如何通知maxlength溢出

Question

有没有办法在执行默认处理程序之前连接信号？我正在寻找一种方法来在QLineEdit::textChanged信号之前执行我的函数，以执行关于最大长度限制的通知。

GTK+有connect_before()、connect()和connect_after()。Qt中也有类似的东西吗？

Answer 1

您可以使用keyPressEvent方法发出自定义信号。

#include <QtWidgets>

class LineEdit: public QLineEdit
{
    Q_OBJECT
public:
    using QLineEdit::QLineEdit;
signals:
    void maxLengthSignal();
protected:
    void keyPressEvent(QKeyEvent *event) override{
        if(!event->text().isEmpty() && maxLength() == text().length())
            emit maxLengthSignal();
        QLineEdit::keyPressEvent(event);
    }
};
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    LineEdit w;
    QObject::connect(&w, &QLineEdit::textEdited, [](const QString & text){
        qDebug()<< text;
    });
    QObject::connect(&w, &LineEdit::maxLengthSignal, [](){
        qDebug()<< "maxLength signal";
    });
    w.setMaxLength(10);
    w.show();
    return a.exec();
}
#include "main.moc"