Highlight text that matches a search string in Qt5

Question

First let me explain what I'm trying to achieve:

In your browser, hit Ctrl + f and type "q". The result of that operation is what I'm trying to achieve. It seems like this should be a solved problem, but despite the many hours I've spent researching, reading documentation, and asking around in the Qt IRC, I'm still stuck. Maybe somebody here will be able to give me a hand.

The following Qt classes are what I'm currently dealing with, in case you want to brush up a bit.: Qt::DisplayRole
QModelIndex::data
QAbstractItemView::setDelegate
QTextEdit::setExtraSelections (only as a reference)
QAbstractDocumentLayout::Selection
QTextDocument::drawContents
QStyledItemDelegate::paint
QStyleOptionViewItem
QStyleOption::rect
QTableView
QAbstractItemModel::match
Learn to Model/View Program with Qt

So now, let me explain how I have things set up, and what the results of my research and interactions with the Qt IRC has lead me to believe I should do.

I am using a QStandardItemModel along with a QTableView. Each row that gets appended to the QStandardItemModel has several columns. As we know, each of these columns is represented in the QStandardItemModel as a QModelIndex. We can pull its displayed text by accessing its data(Qt::DisplayRole).

Conveniently, given a search string, QStandardItemModel::match will return a QModelIndexList of every QModelIndex that, well, matched in the column. Of course, if you have several columns in every row, you would need to preform this match over each column, but I can worry about performance here later.

So cool, that problem is solved. We have our list of every QModelIndex that had a matching string. Now, what I want to do is highlight that string in each column. For example:

Search string: "col"
This is a column | This is aColumn | This is aColumn | This is a co

Parts that would appear highlighted are what I have in bold above. In order to achieve this, I knew from reading the Model/View docs that I needed to subclass a QStyledItemDelegate and reimplement its paint function. So I started there.

The next problem to solve would be, how in the world to select a specific piece of text in the DisplayRole and only highlight it? You can't use Qt::BackgroundRole, that would set the entire index's background color. Enter QTextDocument.

I still need to do more digging to see how exactly I can implement that behavior, but from what I was told in Qt IRC, QTextEdit has a function called setExtraSelections. Looking at how that is implemented, it leverages QAbstractTextDocumentLayout::Selection and various cursor functions available to me in QTextDocument.

However, sadly I cannot even begin to solve that problem, because the first step is to actually ensure that I can render a QTextDocument to the QTableView with my reimplemented custom delegate paint function. This is where I'm stuck currently. I've seen this post, here and the one it references: Similar question that doesn't solve my issue

Its not exactly what I was looking for, but I figured it would help me at least get something rendering. It looks though like in his code, he's drawing the control (which is literally what QStyledItemDelegate does), and then tries to paint his QTextDocument over it. Maybe that's not what that is doing but it sure looks like it.

In any case, when I tried that, it looked like the QTextDocument had no effect. If I commented out the drawControl call, then no text would be rendered at all. Here is my code for that:

void CustomDelegate::paint(QPainter* painter, const QStyleOptionViewItem& option, const QModelIndex& index) const 
{
    QStyleOptionViewItem opt = option;
    initStyleOption(&opt, index);

    if (index.column() == contentColumn)
    {
        painter->save();

        QTextDocument contentDocument;
        //opt.widget->style()->drawControl(QStyle::CE_ItemViewItem, &opt, painter);

        opt.text = "Things";
        painter->setBrush(QBrush(Qt::darkCyan));
        contentDocument.drawContents(painter, opt.rect);

        painter->restore();
    }
    else
    {
        QStyledItemDelegate::paint(painter, option, index);
    }
}

You can see I just typed text "Things" to see if I could just get that to render. To no avail.

Before summarizing my question I want to mention that no, I cannot use a QTextEdit. There are specific columns that I need to word wrap, while others I do not want to word wrap. QTableView displays the data exactly as I expect to see it.

So my question is this: Is the QTextDocument rendering through the paint event of a subclassed QStyledItemDelegate the preferred way to handle this? If not, how else should I deal with it?

If so, how do I make it work as I expect? What is wrong with the code that I have?

