Qt connectSlotsByName与信号槽

在 Qt connect与信号槽一文中提到，信号槽主要通过 connect 进行连接，并且有好几种不同的写法。
新建一个Demo Widgets 程序。在 Qt 设计师界面中，拖入一个 QPushButton 控件，在其上右键 - 转到槽 - 选择信号 clicked()，会自动生成一个槽函数 on_pushButton_clicked()。发现点击按钮会触发该槽，但在代码中并没有找到 connect 的存在。也就是说信号槽的连接还存在其他的方式。

二 setupUi

探究代码，在主界面类的构造函数中，除了 setupUi 函数，并没有其他特殊的地方。setupUi 如下：

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
      
      
    ui->setupUi(this);
}

展开 setupUi

......
class Ui_MainWindow
{
      
      
public:
    QWidget *centralWidget;
    QPushButton *pushButton;
    QMenuBar *menuBar;
    QToolBar *mainToolBar;
    QStatusBar *statusBar;

    void setupUi(QMainWindow *MainWindow)
    {
      
      
        if (MainWindow->objectName().isEmpty())
            MainWindow->setObjectName(QStringLiteral("MainWindow"));
        MainWindow->resize(400, 300);
        centralWidget = new QWidget(MainWindow);
        centralWidget->setObjectName(QStringLiteral("centralWidget"));
        pushButton = new QPushButton(centralWidget);
        pushButton->setObjectName(QStringLiteral("pushButton"));
        pushButton->setGeometry(QRect(140, 80, 75, 23));
        MainWindow->setCentralWidget(centralWidget);
        menuBar = new QMenuBar(MainWindow);
        menuBar->setObjectName(QStringLiteral("menuBar"));
        menuBar->setGeometry(QRect(0, 0, 400, 23));
        MainWindow->setMenuBar(menuBar);
        mainToolBar = new QToolBar(MainWindow);
        mainToolBar->setObjectName(QStringLiteral("mainToolBar"));
        MainWindow->addToolBar(Qt::TopToolBarArea, mainToolBar);
        statusBar = new QStatusBar(MainWindow);
        statusBar->setObjectName(QStringLiteral("statusBar"));
        MainWindow->setStatusBar(statusBar);

        retranslateUi(MainWindow);

        QMetaObject::connectSlotsByName(MainWindow); // 注意此处
    } // setupUi
......

在 setupUi 中唯一的特殊之处为调用 QMetaObject::connectSlotsByName。

三 connectSlotsByName

在 Qt 助手中搜索该函数，描述如下：

Searches recursively for all child objects of the given object, and connects matching signals from them to slots of object that follow the following form:
```
  void on_<object name>_<signal name>(<signal parameters>);
```
Let’s assume our object has a child object of type QPushButton with the object name button1. The slot to catch the button’s clicked() signal would be:
```
  void on_button1_clicked();
```
If object itself has a properly set object name, its own signals are also connected to its respective slots.See also QObject::setObjectName().
简单来说，按照规则书写槽函数的名称，并且调用 QMetaObject::connectSlotsByName，能够自动将子对象的信号与槽进行连接。

举例

// mainwindow.h
......
private slots:
    void on_btn_clicked();
......

// mainwindow.cpp
......
QPushButton* button = new QPushButton(this);
button->setObjectName(QStringLiteral("btn"));
qDebug() << button->objectName();
QMetaObject::connectSlotsByName(this);
......

按照 on_< objectName >_< signal > 形式创建槽函数名称。
在代码中直接调用 QMetaObject::connectSlotsByName，可以实现信号槽连接。

四源码

QMetaObject::connectSlotsByName 的实现在源文件 qobject.cpp中，如下：

void QMetaObject::connectSlotsByName(QObject *o)
{
      
      
    if (!o)
        return;
    const QMetaObject *mo = o->metaObject();
    Q_ASSERT(mo);
    const QObjectList list = // list of all objects to look for matching signals including...
            o->findChildren<QObject *>(QString()) // all children of 'o'...
            << o; // and the object 'o' itself

    // for each method/slot of o ...
    for (int i = 0; i < mo->methodCount(); ++i) {
      
      
        const QByteArray slotSignature = mo->method(i).methodSignature();
        const char *slot = slotSignature.constData();
        Q_ASSERT(slot);

        // ...that starts with "on_", ...
        if (slot[0] != 'o' || slot[1] != 'n' || slot[2] != '_')
            continue;

