QScrollArea signal or virtual function for "about to show scrollbar"

[mrjj]

Hi
Is it possible to catch a signal/virtual override to know when QScrollArea
is about to show scrollbar?

I want to disable the scrollbar and have buttons in
top/bottom to show when normally the side scrollbar would appear.

i have tried 

class ScrollbarProxy : public QScrollBar {
  Q_OBJECT
public:
  explicit ScrollbarProxy(QWidget* parent = nullptr);
signals:

public slots:

protected:
  void changeEvent(QEvent* e) override {
    qDebug() << "changeEvent" << e->type();
  }
  void setVisible(bool visible) override {
    qDebug() << "setVisible" << visible ;    
  }
};

with 
ui->scrollArea->setVerticalScrollBar( new ScrollbarProxy() );
ui->scrollArea->setHorizontalScrollBar( new ScrollbarProxy() );

In the hope i could just use setVisible but even i DONT call baseclass, the scrollbars
are shown never the less?

Also changeEvent did not show any info when scrollbars are shown.

So any hints of how to intercept it if possible?

( mockup )

Basically i want to show the up/down buttons when ever the right side scrollbar would show it self.

[raven-worx]

@mrjj

1. scrollArea->setVerticalScrollBarPolicy( Qt::ScrollBarAlwaysOff );
2. button->setVisible( scrollArea->verticalScrollBar()->minimum() < scrollArea->verticalScrollBar()->maximum() );
3. connect the corresponding button to the scrollbar's triggerAction() method. Either with QAbstractSlider::SliderSingleStepAdd or QAbstractSlider::SliderPageStepAdd to scroll down.

[mrjj]

@raven-worx

Ah, so there is no signal.

The issue is when to call
button->setVisible(xxx);

I guess i must do in all cases where i insert/remove items,
scale window etc.

There really is no lesser hack-ish way ?

Thank you

[raven-worx]

@mrjj
times to check (with the code i've provided) can be when

1. QScrollArea receives QEvent::StyleChange or QEvent::LayoutRequest events
2. the viewport widget resizes
3. content widget resizes

So you could use an event-filter, but make sure to make the check delayed.

[mrjj]

@raven-worx

1: Are they recieved via changeEvent(QEvent* e) ?

Thank you, i will try with eventfilter and see.

[VRonin]

can't you just use QAbstractSlider::rangeChanged signal?

[mrjj]

Hi
thx for both suggestions.
rangeChanged works to some degree but fires 2 signal before even shown
rangeChange 0 , 256
rangeChange 0 , 0

but Im pretty sure i can work something out if i can guess from values when it should be hidden again.

This test code seems to do what i need and show hide the buttons as expected.

void MainWindow::rangeChanged(int min, int max) {
  
  qDebug() << "rangeChange" << min << " , " << max;
  
  if (min < max)  {
    ui->pushButton_4->show();
    ui->pushButton_5->show();
    qDebug() << "SHOW";
  }

  if  (min == 0 && max == 0) {
    ui->pushButton_4->hide();
    ui->pushButton_5->hide();
    qDebug() << "HIDE";
  }

}