trait ListContentProvider<Item> {

    /// Get the number of items provided
    fn get_count(&self) -> i64;

    fn get_item(&self, index: i64) -> Item;
}

trait ListLabelProvider<Item> {

    fn get_label(&self, item: &Item) -> str;
}

trait CellRenderer {
    fn new() -> Self;

    /// Get the height of a single cell
    fn height(&self) -> i64;
}


struct ListView<'a, Item, R: CellRenderer> {
    content_provider: &'a dyn ListContentProvider<Item>,
    label_provider: &'a dyn ListLabelProvider<Item>,
    cell_renderer: R,
    x: i64,
}



/// A ListView is responsible for:
/// - get number of objects from the content provider
/// - manage the range of visible objects
/// - retrieve objects to be rendered from the content provider
///
impl<'a, Item, R: CellRenderer> ListView<'a, Item, R> {
    pub fn new(content_provider: &'a dyn ListContentProvider<Item>, label_provider: &'a dyn ListLabelProvider<Item>) -> Self {
        ListView::<'a, Item, R> {
            content_provider,
            label_provider,
            cell_renderer: CellRenderer::new(),
            x: 0,
        }
    }

    pub fn refresh(&mut self) {
        let list_height = self.content_provider.get_count() * self.cell_renderer.height();
        if self.x > list_height {
            self.x = list_height;
        }
    }

    pub fn top_item(&self) -> Item {
        let index = self.x / self.cell_renderer.height();
        self.content_provider.get_item(index)
    }

    /// move the viewport on the list by the given amount
    pub fn move_by(&mut self, delta: i64) {
        self.x += delta;
        self.refresh();
    }

}