Bonus 2 parter question Once I can render, how can I actually leverage the QTextDocument API to highlight only specific pieces, given that the list of model indexes that contain the text to be highlighted is discovered in a completely different function and at a different time than when the paint function executes?

Update 0 Using the QTextDocument API to make multiple selections is looking like an impossible feat. The column that I would be drawing contents to needs to word wrap and expand.

From what I can tell, I calling drawContents manually means handling resizing, word wrapping, and many other things manually which is not a path I want to go down if that is indeed the case.

I've got another way in mind, I will update if it works.

Update 1 I have accomplished what I want in another way. Unfortunately, I cannot use Qt for what I want. Qt's Model/View system is entirely too inefficient and getting it to do what I need causes it to be extremely and unacceptably slow. How did I accomplish the highlighting?

I will describe in an answer. If nobody gives me a better answer, I will pick mine as the best.

Answer 1

I believe what you need to do here is override this method in your QItemDelegate subclass:

void QItemDelegate::drawDisplay(QPainter *painter, const QStyleOptionViewItem &option, const QRect &rect, const QString &text) const

The problem with QItemDelegate's implementation of this method is at the end, where we see this method call (it's actually called in two places, but since they are identical calls I'm just showing it once):

d->textLayout.draw(painter, layoutRect.topLeft(), QVector<QTextLayout::FormatRange>(), layoutRect);

It's that third argument that you'd want to change -- in QItemDelegate::drawDisplay(), it's hard-coded to always be an empty QVector, which means that all of the characters in the drawn text-string will be always have the same formatting. If you could somehow get it to be called with a QVector<QTextLayout::FormatRange> that contained your per-substring formatting preferences instead, you would get the effect you want.

Of course, the devil is in the details. If you don't mind hacking the Qt source code to fit your purpose, you could do that; but that would be an ugly solution since it would mean your program would no longer work correctly when compiled with a non-customized Qt release (and you'd have to re-patch every time you upgraded to a new Qt release).

So instead you'd probably want to start by copying the contents of the QItemDelegate::drawDisplay() method over to your subclass's method, then modifying the copied version as necessary. (Of course that has its own problems, since QItemDelegate::drawDisplay() references private member variables that your subclass doesn't have access to, but you can probably work around those. You might also want to check with the Qt people to see if there are any legal problems with copying out a method body like that; I'm not sure if there are or not. If you're not comfortable with copying it, you can at least look at it for inspiration about the sorts of things your implementation of drawDisplay() will probably also need to do)

Answer 2

Essentially, I found that getting the QTableView to display rich text was a common use case that people on forums were trying to accomplish. Since this was a solved problem, I tried to see how I could leverage HTML.

First, I set up my custom delegate to handle rich text. Then I had an algorithm that went a bit like this:

   clear all html tags from the display role in every row of the specified column

for every row in the specified column
    populate a list of QModelIndex with matching text in the Display Role

for every QModelIndex with matching text in the display role
    while there is another occurance of the matching string in the display role
        inject html highlight (span) around the matching word

This is, of course, extremely, painfully, unacceptably slow. It does work though, has the exact same effect as hitting ctrl + f. But, I cannot use it. Its a shame that Qt doesn't support something as basic as this. Oh well.

Answer 3

My approach to that problem was to use the paint function from the delegate to render one or several positions from a cursor in a QTextDocument.

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


    // Higlight some text
    {
        QString dataHighlight QString("col"); // The text to highlight.    
        QString value = index.model()->data(index, Qt::DisplayRole).toString();

        QTextDocument *doc = new QTextDocument(value);
        QTextCharFormat selection;

        int position = 0;
        QTextCursor cur;

        // We have to iterate through the QTextDocument to find ALL matching places
        do {

            cur = doc->find(dataHighlight,position);            
            cur.movePosition(QTextCursor::Left, QTextCursor::KeepAnchor);
            cur.selectionStart();
            cur.movePosition(QTextCursor::Right, QTextCursor::KeepAnchor);
            cur.selectionEnd();
            position = cur.position();
            selection.setBackground(Qt::yellow);
            cur.setCharFormat(selection);

        } while (!cur.isNull());

        painter->save();
        painter->translate(option.rect.x(), option.rect.y());
        doc->drawContents(painter);
        painter->restore();

        delete doc;
    }
}