Which Line Is It, Anyway?

The editview module is getting pretty complete. The only missing function is the dreaded syntax-highlighter to highlight scannos or spelling errors. Here's what it looks like now.

Today I added the code to highlight the current line. That's why one line has a sort of pale-lemon background. In V1, there was no current line highlight, and it was quite easy to lose sight of the cursor, and have to rattle the arrow keys to find it. (The string shown in dark gray is selected text and is actually bright yellow; the Grab utility did something to the colors.)

Qt's method of doing this was surprising to me.

In a QPlainTextEdit, there is a 1:1 correspondence between text blocks and logical lines. Each line of text is in one QTextBlock. Now, QTextBlock has a property blockFormat which is a QTextBlockFormat, which is itself a QTextCharFormat derivative, i.e. it can be used to set the font, color, background brush and so on. So when I started looking at how to make the current line a different color, I saw this and supposed it would be a matter of, each time the cursor moved:

• Get the text block containing the cursor, a single method call,
• Clear the background brush of the previous line's text block,
• Set the current text block's blockFormat to a different background brush

But in fact QTextBlock lacks anything like a setBlockFormat, so the property is read-only. And setting the background property of the returned QTextBlockFormat object was accepted but had no visible effect.

Sigh, back to the googles to find a number of places in the Qt docs, stackoverflow and the like, where the question is raised and answered.

QPlainTextEdit supports a property extraSelections, which is a list of QTextEdit::ExtraSelection objects. This is the first and I think only time I've seen a class documented as child of another class. And it's a weird little class; it has no methods (not even a constructor), just two properties, cursor and format. So it's basically the C++ version of a python tuple.

What you do is, you get a QTextCursor to select the entire line, and you build an ExtraSelection object with that cursor and the QTextCharFormat you want to use, and assign that to the edit object's list of extra selections. This is a lot of mechanism to just highlight one line. Apparently the intent is to support an IDE that, for example, wants to put a different color on each line set as a breakpoint, or such.

Note: The following is not the correct way to set a current-line highlight. Do not emulate this code. See this post for the problem with it and a later post for the correct approach.

Anyway for the curious, this is the code that executes every bloody time the cursor moves:

    def _cursor_moved(self):
        tc = QTextCursor(self.Editor.textCursor())
        self.ColNumber.setText(str(tc.positionInBlock()))
        tb = tc.block()
        ln = tb.blockNumber()+1 # block #s are origin-0, line #s origin-1
        if ln != self.last_line_number:
            self.last_line_number = ln
            self.LineNumber.setText(str(ln))
            tc.movePosition(QTextCursor.EndOfBlock)
            tc.movePosition(QTextCursor.StartOfBlock,QTextCursor.KeepAnchor)
            self.current_line_thing.cursor = tc
            self.Editor.setExtraSelections([self.current_line_thing])
            pn = self.page_model.page_index(tc.position())
            if pn is not None : # the page model has info on this position
                self.ImageFilename.setText(self.page_model.filename(pn))
                self.Folio.setText(self.page_model.folio_string(pn))
            else: # no image data, or positioned above page 1
                self.ImageFilename.setText('')
                self.Folio.setText('')

In sequence this does as follows:

• Get a copy of the current edit cursor. A copy because we may mess with it later.
• Set the column number in the column number widget.
• Get the QTextBlock containing the cursor's position property (note 1 below).
• Get the line number it represents.
• If this block is a change from before (note 2):
• Set the line number in the line number widget.
• Make the cursor selection be the entire line ("click" at the end, "drag" to the front)
• Set that cursor in a single ExtraSelection object we keep handy.
• Assign that object as a list of one item to the editor's extra selections.
• Get the filename of the current image file, if any; and if there is one, display it and the logical folio for that page in the image and folio widgets.

Note 1: If there's no selection, a text cursor's position is just where the cursor is. But if the user has made a selection, the position property might be at either end of it. Drag from up-left toward down-right and the position is the end of the selection. Drag the other way, it's at the start. Drag a multi-line selection that starts and ends in mid-line. One of the lines will have the faint current-line highlight: the top line if you dragged up, the bottom line if you dragged down. I don't think anyone will notice, or care if they do. I could add code to set the current line on min(tc.position(),tc.anchor())—but I won't.

Note 2: Initially, there was no "if ln != self.last_line_number" test; everything was done every time the cursor moved. And actually performance was fine. But I just could not stand the idea of all that redundant fussing about happening when it didn't have to.