[GIS] How to time function calls in OpenLayers

javascriptopenlayers-2

We have an existing application that has a function that wraps the OpenLayers.Layer.Vector constructor, which gets its layer info from a KML source over http. Once this function completes, I need to run another function which will read some of the layers data from the map object.

Normally in JavaScript I would of course use a callback when executing a remote AJAX call like this. But in this case — perhaps because I'm still pretty new to JavaScript — I don't know how to delay the execution of my function until the Vector constructor is done. There doesn't seem to be any ability to pass a callback to the OpenLayers.Layer.Vector constructor. Any suggestions? Thanks.

[Addendum]

How do I add the listener to the layer before the KML is loaded?

var newLay = new OpenLayers.Layer.Vector(name, {
    displayInLayerSwitcher: true,
    visibility: visible,
    styleMap: MapLayers.styleMaps.polygonMap,
    strategies: [new OpenLayers.Strategy.Fixed()],
    protocol: new OpenLayers.Protocol.HTTP({
        url: url,
        format: new OpenLayers.Format.KML({
            maxDepth: 2,
            extractAttributes: true,
            extractStyles: useStyles
        })
    })
});

function loadEndListener() {
  alert('loading ended);
}

var events = new OpenLayers.Events(newLay);
events.on({"loadend": loadEndListener});

Best Answer

You can use register event loadend on your layer. More on:

http://dev.openlayers.org/docs/files/OpenLayers/Events-js.html#OpenLayers.Events.on

You should call this that:

newLay.events.on({"loadend":loadEndListener});

I make it a long time ago, but probably that should work. On another site I found that should be tilesloaded listener, but I don't check it.

I'm sorry that so late, but I have a free.

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I think you hit a very common problem of imperative languages that have first-class functions as functional languages do but lack referential transparency. The result are (as a functional programmer would call it) broken closures. I hit this problem in Python some years ago, too and is was quite frustrating. Some weeks ago, this blog post hit my Google+ stream discussing the same issue.

As a sample, take the following code (please excuse I'm not demonstrating it using OpenLayers since I know nothing about OpenLayers and nearly nothing about JavaScript (so excuse the possible use of JS anti-patterns))

function foo_broken() {
    var xs = [];

    for (var i = 0; i < 10; i++) {
        var bar = function() { return i; }
        xs.push(bar);
    }

    return xs;
}

function foo_working() {
    var xs = [];

    for (var i = 0; i < 10; i++) {
        var bar = function(t) { return function() { return t; } };
        xs.push(bar(i));
    }

    return xs;
}

I think most users would expect foo_broken() and foo_working() to do exact the same thing, but no, not in JavaScript. The test code below alerts 10,10,10,10,10,10,10,10,10,10 when evaluating the list returned by foo_broken() and 0,1,2,3,4,5,6,7,8,9 for foo_working().

The problem is that the closure for the anonymous function assigned to bar does not contain the current value of i but a reference to the variable i. That's the reason it's always the last value assigned to that variable. It's IMHO broken but it's like JavaScript, Python 2 and some other languages do it :-(.

The pattern to fight this issue I always use is to define another function which takes as many arguments as the values I'd like to force. In the demo above it's only one parameter (t) because I simply want the value of i. So to force the evaluation of variable i I pass it as a parameter to the anonymous function inside the loop and so the JavaScript interpreter is forced to evaluate it inside the loop.

function alert_values(fun) {
    var bars = fun();
    value_strs = "";
    for (var i = 0; i < 10; i++) {
        value_strs += "" + bars[i]() + ",";
    }
    alert(value_strs);
}

alert_values(foo_broken);
alert_values(foo_working);

Best Answer

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