        // ...we check each object in our list, ...
        bool foundIt = false;
        for(int j = 0; j < list.count(); ++j) {
      
      
            const QObject *co = list.at(j);
            const QByteArray coName = co->objectName().toLatin1();

            // ...discarding those whose objectName is not fitting the pattern "on_<objectName>_...", ...
            if (coName.isEmpty() || qstrncmp(slot + 3, coName.constData(), coName.size())    
                || slot[coName.size()+3] != '_')
                continue;

            const char *signal = slot + coName.size() + 4; // the 'signal' part of the slot name

            // ...for the presence of a matching signal "on_<objectName>_<signal>".
            const QMetaObject *smeta;
            int sigIndex = co->d_func()->signalIndex(signal, &smeta);
            if (sigIndex < 0) {
      
      
                // if no exactly fitting signal (name + complete parameter type list) could be found
                // look for just any signal with the correct name and at least the slot's parameter list.
                // Note: if more than one of thoses signals exist, the one that gets connected is
                // chosen 'at random' (order of declaration in source file)
                QList<QByteArray> compatibleSignals;
                const QMetaObject *smo = co->metaObject();
                int sigLen = qstrlen(signal) - 1; // ignore the trailing ')'
                for (int k = QMetaObjectPrivate::absoluteSignalCount(smo)-1; k >= 0; --k) {
      
      
                    const QMetaMethod method = QMetaObjectPrivate::signal(smo, k);
                    if (!qstrncmp(method.methodSignature().constData(), signal, sigLen)) {
      
      
                        smeta = method.enclosingMetaObject();
                        sigIndex = k;
                        compatibleSignals.prepend(method.methodSignature());
                    }
                }
                if (compatibleSignals.size() > 1)
                    qWarning() << "QMetaObject::connectSlotsByName: Connecting slot" << slot
                               << "with the first of the following compatible signals:" << compatibleSignals;
            }

            if (sigIndex < 0)
                continue;

            // we connect it...
            if (Connection(QMetaObjectPrivate::connect(co, sigIndex, smeta, o, i))) {
      
      
                foundIt = true;
                // ...and stop looking for further objects with the same name.
                // Note: the Designer will make sure each object name is unique in the above
                // 'list' but other code may create two child objects with the same name. In
                // this case one is chosen 'at random'.
                break;
            }
        }
        if (foundIt) {
      
      
            // we found our slot, now skip all overloads
            while (mo->method(i + 1).attributes() & QMetaMethod::Cloned)
                  ++i;
        } else if (!(mo->method(i).attributes() & QMetaMethod::Cloned)) {
      
      
            // check if the slot has the following signature: "on_..._...(..."
            int iParen = slotSignature.indexOf('(');
            int iLastUnderscore = slotSignature.lastIndexOf('_', iParen-1);
            if (iLastUnderscore > 3)
                qWarning("QMetaObject::connectSlotsByName: No matching signal for %s", slot);
        }
    }
}

过程：
- findChildren 获取对象 o 所有子对象存入 list；
- 遍历对象 o 的所有方法，并仅对 on_< objectname >_< signal > 形式的方法进行处理；
- 遍历子对象，先找到 objectname 匹配的子对象，然后通过 signalIndex 查找该子对象完全匹配的信号
- 若未找到完全匹配信号(fitting signal : name + complete parameter type list)，则遍历该子对象的信号，查找兼容匹配信号(compatible signal : any signal with the correct name and at least the slot’s parameter list.)
- 找到信号，则通过 QMetaObjectPrivate::connect 进行连接。

注意

若查到到多个匹配信号，只会对第一个信号进行连接；

未找到信号，则会有 No matching signal 提示信息，注意观察。

// 将 on_pushButton_clicked 更改为 on_pushButton1_clicked 或 on_pushButton_clicked1
// 会出现以下输出信息：
QMetaObject::connectSlotsByName: No matching signal for on_pushButton1_clicked()
QMetaObject::connectSlotsByName: No matching signal for on_pushButton_clicked1()

五注意

根据需要，connect 和 QMetaObject::connectSlotsByName 可以一同使用，以提高效率。

Qt connectSlotsByName与信号槽

目录

一场景

二 setupUi

三 connectSlotsByName

四源码

五注意

猜你喜欢

Qt connectSlotsByName与信号槽

目录

一 场景

二 setupUi

三 connectSlotsByName

四 源码

五 注意

猜你喜欢

一场景

四源码

五注意