[英]How can I make a program wait for a variable change in javascript?
我想强制 JavaScript 程序在其执行的某些特定点等待,直到变量发生更改。 有没有办法做到这一点? 我已经找到了一个名为“narrative JavaScript”的扩展,它强制程序等待事件发生。 有没有办法创建一个新事件,例如一个“变量更改事件”,其行为类似于 onclick 事件..
2018 年编辑:请查看Object getter 和 setter以及Proxies 。 下面的旧答案:
一个快速简单的解决方案是这样的:
var something=999;
var something_cachedValue=something;
function doStuff() {
if(something===something_cachedValue) {//we want it to match
setTimeout(doStuff, 50);//wait 50 millisecnds then recheck
return;
}
something_cachedValue=something;
//real action
}
doStuff();
JavaScript 解释器是单线程的,所以当代码在其他不改变变量的代码中等待时,变量永远不会改变。
在我看来,将变量包装在某种具有 getter 和 setter 函数的对象中将是最好的解决方案。 然后可以在调用对象的 setter 函数时调用的对象中注册回调函数。 然后您可以在回调中使用 getter 函数来检索当前值:
function Wrapper(callback) {
var value;
this.set = function(v) {
value = v;
callback(this);
}
this.get = function() {
return value;
}
}
这可以像这样轻松使用:
<html>
<head>
<script type="text/javascript" src="wrapper.js"></script>
<script type="text/javascript">
function callback(wrapper) {
alert("Value is now: " + wrapper.get());
}
wrapper = new Wrapper(callback);
</script>
</head>
<body>
<input type="text" onchange="wrapper.set(this.value)"/>
</body>
</html>
我会推荐一个包装器来处理被改变的值。 例如,您可以拥有 JavaScript 函数,如下所示:
function Variable(initVal, onChange)
{
this.val = initVal; //Value to be stored in this object
this.onChange = onChange; //OnChange handler
//This method returns stored value
this.GetValue = function()
{
return this.val;
}
//This method changes the value and calls the given handler
this.SetValue = function(value)
{
this.val = value;
this.onChange();
}
}
然后您可以从中创建一个对象,该对象将保存您想要监视的值,以及一个将在值更改时调用的函数。 例如,如果您想在值更改时收到警报,并且初始值为 10,您可以编写如下代码:
var myVar = new Variable(10, function(){alert("Value changed!");});
Handler function(){alert("Value changed!");}
将在调用SetValue()
时被调用(如果你查看代码)。
您可以像这样获得价值:
alert(myVar.GetValue());
您可以像这样设置值:
myVar.SetValue(12);
紧接着,屏幕上将显示警报。 看看它是如何工作的: http : //jsfiddle.net/cDJsB/
这个问题很久以前就贴出来了,很多答案会周期性地汇集目标,如果目标不变,就会产生不必要的资源浪费。 此外,大多数答案在等待原始帖子要求的更改时不会阻止程序。
我们现在可以应用纯事件驱动的解决方案。
该解决方案使用 onClick 事件来传递由值更改触发的事件。
该解决方案可以在支持 Promise 和 async/await 的现代浏览器上运行。 如果您使用 Node.js,请考虑将EventEmitter作为更好的解决方案。
<!-- This div is the trick. --> <div id="trick" onclick="onTrickClick()" /> <!-- Someone else change the value you monitored. In this case, the person will click this button. --> <button onclick="changeValue()">Change value</button> <script> // targetObj.x is the value you want to monitor. const targetObj = { _x: 0, get x() { return this._x; }, set x(value) { this._x = value; // The following line tells your code targetObj.x has been changed. document.getElementById('trick').click(); } }; // Someone else click the button above and change targetObj.x. function changeValue() { targetObj.x = targetObj.x + 1; } // This is called by the trick div. We fill the details later. let onTrickClick = function () { }; // Use Promise to help you "wait". This function is called in your code. function waitForChange() { return new Promise(resolve => { onTrickClick = function () { resolve(); } }); } // Your main code (must be in an async function). (async () => { while (true) { // The loop is not for pooling. It receives the change event passively. await waitForChange(); // Wait until targetObj.x has been changed. alert(targetObj.x); // Show the dialog only when targetObj.x is changed. await new Promise(resolve => setTimeout(resolve, 0)); // Making the dialog to show properly. You will not need this line in your code. } })(); </script>
您可以使用属性:
示例:
function def(varName, onChange) {
var _value;
Object.defineProperty(this, varName, {
get: function() {
return _value;
},
set: function(value) {
if (onChange)
onChange(_value, value);
_value = value;
}
});
return this[varName];
}
def('myVar', function (oldValue, newValue) {
alert('Old value: ' + oldValue + '\nNew value: ' + newValue);
});
myVar = 1; // alert: Old value: undefined | New value: 1
myVar = 2; // alert: Old value: 1 | New value: 2
对我有用的(我到处查看并最终使用了某人的 jsfiddler / 对其进行了很小的修改 - 效果很好)是将该变量设置为具有 getter 和 setter 的对象,然后 setter 触发正在等待的函数变量变化。
