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问题:

I want to force a JavaScript program to wait in some particular points of its execution until a variable has changed. Is there a way to do it? I have already found an extension that is called "narrative JavaScript" that force the program to wait until an event to happen. Is there a way to create a new event, a "variable change event" for example that behaves like onclick event..

回答1:

Edit 2018: Please look into Object getters and setters and Proxies. Old answer below:

a quick and easy solution goes like this:

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();

回答2:

JavaScript interpreters are single threaded, so a variable can never change, when the code is waiting in some other code that does not change the variable.

In my opinion it would be the best solution to wrap the variable in some kind of object that has a getter and setter function. You can then register a callback function in the object that is called when the setter function of the object is called. You can then use the getter function in the callback to retrieve the current value:

function Wrapper(callback) {
    var value;
    this.set = function(v) {
        value = v;
        callback(this);
    }
    this.get = function() {
        return value;
    }  
}

This could be easily used like this:

<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>

回答3:

I would recommend a wrapper that will handle value being changed. For example you can have JavaScript function, like this:

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();
    }


}

And then you can make an object out of it that will hold value that you want to monitor, and also a function that will be called when the value gets changed. For example, if you want to be alerted when the value changes, and initial value is 10, you would write code like this:

var myVar = new Variable(10, function(){alert("Value changed!");});

Handler function(){alert("Value changed!");} will be called (if you look at the code) when SetValue() is called.

You can get value like so:

alert(myVar.GetValue());

You can set value like so:

myVar.SetValue(12);

And immediately after, an alert will be shown on the screen. See how it works: http://jsfiddle.net/cDJsB/

回答4:

You can use properties:

Object.defineProperty MDN documentation

Example:

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