Function application 

pub fn application<State, Message, Theme, Renderer>(
    boot: impl BootFn<State, Message>,
    update: impl UpdateFn<State, Message>,
    view: impl for<'a> ViewFn<'a, State, Message, Theme, Renderer>,
) -> Application<impl Program<Message = Message, Theme = Theme, State = State>>
where
    State: 'static,
    Message: 'static + TryInto<UnLockAction, Error = Message> + Send + Debug,
    Theme: DefaultStyle,
    Renderer: Renderer,

The renderer of iced program.