Rotate a Label in Gtk4

Gtk3 had GtkLabel.set_angle which was removed with Gtk4.

I am aware of this response, however I do not want to have to manually render a pango layout rather than just operating on GtkLabel like before. set_angle did not need any interaction with the underlying pango layout, I want the object rendering the text to stay being a GtkLabel.

I tried the following (in C++ here, though I am using the C binding)

auto* label = GTK_LABEL(gtk_label_new("test"));
auto* layout = gtk_label_get_layout(label);
auto* context = pango_layout_get_context(layout);
auto* matrix = new PangoMatrix(PANGO_MATRIX_INIT);
pango_matrix_rotate(matrix, 90);
pango_context_set_matrix(context, matrix);

Which had no effect, I assume because the labels layout is read-only.

I am also aware of GtkFixed.set_child_transform, which lets me rotate any widget, but only in a fixed context. I want the label to align normally, respecting it’s halign and valign properties. The only way to apply a transform to a widget I could find was through GtkFixed.

The Pango attributes related to gravity almost got me there, however they only rotate the letters, not the entire text:

auto* label = GTK_LABEL(gtk_label_new("<span gravity=\"east\" gravity_hint=\"strong\">test</span>");

Prints letter horizontally, with each letter rotate by 90°. I want the entire label to be rotated by 90° (or an arbitrary number of degrees), not just every letter.

How do I rotate a GtkLabel around it’s centroid in Gtk4? A code snippet would help a lot, thank you in advance.

If a static approach is enough for you, you can use CSS to rotate the label and let Gsk do the underlying work for you.

I think the following approach will do it: Add a style class to the StyleContext of the label by calling get_style_context and gtk_style_context_add_class and then provide the style with gtk_style_context_add_provider.

If you choserotated as style class name the appropriate css string to rotate the label would be

label.rotated { 	transform: rotate(90deg); }

It is easy to test the effects of the transformation without editing code by invoking the GTK Inspector. See the following picture

You can also use gtk_snapshot_rotate in the snapshot function of the parent widget if you need more control than what CSS gives you. Note the transform should also be rotated when calling gtk_widget_allocate if you want the input coordinates to be correct.

And you’ll need to change the measure() function as well, to provide the correctly transformed width and height.