监听js变量变化

Question

假设在 地方 2

有一个代码

var place2IsReady = true;

在地方1我需要实现下面的逻辑：

Once place2IsReady value was changed (to true) then display alert('ready!');

备注：

place2IsReady 变量在 place 1.
来自 place 1 的代码在 place 2 执行之前执行（或者存在竞争条件）。

解决方案 1

我相信我可以改用 window.place2IsReady 并在位置 1 中使用 setTimeout/setInterval 直到我得到 window.place2IsReady === true.

有更好的选择吗？使用听众？关于变量的变化？

P.S。我只需要跟踪 place2IsReady 的第一个可能的变化。

有没有更好的方法？谢谢。

Answer 1

您可以使用 setTimeout 为变量更改创建侦听器，例如：

let place2IsReady = false;

setReadyListener();

// testing wait 2 seconds to set place2IsReady to true
// so: an alert should occur after 2 seconds
setTimeout(() => place2IsReady = true, 2000);

function setReadyListener() {
  const readyListener = () => {
    if (place2IsReady) {
      return alert("Ready!");
    }
    return setTimeout(readyListener, 250);
  };
  readyListener();
}

更通用的侦听器 'factory' 可以是：

let place2IsReady = false;
let fromObj = {
  place2IsReady: "busy",
  done() { this.place2IsReady = "done"; },
};
const listen = changeListenerFactory();

listen(
  () => place2IsReady, 
  () => console.log("place2IsReady") );
listen(
  () => fromObj.place2IsReady === "done", 
  () => console.log("formObj.place2IsReady done!") );
  
console.log("Listening...");

// test change variables with listeners
setTimeout(() => place2IsReady = true, 1000);
setTimeout(() => fromObj.done(), 3000);

function changeListenerFactory() {
  const readyListener = (condition, callback, delay) => {
    if (!condition || typeof condition !== "function") { return true; }
    if (condition()) {
      return callback();
    }
    setTimeout(() => readyListener(condition, callback, delay), delay);
  };
  
  return (condition, callback = () => {}, delay = 250) => 
    readyListener(condition, callback, delay);
}

或者使用 Proxy（带有 设置陷阱 ）可能对你有效

const readyState = new Proxy({ ready: false }, { 
  set (target, prop, val) {
    console.log(`readyState.ready changed from ${target[prop]} to ${val}`);
    target[prop] = val;
  }
});

console.log("Waiting for changes ...");
setTimeout(() => readyState.ready = true, 2000);

Answer 2

假设您可以用对象替换 place2IsReady：

place2IsReady = {
  state: false,
  set ready(value) {
      this.state = value
      state && place_1_call()
  },
  get ready() { 
    return state
  }
}

place_1_call = () => {
  alert('ready')
}

place2IsReady.ready = true

监听js变量变化

Listen to js variable change

javascript

typescript

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