GTK TextView auto scroll when text is added to text buffer

Question

I am trying to create a very simple log-like GUI application that merely displays text from a log file dynamically and asynchronously. The problem is that when the log file is updated, the text view in the GUI scrolls back up to line 1. Every attempt to fix this has failed and I am wondering if I have stumbled across a bug in GTK. Here is a summary of my code:

using Cairo;
using Gtk;

namespace ServerManager {
    public class ServerManager : Window {
        public TextView text_view;
        public TextIter myIter;
        public TextMark myMark;

        public async void read_something_async (File file) {
            var text = new StringBuilder ();
            var dis = new DataInputStream (file.read ());
            string line;

            while ((line = yield dis.read_line_async (Priority.DEFAULT)) != null) {
                text.append (line);
                text.append_c('\n');
            }
            this.text_view.buffer.text = text.str;
            text_view.buffer.get_end_iter(out myIter);
            text_view.scroll_to_iter(myIter, 0, false, 0, 0);
        }

        public static int main (string[] args) {
        Gtk.init (ref args);


        var window = new ServerManager ();

        // The read-only TextView
        window.text_view = new TextView ();
        window.text_view.editable = false;
        window.text_view.cursor_visible = false;
        window.text_view.wrap_mode = Gtk.WrapMode.WORD;

        // Add scrolling functionality to the TextView
        var scroll = new ScrolledWindow (null, null);
        scroll.set_policy (PolicyType.AUTOMATIC, PolicyType.AUTOMATIC);
        scroll.add (window.text_view);

        // Vbox so that our TextView has someplace to live
        var vbox = new Box (Orientation.VERTICAL, 0);
        vbox.pack_start (scroll, true, true, 0);
        window.add (vbox);

        window.set_border_width (12);
        window.set_position (Gtk.WindowPosition.CENTER);
        window.set_default_size (800, 600);
        window.destroy.connect (Gtk.main_quit);
        window.show_all ();

        File file = File.new_for_path ("/home/user/temp.log");
        FileMonitor monitor = file.monitor (FileMonitorFlags.NONE, null);
        stdout.printf ("Monitoring: %s\n", file.get_path ());

        monitor.changed.connect (() => {
            window.read_something_async(file);
        });

        Gtk.main ();
        return 0;
        }
    }
}

I also tried using TextMarks instead of Iters but that had no affect.

Answer 1

Scroll to 1st row happens because read_something_async() deletes the current contents of the buffer and then writes the new one (this is what setting the text property does). Maybe this is what you want but unless you keep track of the scroll location you will lose it.

The reason your scroll_to_iter() didn't work as expected is probably this:

Note that this function uses the currently-computed height of the lines in the text buffer. Line heights are computed in an idle handler; so this function may not have the desired effect if it’s called before the height computations. To avoid oddness, consider using gtk_text_view_scroll_to_mark() which saves a point to be scrolled to after line validation.

Calling TextView.ScrollToMark() with a "right gravity" TextMark should work for you.