Text in QTableView not visible when setting Qt::BackgroundRole

olowo726

Hi,

Using Qt 6.3.0

I'm subclassing QAbstractTableModel and overriding virtual QVariant data(const QModelIndex& index, int role = Qt::DisplayRole) const override; to provide data to a QTableView. Data is provided both for Qt::DisplayRole (returning a QVariant(QString("sometxt")) and Qt::Backgroundrole (returning QVariant(QBrush(QColorConstants::Yellow)))

For one column which is seldom updated this works fine. But for a different column frequently updated the text isn't show at all, only the background color is painted all over the cell. Updates of the malfunctioning column are triggered with

QVector<int> roles;
roles.push_back(Qt::DisplayRole);
Q_EMIT dataChanged(topLeftValue, bottomRightValue, roles);

topLeftValue and bottorRightValue of course being indexes. Note Qt::DisplayRole, not Qt::Backgroundrole.

Why isn't the text shown at all, only the background, when dataChanged() is frequently emitted? I can add that if data returns QVariant() for the background role it works fine (i.e. the view displays black text on white background) also for frequently updated columns).

SGaist

Hi,

How often is "more frequently" ?
Are you also changing other roles ?
Do you have a different logic for the column that fails ?
Do you have a custom view ?

olowo726

"more frequent" - 50ms timer
"Are you also changing other roles ?" Only working with Display and Background
"Do you have a different logic for the column that fails ?" Don't understand the question - it's a different logic because it doesn't show the same data
"Do you have a custom view ?" - Nope, pure QTableView

olowo726

I shall add that I think this has worked in earlier Qt versions since we have an old feature in our app which also frequently modifies background role. I know this feature has been working before but is now broken.

olowo726

Actually I did have a delegate according to https://stackoverflow.com/questions/64198197/how-to-prevent-too-aggressive-text-elide-in-qtableview.

This delegate is obviously the culprit since disabling it solves the issue. But alas I need the elide....

I guess that

QApplication::style()->drawControl(QStyle::CE_ItemViewItem, &opt, painter);

Overwrites the text when there is a background color. Moving the drawText to below drawControl resulted in no text whatsoever regardless of background color.

So, how can I get both proper elide and background color in a table cell?

ChrisW67

The delegate on the SO post sets the opt.text to an empty string before calling drawControl() which dutifully renders a blanks cell. If drawControl() does not fill a background (because the background role is null) then the already painted text is left untouched. In short, I think that delegate worked by accident not design.

Something like this is what I would have tried:

void Delegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                      const QModelIndex &index) const
 {
     if (!index.isValid())
         return;

     QStyleOptionViewItem opt = option;
     initStyleOption(&opt, index);
    
// elide and store the text if needed.
    opt.text = opt.fontMetrics.elidedText(opt.text, Qt::ElideRight, opt.rect.width() ) );
// make sure the style does not try to elide the elided text
    opt.textElideMode = Qt::ElideNone;
     QApplication::style()->drawControl(QStyle::CE_ItemViewItem, &opt, painter);
 }

olowo726

I tried this suggestion. It works fine - for negative numbers only! For positive numbers opt.text is elided properly, i.e. after call to elideText it contains the properly elided text string but that's displayed is "0...." with really a lot of blank space available in the cell. I guess that's related to https://bugreports.qt.io/browse/QTBUG-87178

So, paint the background in the elide?

olowo726

This post is deleted!

ChrisW67

What is interesting is that this behaviour is only if the value looks like a number (even it is QVariant(String)) and correct otherwise:

Have not got a good workaround yet.

Christian Ehrlicher

You should check opt.rect.width() if it's different for your cells.

olowo726

@Christian-Ehrlicher It's the same for both working and non-working strings in my case.

olowo726

I think that this thread has became a bit messy due to me not realizing from start that I had a delegate. To summarize:
I need a solution which fulfills

Proper elide
Possible to set background color by emitting dataChanged() from the model in a method called by a timer at ~50ms
Possible to edit value as normal.

QTBUG: https://bugreports.qt.io/browse/QTBUG-87178
Solution which fulfills req 1 but not 2 and 3: https://stackoverflow.com/questions/64198197/how-to-prevent-too-aggressive-text-elide-in-qtableview

Start a new thread which makes this more clear for anyone else looking for a solution to this problem?

