如何在 Swing-GUI 和 WebApplication (如 Vaadin 或本机 JS)之间同步倒计时

Question

我正在尝试将倒计时从Java应用程序同步到浏览器。倒计时可以随时停止、启动和重置。

我尝试在 Vaadin 13 中实现这一点，但无法访问 UI 访问 Mehtod 以锁定 vaadin 会话。现在我正在尝试使用本机 JS 和 Ajax 请求实现这一点，但我不确定如何在不每秒发出 ajax 请求的情况下同步停止/启动和重置事件。

这是计数器的摆动实现

public void timer() {
Timer timer = new Timer(1000, new ActionListener() {
public void actionPerformed(ActionEvent e) {
if (seconds == 0  && minutes > 0) {
minutes--;
seconds = 59;   
} else {
seconds--;
}
label.setText(minutes+":"+seconds);
repaint();
}
});
timer.start();
}

现在，我将为JS代码提供一个Spring Boot Rest API，以请求剩余的分钟和秒数。

setInterval(test, 1000);
async function test() {
var xhttp = new XMLHttpRequest();
xhttp.open("GET", "http://10.0.1.17/countdown", false);
xhttp.send();
//console.log(JSON.parse(xhttp.responseText));
//Do Something with it
}

这似乎是一种不可靠且效率低下的方式

Answer 1

看看这个类，从我写的另一个答案

/*
a (pausable) linear equation over real time
value = _speed * Date.now() + _offset;
//when paused, it's simply: 
value = _offset;
so basically a clock, a stopwatch, a countdown, a gauge, ...
since it is only a linear equation over time, it is independant of any interval.
It computes the value (using Date.now()) whenever you ask for it. Wether this is ever frame or every hour.
*/
class Clock {
constructor(value=Date.now(), speed=1){
//state; changes only when YOU set one of the properties (value, paused or speed)
this._offset = +value || 0;
this._speed = +speed || 0;
this._paused = true;
//preparing a simple hook to get notified after the state has been updated (maybe to store the new state in the localStorage)
this.onStateChange = undefined;
}
get value(){ 
return this._paused? this._offset: this._speed*Date.now() + this._offset 
}
set value(arg){
let value = +arg || 0;
let offset = this._paused? value: value - this._speed * Date.now();
if(this._offset !== offset){
this._offset = offset;
if(typeof this.onStateChange === "function") 
this.onStateChange(this);
}
}
get speed(){
return this._speed
}
set speed(arg){
let speed = +arg || 0;
if(this._speed !== speed){
if(!this._paused)
this._offset += Date.now() * (this._speed - speed);
this._speed = speed;
if(typeof this.onStateChange === "function")
this.onStateChange(this);
}
}
get paused(){
return this._paused
}
set paused(arg){
let pause = !!arg;
if(this._paused !== pause){
this._offset += (pause? 1: -1) * this._speed * Date.now();
this._paused = pause;
if(typeof this.onStateChange === "function")
this.onStateChange(this);
}
}
time(){
let value = this.value,v = Math.abs(value);
return {
value,
//sign: value < 0? "-": "",
seconds: Math.floor(v/1e3)%60,
minutes: Math.floor(v/6e4)%60,
hours: Math.floor(v/36e5)%24,
days: Math.floor(v/864e5)
}
}
valueOf(){
return this.value;
}   
start(){
this.paused = false;
return this;        
}
stop(){
this.paused = true;
return this;
}
}

我展示这个是因为如果你仔细观察它，你会发现这个东西的整个状态由两个数字和一个布尔值组成，它们只有在你做某事时才会改变，比如启动/停止它。

实际值是根据此状态和计算机内部时钟计算的。

因此，如果在前端和后端之间同步此状态，它们(大部分)都同步运行。

为什么是大多数？因为在另一端收到新状态之前几乎没有延迟。对于这几毫秒，两者不同步。一旦另一端更新了其状态，它们就会再次同步。