Function button 

pub fn button(
    input: &InputState,
    ui: &mut UiState,
    theme: &UiTheme,
    label: &str,
    rect: Rect,
) -> UiInteraction

Draw a button and return whether it was clicked.

Example

if ui::button(ctx, "Start Game", Rect::new(100, 200, 200, 50)) {
    start_game();
}