Christian Ehrlicher

@olowo726 said in Text in QTableView not visible when setting Qt::BackgroundRole:

Possible to set background color by emitting dataChanged() from the model in a method called by a timer at ~50ms

As I saidf in the bug report - such update rates are useless / stupid for a gui showing numbers...

olowo726

@Christian-Ehrlicher said in Text in QTableView not visible when setting Qt::BackgroundRole:

As I saidf in the bug report - such update rates are useless / stupid for a gui showing numbers...

In this case it isn't about the number, it's about the background color. The user will very quickly perceive when the color box moves on the screen and if that's a lot slower than the physical machine which the user can observe simultaneously then the user experience will be suffering. Humans are quite sensitive to time lag in user experience. One could go to 100ms but not 1s.

Anyway, let's say 1s is possible, then the elide delegate from the SO post still prevents the user to edit the value because it repaints the control with the existing value from the model, not the digits the user has entered this far, when a dataChanged() for background color is emitted. Can the delegate detect that user editing is in progress?

ChrisW67

If user editing of a value is in progress then the display within the cell's region of the view is driven by the editor widget that is created when the cell switch to edit mode. You are going to need to post a complete program that demonstrates the new problem.

olowo726

@ChrisW67 Here you are: https://udokaelectronics.com/files/untitled.zip

Comment out

ui->tableView->setItemDelegate(elideDelegate);

To see the result with and without the delegate.

Without the elide delegate all works fine except the elide. With the elide delegate the background overwrites the text and the existing text is continuously redrawn during editing of cell content.

SGaist

Why are you drawing the text and then on top of it call the style's drawControl method with no text ?

What would make more sense is to change the text and elide value of the option and pass the modified option to the base class paint method.

~~Also, why nuke the options object you just got by calling init on it ?~~

Note I have just read the delegate's code you provided.

[edit: options must be updated as done, see base class implementation SGaist]

olowo726

I have tried the straightforward implementation

void ElideDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                          const QModelIndex &index) const {
  if (!index.isValid()) {
    return;
  }

  QStyleOptionViewItem opt = option;
  initStyleOption(&opt, index);

  opt.text= opt.fontMetrics.elidedText(opt.text, Qt::ElideRight, opt.rect.width());
  opt.textElideMode = Qt::ElideNone;

  QApplication::style()->drawControl(QStyle::CE_ItemViewItem, &opt, painter);
}

opt.text is for some reason blank until initStyleOption(&opt, index); returns. Hence "nuking" opt by initStyleOption(&opt, index); is necessary.

With the code above and string "1.234567890" the string gets properly elided by opt.fontMetrics.elidedText to for example "1.2345..." but the view still shows "1....". Note that this problem is only present for positive numbers. It works fine without the delegate for negative numbers.

Substituting QApplication::style()->drawControl(QStyle::CE_ItemViewItem, &opt, painter); with QStyledItemDelegate::paint(painter, opt, index); doesn't help. Perhaps related to that one needs to call initStyleOption() to get the actual text so perhaps QStyledItemDelegate::paint does the same and hence discards all changes to the text.

olowo726

I just discovered by exploratory testing that giving opt.fondMetrics.elidedText some margin seems to do the trick, i.e. replace opt.text = opt.fontMetrics.elidedText(opt.text, Qt::ElideRight,opt.rect.width() ); with opt.text = opt.fontMetrics.elidedText(opt.text, Qt::ElideRight,opt.rect.width() - 10);.

That gives me both background color and elided text. No idea of how portable it is though.

But it doesn't solve the issue with text being overwritten when the model emits dataChanged() with role Backgroundrole when user edit is in progress. Is is possible to somehow detect when user edit is in progress? Override QAbstractItemView:edit() to detect start of user edit, but how detect when user edit is finished? Override closeEditor() and assume there is only one edit active at once?

SGaist

You were right about the use of initStyleOption, I had it at the wrong place.

Can you explain the use case of a user editable value that can be replaced under the hood programmatically while being edited ?

It looks like it should rather be a independent way to input data to avoid collision or an "edit mode" where the other updates are disabled.