II. ASP.NET Ajax Imperative Drag and Drop


 



To accomplish the same thing using a programmatic model requires a bit more code, but not much more.  It is important to understand that when you add an Ajax Extensions Script Manager component to your page, you are actually giving instructions to have the Ajax Extensions javascript library loaded into your page.  The Ajax Extensions library, among other things, provides client-side classes that extend the DOM and provide you with tools that allow you to code in a browser agnostic manner (though there currently are still issues with Safari compatibility).  These client-side classes also allow you to add behaviors to your html elements.

To switch to an imperative model, you will need to replace the XML markup with two javascript functions.  The first one is generic script to add floating behavior to an html element.  It leverages the Ajax Extensions client-side classes to accomplish this:



<script type=”text/javascript”>
        function addFloatingBehavior(ctrl, ctrlHandle){
              $create(Sys.Preview.UI.FloatingBehavior, {‘handle’: ctrlHandle}, null, null, ctrl);


              }
</script>



The function takes two parameter values; the html element that you want to make draggable, and the html element that is the drag handle for the dragging behavior.  The new $create function encapsulates the instantiation and initialization routines for the behavior.  The addFloatingBehavior utility function will be used throughout the rest of this tutorial.

Now you need to call the “addFloatingBehavior” function when the page loads.  This, surprisingly, was the hardest part about coding this example.  The Script Manager doesn’t simply create a reference to the Ajax Extensions javascript libraries, and I have read speculation that it actually loads the library scripts into the DOM.  In any case, what this means is that the libraries get loaded only after everything else on the page is loaded.  The problem for us, then, is that there is no standard way to make our code that adds the floating behavior run after the libraries are loaded; and if we try to run it before the libraries are loaded, we simply generate javascript errors, since all of the Ajax Extensions methods we call can’t be found.

There are actually a few workarounds for this, but the easiest one is to use a custom Ajax Extensions event called “pageLoad()” that  only gets called after the libraries are loaded.  To add the floating behavior to your div tag when the page is first loaded (but after the library scripts are loaded) you just need to write the following:


<script type=“text/javascript”>
function pageLoad(){
addFloatingBehavior(document.getElementById(‘draggableDiv’),
document.getElementById(‘handleBar’));
}
</script>

which, in turn, can be written this way, using an Ajax Extensions scripting shorthand that replaces “document.getElementById()” with “$get()“:


<script type=“text/javascript”>
function pageLoad(){
addFloatingBehavior($get(‘draggableDiv’),$get(‘handleBar’));
}
</script>

And once again, you have a draggable div that behaves exactly the same as the draggable div you wrote using the declarative model.

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