programmatically moving the scrollbar in a scrollarea

janK

I have a QScrollArea with in it a Table.
The table is way too large to be fully shown in the (relatively small) window.
From time to time I want to highlight a table item, selection and altering the fonts used there iis no problem at all, but due to the
limited size of the enclosing window, the high lighted element is not automatically in the viewable part of the scrollarea

In order to make the selected element visible I want to programatically set the scrollbar such that the area with the
selected element is visible. Having set the minimum and maximum values for the scrollbar to 0 ..100,
computing is no problem.
I compute a value to set the scrollbar and issue a command

.... -> theWidget -> verticalScrollBar () -> setValue ( (int)(yVal * 100));
where yVal is fraction between 0 and 1 resulting from my computation
However, the scrollbar does not move and inquiring the scrollbar value
theWidget -> verticalScrollBar () -> value ());
always gives the value 0

Apparently I am doing something wrong
Any help would be appreciated

best
jan

jeremy_k

Have you tried QScrollArea::ensureVisible() instead of manipulating the scrollbar?

Christian Ehrlicher

@janK said in prorammatically moving the scrollbar in a scrollarea:

theWidget -> verticalScrollBar () -> setValue ( (int)(yVal * 100));

Is the range of your scrollbar really only from 0 to 100 ? I doubt this.

janK

Yes, I have tried ensureVisibility that did not work
Ans I continuous verify that the range of (int)(yVal * 100) is between 0 .. 100,
yVal is a value between 0.1

Christian Ehrlicher

@janK said in prorammatically moving the scrollbar in a scrollarea:

Ans I continuous verify that the range of (int)(yVal * 100) is between 0 .. 100,

This was not my question. Why do you think the range of your scrollbar is between 0 and 100? Please read my link.

janK

OK
The text in the original question states that I did set the min en max for th scrollbar to 0 and 100

Christian Ehrlicher

Did you actually check if QScrollArea did not modify the scrollbar? QScrollArea is the owner of the QScrollBar(s) so it also changes min/max to match it's interal logic.

jeremy_k

@janK said in programmatically moving the scrollbar in a scrollarea:

Yes, I have tried ensureVisibility that did not work
Ans I continuous verify that the range of (int)(yVal * 100) is between 0 .. 100,
yVal is a value between 0.1

What does "did not work" mean? And where does this value 100 come from? Is the table height 100 pixels + the height of the QScrollAreal?

SGaist

Hi,

@janK said in programmatically moving the scrollbar in a scrollarea:

I have a QScrollArea with in it a Table.

Something is not fully clear here. QTableView already is a scroll area (albeit not a QScrollArea) so from your description it seems you have a QTableView within a QScrollArea and you try to move your QTableView content but are doing that by modifying your QScrollArea.

Can you show your exact setup ?

SimonSchroeder

Also, if you restrict the scrollbar to the range 0...100, your new position might come out as the same value within that range and thus not move the scrollbar one bit.

C_H_E_N

As far as I understand, manually setting minimum/maximum may cause unexpected result. See:
https://github.com/qt/qtbase/blob/dev/src/widgets/widgets/qscrollarea.cpp#L191
The min/max values are most likely measured in pixels. Otherwise, I don't understand why QScrollArea does that by default

vbar->setRange(0, v.height() - p.height());

So if you limit the maximum to 100, you are probably going to only see very limited part of your table, which may result in looking like "nothing happened" because <100 pixels is perhaps too unnoticeable. You may try unsetting min/max value and just try something like

QScrollBar* scrollBar = <whatever_your_scroll_bar_is>;
scrollBar->setValue(int(scrollBar->maximum() * 0.1f));

And see if you can notice something.

And another recommendation is that a minimal reproducing sample can really help here.