QInputDialog in mouse event

Question

In example code:

class MyWidget : public QWidget
{
Q_OBJECT
protected:
    void mousePressEvent(QMouseEvent *event)
    {
        qDebug() << event;
        event->accept();
        QInputDialog::getText(NULL, "", "");
    }
};

When I click Right mouse button on widget Input dialog appear on screen. After I click any button on dialog it closed and mousePressEvent call again and again and show dialog. If I click Left mouse button or Ctrl+Left Mouse button on widget all work fine. This bug apper only on Mac OS (under Windows work fine).

Please help me to avoid this bug.

Answer 1

Those static/synchronous dialog functions always seemed a bit dubious to me -- they are implemented by recursively re-invoking the Qt event loop routine from within the getText() call, and so it's easy to get "interesting" re-entrancy issues when you use them. (For example, if some event in your program were to delete the MyWidget object before the user had dismissed the QInputDialog, then after QInputDialog::getText() returned, the program would be executing from within the mousePressEvent() method of a deleted MyWidget object, which is a situation that is just begging to cause undefined behavior)

In any case, my recommended fix is to avoid the static/synchronous getText() call and use signals instead, something like this:

#include <QWidget>
#include <QInputDialog>
#include <QMouseEvent>

class MyWidget : public QWidget
{
Q_OBJECT
public:
    MyWidget() : dialog(NULL) {}
    ~MyWidget() {delete dialog;}

protected:
    void mousePressEvent(QMouseEvent *event)
    {
        event->accept();

        if (dialog)
        {
           dialog->raise();
           dialog->setFocus();
        }
        else
        {
           dialog = new QInputDialog;
           connect(dialog, SIGNAL(finished(int)), this, SLOT(dialogDismissed()));
           dialog->show();
        }
    }

private slots:
    void dialogDismissed()
    {
       if (dialog)
       {
          int result = dialog->result();
          QString t = dialog->textValue();

          printf("Dialog finished, result was %i, user entered text [%s]\n", result, t.toUtf8().constData());
          dialog->deleteLater();  // can't just call delete because (dialog) is what is calling this function (via a signal)!
          dialog = NULL;
       }
    }

private:
    QInputDialog * dialog;
};