Scrolling to the bottom of the TextView in gtkmm

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The layout is the following: There is a Gtk::ScrollWindow and inside of it is Gtk::TextView, the latter is of a derived class called TextArea.

As a test, there is a button that adds some texgt to the TextView one line at the time and attempts to immediately scroll to the bottom.

The code:

void MainWindow::onButtonPress()
{
    Glib::RefPtr<Gtk::TextBuffer::Tag> boldTag = Gtk::TextBuffer::Tag::create();
    boldTag->property_weight()=800;

    textarea->get_buffer()->get_tag_table()->add(boldTag);

    textarea->get_buffer()->insert_with_tag(textarea->get_buffer()->end(), "\ntest text", boldTag);

    auto iter = textarea->get_buffer()->end();
    iter.set_line_offset(0);
    textarea->scroll_to(iter);
}

The funny thing is that the scrolling does take place, but not to the last line, only to the line second to last. A silly attempt to add -1 as the offset creates an instant error, since the value must be non-negative.

gtkmm3 gtkmm gtk3 gtk c++
1 Answers

I believe your problem is that you use an iterator to move across the buffer in which you are inserting text. As the documentation suggests:

Iterators are not valid indefinitely; whenever the buffer is modified in a way that affects the number of characters in the buffer, all outstanding iterators become invalid.

Instead of using iterators, I suggest using a Gtk::TextBuffer::Mark that refers to the end of the buffer. Unlike iterators, marks represent:

A position in the buffer, preserved across buffer modifications.

The Gtk::TextView widget also has overloads of its scroll_to method that deal with marks, one of which is:

void Gtk::TextView::scroll_to(const Glib::RefPtr< TextBuffer::Mark >& mark,
                              double within_margin = 0 
                             )

You can see it in action in the following simplified example:

#include <iostream>
#include <string>
#include <gtkmm.h>

// Move mark to the right of newly added text (see below):
constexpr bool RIGHT_GRAVITY = false;

class MyWindow : public Gtk::ApplicationWindow
{

public:

    MyWindow()
    {
        m_button.signal_clicked().connect([this](){OnButtonPressed();});

        // Create a mark that "points" to the end of the buffer. This
        // mark will be updated accordinly as the buffer is modified:
        m_endMark = m_textArea.get_buffer()->create_mark(m_textArea.get_buffer()->end(), RIGHT_GRAVITY);

        m_scrolledWindow.add(m_textArea);

        m_layout.attach(m_scrolledWindow, 0, 0, 1, 1);
        m_layout.attach(m_button, 0, 1, 1, 1);

        add(m_layout);
    }

    void OnButtonPressed()
    {
        // Insert new line at the end of the Gtk::TextView:
        static int lineNumber = 0;
        m_textArea.get_buffer()->insert(m_textArea.get_buffer()->end(), "\n" + std::to_string(lineNumber) +" - test text");
        ++lineNumber;

        // Scroll down to the mark:
        m_textArea.scroll_to(m_endMark);
    }

private:

    Gtk::Grid m_layout;

    Gtk::ScrolledWindow m_scrolledWindow;
    Gtk::TextView m_textArea;

    Glib::RefPtr<Gtk::TextBuffer::Mark> m_endMark;

    Gtk::Button m_button{"Add line at the end..."};

};

int main(int argc, char* argv[]) 
{
    auto app = Gtk::Application::create(argc, argv, "so.question.q66329582");

    MyWindow window;
    window.show_all();

    return app->run(window);
}
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