Force GtkLabel to clip its center-aligned text

Question

I`ve got a GtkLabel whose text is to remain centered regardless of whether it is shorter or longer than the label.

For example, a Win32 static control that has the SS_CENTER style flag set behaves like that:

             ┌===========================┐
             │     Lorem ipsum dolor     │
             └===========================┘

— when the text is shorter than the control;

             ┌===========================┐
  Lorem ipsum│dolor sit amet, consectetur│adipiscing
             └===========================┘

— when the text is longer than the control.

N.B.: The only part of the text seen by the user is inside the frame.

I expected GtkLabel-s to do the same thing, but actually they render centered text differently:

             ┌===========================┐
             │     Lorem ipsum dolor     │
             └===========================┘

— when the text is shorter than the control;

             ┌===========================┐
             │Lorem ipsum dolor sit amet,│consectetur adipiscing
             └===========================┘

— when the text is longer than the control.

How do I make centered text in a GtkLabel remain centered even when it is long?

Just in case: the actual text the user sees is updated at ~4 FPS and is unknown prior to runtime.

Answer 1

That may be doable, but showing a semi-random part of a sentence doesn't really make sense from a user experience point of view.

Maybe give a look to gtk_label_set_ellipsize? They use a PangoEllipsizeMode to tell which part of the sentence to hide. You can then hide the beginning, middle, or end part of the label. It's an enum though, not a flag, so you won't be able to hide beginning + end to show only the middle part.

So the possible solutions I see is:

• subclass GtkLabel and do the drawing yourself, possibly with PangoCairo
• or add new wrap mode in GTK+ and implement it (inside pango? PangoCairo? Both?)

If this is not a strong requirement, just use a saner comportment and use existing elipsize modes.

Answer 2

At last the solution is ready.

#include <gtk/gtk.h>

struct SCTX {
    GtkWidget *text;
    GdkRectangle rect;
};

void Resize(GtkWidget *view, GdkRectangle *rect, gpointer user) {
    struct SCTX *sctx = user;
    GtkRequisition requ;

    if ((sctx->rect.width  != rect->width )
    ||  (sctx->rect.height != rect->height)) {
        sctx->rect = *rect;
        gtk_widget_size_request(sctx->text, &requ);
        gtk_layout_move(view, sctx->text, (rect->width  - requ.width ) / 2,
                                          (rect->height - requ.height) / 2);
    }
}

int main(int argc, char *argv[]) {
    GtkWidget *hwnd, *view;
    struct SCTX sctx = {0};

    gtk_init(&argc, &argv);
    hwnd = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(hwnd, "destroy", gtk_main_quit, 0);

    view = gtk_layout_new(0, 0);
    g_signal_connect(view, "size-allocate", G_CALLBACK(Resize), &sctx);
    gtk_widget_set_size_request(view, 320, 200);

    sctx.text = gtk_label_new("Lorem ipsum dolor sit amet, consectetur "
                              "adipiscing elit, sed do eiusmod tempor "
                              "incididunt ut labore et dolore magna aliqua");
    gtk_container_add(view, sctx.text);

    gtk_container_add(hwnd, view);
    gtk_container_set_border_width(GTK_CONTAINER(hwnd), 32);
    gtk_window_set_position(GTK_WINDOW(hwnd), GTK_WIN_POS_CENTER);
    gtk_widget_show_all(hwnd);
    gtk_main();
    return 0;
}

The crucial part here is wrapping a GtkLayout around our GtkLabel so the former could move the latter around inside its bounds.