QTextEdit and cursor interaction - qt

Question

I'm modifying the Qt 5 Terminal example and use a QTextEdit window as a terminal console. I've encountered several problems.
Qt does a strange interpretation of carriage return ('\r') in incoming strings. Ocassionally, efter 3-7 sends, it interprets ('\r') as new line ('\n'), most annoying. When I finally found out I choose to filter out all '\r' from the incoming data.
Is this behaviour due to some setting?
Getting the cursor interaction to work properly is a bit problematic. I want the console to have autoscroll selectable via a checkbox. I also want it to be possible to select text whenever the console is running, without losing the selection when new data is coming.
Here is my current prinout function, that is a slot connected to a signal emitted as soon as any data has arrived:
void MainWindow::printSerialString(QString& toPrint)
{
static int cursPos=0;
//Set the cursorpos to the position from last printout
QTextCursor c = ui->textEdit_console->textCursor();
c.setPosition(cursPos);
ui->textEdit_console->setTextCursor( c );
ui->textEdit_console->insertPlainText(toPrint);
qDebug()<<"Cursor: " << ui->textEdit_console->textCursor().position();
//Save the old cursorposition, so the user doesn't change it
cursPos= ui->textEdit_console->textCursor().position();
toPrint.clear();
}
I had the problem that if the user clicked around in the console, the cursor would change position and the following incoming data would end up in the wrong place. Issues:
If a section is marked by the user, the marking would get lost when new data is coming.
When "forcing" the pointer like this, it gets a rather ugly autoscroll behaviour that isn't possible to disable.
If the cursor is changed by another part of the program between to printouts, I also have to record that somehow.
The append function which sound like a more logical solution, works fine for appending a whole complete string but displays an erratic behaviour when printing just parts of an incoming string, putting characters and new lines everywhere.
I haven't found a single setting regarding this but there should be one? Setting QTextEdit to "readOnly" doesn't disable the cursor interaction.
3.An idea is to have two cursors in the console. One invisible that is used for printouts and that is not possible at all to manipulate for the user, and one visible which enables the user to select text. But how to do that beats me :) Any related example, FAQ or guide are very appreciated.

Answer 1

I've done a QTextEdit based terminal for SWI-Prolog, pqConsole, with some features, like ANSI coloring sequences (subset) decoding, command history management, multiple insertion points, completion, hinting...
It runs a nonblocking user interface while serving a modal REPL (Read/Eval/Print/Loop), the most common interface for interpreted languages, like Prolog is.
The code it's complicated by the threading issues (on user request, it's possible to have multiple consoles, or multiple threads interacting on the main), but the core it's rather simple. I just keep track of the insertion point(s), and allow the cursor moving around, disabling editing when in output area.
pqConsole it's a shared object (I like such kind of code reuse), but for deployment, a stand-alone program swipl-win is more handy.
Here some selected snippets, the status variables used to control output are promptPosition and fixedPosition.
/** display different cursor where editing available
*/
void ConsoleEdit::onCursorPositionChanged() {
QTextCursor c = textCursor();
set_cursor_tip(c);
if (fixedPosition > c.position()) {
viewport()->setCursor(Qt::OpenHandCursor);
set_editable(false);
clickable_message_line(c, true);
} else {
set_editable(true);
viewport()->setCursor(Qt::IBeamCursor);
}
if (pmatched.size()) {
pmatched.format_both(c);
pmatched = ParenMatching::range();
}
ParenMatching pm(c);
if (pm)
(pmatched = pm.positions).format_both(c, pmatched.bold());
}
/** strict control on keyboard events required
*/
void ConsoleEdit::keyPressEvent(QKeyEvent *event) {
using namespace Qt;
...
bool accept = true, ret = false, down = true, editable = (cp >= fixedPosition);
QString cmd;
switch (k) {
case Key_Space:
if (!on_completion && ctrl && editable) {
compinit2(c);
return;
}
accept = editable;
break;
case Key_Tab:
if (ctrl) {
event->ignore(); // otherwise tab control get lost !
return;
}
if (!on_completion && !ctrl && editable) {
compinit(c);
return;
}
break;
case Key_Backtab:
// otherwise tab control get lost !
event->ignore();
return;
case Key_Home:
if (!ctrl && cp > fixedPosition) {
c.setPosition(fixedPosition, (event->modifiers() & SHIFT) ? c.KeepAnchor : c.MoveAnchor);
setTextCursor(c);
return;
}
case Key_End:
case Key_Left:
case Key_Right:
case Key_PageUp:
case Key_PageDown:
break;
}
you can see that most complexity goes in keyboard management...
/** \brief send text to output
*
* Decode ANSI terminal sequences, to output coloured text.
* Colours encoding are (approx) derived from swipl console.
*/
void ConsoleEdit::user_output(QString text) {
#if defined(Q_OS_WIN)
text.replace("\r\n", "\n");
#endif
QTextCursor c = textCursor();
if (status == wait_input)
c.setPosition(promptPosition);
else {
promptPosition = c.position(); // save for later
c.movePosition(QTextCursor::End);
}
auto instext = [&](QString text) {
c.insertText(text, output_text_fmt);
// Jan requested extension: put messages *above* the prompt location
if (status == wait_input) {
int ltext = text.length();
promptPosition += ltext;
fixedPosition += ltext;
ensureCursorVisible();
}
};
// filter and apply (some) ANSI sequence
int pos = text.indexOf('\x1B');
if (pos >= 0) {
int left = 0;
...
instext(text.mid(pos));
}
else
instext(text);
linkto_message_source();
}
I think you should not use a static variable (like that appearing in your code), but rely instead on QTextCursor interface and some status variable, like I do.

Answer 2

Generally, using a QTextEdit for a feature-rich terminal widget seems to be a bad idea. You'll need to properly handle escape sequences such as cursor movements and color mode settings, somehow stick the edit to the top-left corner of current terminal "page", etc. A better solution could be to inherit QScrollArea and implement all the needed painting–selection-scrolling features yourself.
As a temporary workaround for some of your problems I can suggest using ui->textEdit_console->append(toPrint) instead of insertPlainText(toPrint).
To automatically scroll the edit you can move the cursor to the end with QTextEdit::moveCursor() and call QTextEdit::ensureCursorVisible().