var myVariableImWaitingOn = function (methodNameToTriggerWhenChanged){
triggerVar = this;
triggerVar.val = '';
triggerVar.onChange = methodNameToTriggerWhenChanged;
this.SetValue(value){
if (value != 'undefined' && value != ''){
triggerVar.val = value; //modify this according to what you're passing in -
//like a loop if an array that's only available for a short time, etc
triggerVar.onChange(); //could also pass the val to the waiting function here
//or the waiting function can just call myVariableImWaitingOn.GetValue()
}
};
this.GetValue(){
return triggerVar.val();
};
};
超级过时,但肯定是适应这一点的好方法。 刚刚为一个项目写了这个,并认为我会分享。 与其他一些类似,风格各异。
var ObjectListener = function(prop, value) {
if (value === undefined) value = null;
var obj = {};
obj.internal = value;
obj.watcher = (function(x) {});
obj.emit = function(fn) {
obj.watch = fn;
};
var setter = {};
setter.enumerable = true;
setter.configurable = true;
setter.set = function(x) {
obj.internal = x;
obj.watcher(x);
};
var getter = {};
getter.enumerable = true;
getter.configurable = true;
getter.get = function() {
return obj.internal;
};
return (obj,
Object.defineProperty(obj, prop, setter),
Object.defineProperty(obj, prop, getter),
obj.emit, obj);
};
user._licenseXYZ = ObjectListener(testProp);
user._licenseXYZ.emit(testLog);
function testLog() {
return function() {
return console.log([
'user._licenseXYZ.testProp was updated to ', value
].join('');
};
}
user._licenseXYZ.testProp = 123;
JavaScript是有史以来最糟糕的程序\\脚本语言之一!
JavaScript中似乎不可能“等待”! (是的,就像在现实生活中一样,有时等待是最好的选择!)
我尝试了“while”循环和“Recursion”(一个函数重复调用自己直到...),但JavaScript无论如何都拒绝工作! (这是令人难以置信的,但无论如何,请参阅下面的代码:)
while循环:
<!DOCTYPE html>
<script>
var Continue = "no";
setTimeout(function(){Continue = "yes";}, 5000); //after 5 seconds, "Continue" is changed to "yes"
while(Continue === 'no'){}; //"while" loop will stop when "Continue" is changed to "yes" 5 seconds later
//the problem here is that "while" loop prevents the "setTimeout()" to change "Continue" to "yes" 5 seconds later
//worse, the "while" loop will freeze the entire browser for a brief time until you click the "stop" script execution button
</script>
递归:
<!DOCTYPE html>
1234
<script>
function Wait_If(v,c){
if (window[v] === c){Wait_If(v,c)};
};
Continue_Code = "no"
setTimeout(function(){Continue_Code = "yes";}, 5000); //after 5 seconds, "Continue_Code" is changed to "yes"
Wait_If('Continue_Code', 'no');
//the problem here, the javascript console trows the "too much recursion" error, because "Wait_If()" function calls itself repeatedly!
document.write('<br>5678'); //this line will not be executed because of the "too much recursion" error above!
</script>
或者,您可以创建一个函数,根据其“静态”变量的值执行任务,示例如下:
<!DOCTYPE html>
<div id="Time_Box"> Time </div>
<button type="button" onclick='Update_Time("on")'>Update Time On</button>
<button type="button" onclick='Update_Time("off")'>Update Time Off</button>
<script>
var Update_Time = (function () { //_____________________________________________________________
var Static = []; //"var" declares "Static" variable as static object in this function
return function (Option) {
var Local = []; //"var" declares "Local" variable as local object in this function
if (typeof Option === 'string'){Static.Update = Option};
if (Static.Update === "on"){
document.getElementById("Time_Box").innerText = Date();
setTimeout(function(){Update_Time()}, 1000); //update every 1 seconds
};
};
})();
Update_Time('on'); //turns on time update
</script>
基于更多功能承诺(async/await)的方法的示例:
var delay = require('delay');
var obj = {
k: null
};
function notAwaitable() {
return obj.k;
}
async function waitFor(f) {
let r = f();
while (!r) {
console.log('Not yet, waiting more');
await delay(1000);
r = f();
}
return r;
}
(async function() {
await delay(5000);
obj.k = 42;
})();
(async function() {
let result = await waitFor(notAwaitable);
console.log(result);
})();
不,您必须创建自己的解决方案。 比如使用观察者设计模式什么的。
如果你无法控制变量或谁在使用它,恐怕你就完蛋了。 编辑:或使用 Skilldrick 的解决方案!
迈克
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