Original Blog Entry: ASP.NET AJAX: Half-way between createDelegate() and createCallback()

Speednet — Mon, 14 Jan 2008 19:47:12 GMT

I stumbled on a use of Function.createDelegate() method today that seems to be a cross between Function.createDelegate() and Function.createCallback().

I had the need to pass a context to a handler function, with the context being a JSON object containing the state of a few variables. I tried using createCallback(), which is designed for that purpose, but it did not seem to work well because I was not using it together with a DOM event — just with an internal function call.

So I turned to createDelegate(), using the JSON object as the this argument, and it worked quite nicely.

An example of this syntax is as follows. The code assigns a function to the variable fn that (when called) displays an alert box containing the element number and id of a DIV with an ID of "ThisOne". There are certainly more useful uses for this method, but it should provide a clear example of how it works. The key point here is the function() assignment that uses this to utilize the values passed in the delegate.

var a = document.getElementsByTagName("div");
var fn;

for (var x=0; x < a.length; x++) {
if (a[x].id == "ThisOne") {
fn = Function.createDelegate({
e: a[x],
num: x
}, function () {
alert("DIV #"+this.num+": "+this.e.id);
});
}
}

fn();

If you're not sure about the value of the above code, ask yourself, "How would I code the same thing without the createDelegate()?" Also a....

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