BlockingQueuedConnection) {
                blocking_activate(sender, signal_absolute_index, c, argv ? argv : empty_argv);
                continue;
            }

摘自qobject.cpp

z众�h皆惊�Q?strong>原来在Auto模式下，如果sender的触发时所处的�U�程和receiver不同�Q�也会是异步调用。此旉��友齐声向柯南喊道“�q�是全部的真�怺��?#8221;�Q�柯南�{�q�头来笑而不语，渐渐又消失在黑暗中�?#8220;有多��无��d��以重�?#8221;漂上了众人的心头。望着�q�处的雨后阳光，一个大大的问号也出现在众�h头顶,"Qt你到底有多无耻？�Q�？"。众人又陷入了沉思�?/p>

�Ʋ知后事如何�Q�请听下回分解�?/p>

此篇已在CNBLOG同时发布

樱桃��锤�?/a> 2011-08-26 10:41 发表评论

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�W�一�?Signal和Slot是同步的�q�是异步的？