Centering arbitrary UIs in egui

egui_centerer.rs

  // Helper function to center arbitrary widgets. It works by measuring the width of the widgets after rendering, and
  // then using that offset on the next frame.
  fn centerer(ui: &mut Ui, add_contents: impl FnOnce(&mut Ui)) {
    ui.horizontal(|ui| {
      let id = ui.id().with("_centerer");
      let last_width: Option<f32> = ui.memory_mut(|mem| mem.data.get_temp(id));
      if let Some(last_width) = last_width {
        ui.add_space((ui.available_width() - last_width) / 2.0);
      }
      let res = ui
        .scope(|ui| {
          add_contents(ui);
        })
        .response;
      let width = res.rect.width();
      ui.memory_mut(|mem| mem.data.insert_temp(id, width));

      // Repaint if width changed
      match last_width {
        None => ui.ctx().request_repaint(),
        Some(last_width) if last_width != width => ui.ctx().request_repaint(),
        Some(_) => {}
      }
    });
  }