Question

I want to indent text of a QPlainTextEdit whan a menu button is pressed. When the button is pressed I ask if there is a selection if not I just indent the current line, if there is I want to indent all the lines in the selection. Right now the code works for the single line, but when indenting a selection is like the last part of the line disappears. For example, if I have the line: "Artificial Intelligence stands no chance against Natural Stupidity.", after the indentation it’s just: " Artificial Intelligence stands no chance against Natural Stupidi and after that if I start writing in that line the text starts to disappear when it reaches what now is the end of the sentence. Also, the program crashes if I click or put the cursor in that line after the part of the sentence that disappear.

The code:

void MainWindow::on_action_Indent_triggered()
{
    Document* doc = dynamic_cast<Document*>(ui->tabsManager->currentWidget());
    QTextCursor cursor = doc->textArea->textCursor();
    cursor.beginEditBlock();

    // If ther is no text selected...
    if (cursor.selection().isEmpty()) {
        cursor.movePosition(QTextCursor::StartOfLine);
        cursor.insertText(this->tabLength);
    } else { // If the selection is not empty...
        cursor.beginEditBlock();

        // Save selection start and end
        int start = cursor.selectionStart();
        int end = cursor.selectionEnd();
        cursor.clearSelection();

        // Set end to the end of line of the selected line
        cursor.setPosition(end);
        cursor.movePosition(QTextCursor::EndOfLine);
        end = cursor.position();

        // Set cursor to the start of the first selected line
        cursor.setPosition(start);
        cursor.movePosition(QTextCursor::StartOfLine);
        start = cursor.position();

        // While still in the selection, add "    " to the start of each line
        do {
            cursor.movePosition(QTextCursor::StartOfLine);
            cursor.insertText(this->tabLength);
            end += this->tabLength.count();
            cursor.movePosition(QTextCursor::EndOfLine);
        } while (cursor.position() < end && cursor.movePosition(QTextCursor::Down));

        // Select the changed areatabLenght
        cursor.clearSelection();
        cursor.setPosition(start);
        while (cursor.position() < end)
            cursor.movePosition(QTextCursor::NextCharacter, QTextCursor::KeepAnchor);
    }
    // Set the cursor in the GUI
    doc->textArea->setTextCursor(cursor);
    cursor.endEditBlock();
}

Document is a Class and textArea is a QTextPlainEdit. this->tabLength is a QString with a value of ” “

Answer 1

The problem is quite simple: you’re calling beginEditBlock() more times than you call endEditBlock(). Remove the beginEditBlock() call right after } else {. I could reproduce this problem, and matching [begin|end]EditBlock() calls indeed fixes it.

Below is a self-contained example.

# indenttest.pro
CONFIG += qt gui
SOURCES += indenttest.cpp

// indenttest.cpp
#include <cmath>
#include <QWidget>
#include <QVBoxLayout>
#include <QPlainTextEdit>
#include <QPushButton>
#include <QTextCursor>
#include <QTextDocumentFragment>
#include <QApplication>

//

class QPlainTextEdit;
class MainWindow : public QWidget
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);

public slots:
    void on_indent();

private:
    const QString tabLength;
    QPlainTextEdit * textArea;
};

//

MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent),
    tabLength("    ")
{
    QVBoxLayout * layout = new QVBoxLayout(this);
    QPushButton * btn = new QPushButton("Indent", this);
    layout->addWidget(btn);
    textArea = new QPlainTextEdit(this);
    textArea->setPlainText("foo\nbar\nbaz\nbat");
    layout->addWidget(textArea);
    connect(btn, SIGNAL(clicked()), SLOT(on_indent()));

}

void MainWindow::on_indent()
{
    QTextCursor cursor = textArea->textCursor();
    cursor.beginEditBlock();

    // If ther is no text selected...
    if (cursor.selection().isEmpty()) {
        cursor.movePosition(QTextCursor::StartOfLine);
        cursor.insertText(this->tabLength);
    } else { // If the selection is not empty...
        //cursor.beginEditBlock();

        // Save selection start and end
        int start = cursor.selectionStart();
        int end = cursor.selectionEnd();
        //cursor.clearSelection();

        // Set end to the end of line of the selected line
        cursor.setPosition(end);
        cursor.movePosition(QTextCursor::EndOfLine);
        end = cursor.position();

        // Set cursor to the start of the first selected line
        cursor.setPosition(start);
        cursor.movePosition(QTextCursor::StartOfLine);
        start = cursor.position();

        // While still in the selection, add "    " to the start of each line
        do {
            cursor.movePosition(QTextCursor::StartOfLine);
            cursor.insertText(this->tabLength);
            end += this->tabLength.count();
            cursor.movePosition(QTextCursor::EndOfLine);
        } while (cursor.position() < end && cursor.movePosition(QTextCursor::Down));

        // Select the changed areatabLenght
        cursor.clearSelection();
        cursor.setPosition(start);
        while (cursor.position() < end)
            cursor.movePosition(QTextCursor::NextCharacter, QTextCursor::KeepAnchor);
    }
    // Set the cursor in the GUI
    textArea->setTextCursor(cursor);
    cursor.endEditBlock();
}

int main(int argc, char** argv)
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

#include "indenttest.moc"