事件处理：永久发送按键事件

问：

我只想在黑框中画一些对象（白色方块）并处理一些事件（按下左、下、右、上箭头）。当按下其中一些键时，物体会沿相应方向移动。但是窗口一遍又一遍地获得相同的信号，直到按下另一个键。我建议，我们有一些事件缓冲区来保存事件和函数 event_key（） 返回上次发生的事件。但是如何处理一次事件，直到再次按下该键。

use fltk::draw::*;
use fltk::{prelude::*, *};
use std::cell::RefCell;
use std::rc::Rc;

fn main() {
    let app = app::App::default(); //create app
    let mut win = window::Window::default() //create window
        .with_size(350, 350);
    let mut frame = frame::Frame::default() //create frame
        .size_of_parent();

    let ww = win.w() / 2; //position of white square in thr middle of width
    let wh = win.h() / 2; //position of white square in the muddle of height
    let offset = Rc::new(RefCell::new(0)); //offset of white square
    let offset1 = offset.clone();

    //drawning wgite square
    frame.draw(move |f| {
        let offset = offset1.borrow();
        draw_rect_fill(0, 0, f.w(), f.h(), enums::Color::Black);
        draw_rect_fill(ww + *offset, wh, 10, 10, enums::Color::White);
    });

    //handle key press events
    frame.handle(move |frame, _| {
        use fltk::enums::Key;

        let mut offset = offset.borrow_mut();
        let key = app::event_key();

        match key {
            Key::Right => {
                //the right arrow is pressed
                println!("{:?}", key);
                *offset += 1;
                frame.redraw();
                app::sleep(0.0500);
                true
            }
            Key::Left => {
                //the left arrow is pressed
                println!("left");
                true
            }
            Key::Up => {
                //the up arrow is pressed
                println!("Up");
                true
            }
            Key::Down => {
                //the down arrow is presed
                println!("Down");
                true
            }
            _ => false, //ignore other events
        }
    });

    win.end();
    win.show();
    app.run().unwrap();
}