QAbstractSpinBox Subclass exhibits wierd behavour

[sunil.thaha]

Hi,

Does anyone know why this simple class exihits a wierd behavior ?

class Test : public QAbstractSpinBox {
Q_OBJECT
public:
    Test( QWidget *parent =0 ) 
        :QAbstractSpinBox(parent){
        lineEdit()->setText( "Hello " );
    }
 
    void setText( const QString &txt){
        lineEdit()->setText( txt );
    }
};

class Test : public QAbstractSpinBox {
Q_OBJECT
public:
    Test( QWidget *parent =0 ) 
        :QAbstractSpinBox(parent){
        lineEdit()->setText( "Hello " );
    }
    
    void setText( const QString &txt){
        lineEdit()->setText( txt );
    }
};

To copy to clipboard, switch view to plain text mode 

I have subclassed a QAbstractSpinBox and in the constructor I have set the SpinBox's lineedit's text to "Hello" So In the code below I expect to see the text "Hello " inside the LineEdit but It remains blank

Test t;
t.show();

Test t;
t.show();

To copy to clipboard, switch view to plain text mode 

Now I tried this, note that the setText() call after the show works, but not before it ?

Test t;
t.setText( "Before show");                // Will not work, to test it comment the setText below show
 t.show();
t.setText( "Hi Hi ");                          // Calls to setText works after show ??

Test t;
t.setText( "Before show");                // Will not work, to test it comment the setText below show
 t.show();
t.setText( "Hi Hi ");                          // Calls to setText works after show ??

To copy to clipboard, switch view to plain text mode 

Any clue why this happens?

[wysota]

Do you want to just show the text instead of numbers? You may reimplement textFromValue from QSpinBox.

[sunil.thaha]

I am Subclassing QAbstractSpinBox, My intension is to make a widget like QTimeEdit with a different functionality. But this does not explain why the setText() works after show() gets called ?

[jpn]

I recall someone else getting bumped into this problem too. The problem lies within QAsbtractSpinBox::showEvent() which updates the content of the line edit like this:

// from qabstractspinbox.cpp:
void QAbstractSpinBox::showEvent(QShowEvent *)
{
    Q_D(QAbstractSpinBox);
 
    d->reset();
    d->updateEdit(); // <-- this is the one that causes the problem
}

// from qabstractspinbox.cpp:
void QAbstractSpinBox::showEvent(QShowEvent *)
{
    Q_D(QAbstractSpinBox);

    d->reset();
    d->updateEdit(); // <-- this is the one that causes the problem
}

To copy to clipboard, switch view to plain text mode 

The problem is that QAbstractSpinBoxPrivate::updateEdit() uses QAbstractSpinBoxPrivate::textFromValue(). In QAbstractSpinBox subclasses like QSpinBox and QDoubleSpinBox the corresponding pimpl method is overridden and the call is redirected to the corresponding method in the public implementation class, QSpinBox::textFromValue() and so on. In my humble opinion this is a little design flaw by the Trolls..

[sunil.thaha]

Does that mean that QAbstractSpinBox is never meant to be overridden by users, since its private implementation is not documented? I am developing a widget like the QTimeEdit with one more field, like the frame, and the value of the frame can be anywhere from 0 - (frameRate -1). So is QDateTimeEdit or QTimeEdit a good candidate to override ?

[jpn]

The particular problem can be avoided by bypassing QAbstractSpinBox::showEvent():

void MySpinBox::showEvent(QShowEvent* event)
{
    // bypass QAbstractSpinBox::showEvent()
    QWidget::showEvent(event);
}

void MySpinBox::showEvent(QShowEvent* event)
{
    // bypass QAbstractSpinBox::showEvent()
    QWidget::showEvent(event);
}

To copy to clipboard, switch view to plain text mode 

Edit: I'm not 100% sure whether it will break something else, though

[sunil.thaha]

Good thinking !!