In this post, I'm going to end my discussion on Animations in ASP.NET AJAX by showing you how they work, internally. At the end of this post you should not only know how Animations work, but also how ExtenderControl's work in general.
What is an ExtenderControl?
An Animation is actually just an ExtenderControl, so it will do us some good to understand what an ExtenderControl is all about.
ExtenderControls are script controls that apply a particular feature from ASP.NET AJAX to an already existing ASP.NET (non-AJAX) control. For example, if you wanted to apply some cool, new feature of ASP.NET AJAX to a standard radio button control, you'd do so through the implementation of an ExtenderControl. Extender control's work by emitting JavaScript that is attached to and extends functionality of an ASP.NET control. You will always need to reference the control to which you are applying the extender because it will need to know which control the emitted script should target. The key thing to remember is that it's an ExtenderControl is an auxiliary to an existing control, and typically the extending features are done through JavaScript.
Exactly how does the script emitted by an ExtenderControl become attached to the target ASP.NET control? Script becomes successfully attached to pre-existing controls through the runtime execution of the ScriptManager on the server, the use of a type known as the ScriptDescriptor, and the bootstrap client-side code of ASP.NET AJAX on the client.
On the server, the process looks something like this
The process starts when the ASP.NET runtime asks the ScriptManager (a native, server-side control) to render itself out. The result of doing so causes the ScriptManager to ask every control that has attached itself to the ScriptManager to render itself out too (an Animation, being an ExtenderControl, is therefore asked to render).
A script control is a special, new type of control introduced into the ASP.NET AJAX framework as a control that relies heavily on the scripting facilities of ASP.NET AJAX. One of the important features of a script control that sets it apart from other controls is that it has a great deal of functionality that must be executed on the client (through JavaScript) for it to function properly. A script control is responsible for giving all scripts and script types it requires to operate to the ScriptManager when it renders; doing so guarantees that the script will be included in the page output and therefore included in the rendered page.
Script controls hand off all the necessary script and script types to the ScriptManager as nicely packaged objects known as ScriptDescriptors and ScriptReferences. We’re more interested in ScriptDescriptors, however, as they are the layout for the JavaScript types and so (if you refer to the diagram above) you see that our extender control (when asked to render) calls back into the ScriptManager to register all its types. Doing so finally calls into ScriptDescriptor.GetScript() which returns back a block of script leveraging the $create() shortcut method and a series of JSON-formatted data.
When the ScriptManager renders, then, it guarantees that the JSON-formatted data of the control will be instantiated and initialized on the client through the $create() method. How? Because the ScriptManager will wrap the output of GetScript around the Sys.Application.add_init() javascript method. Add_init() actually attaches code to the initialization script of ASP.NET AJAX’s client libraries; this guarantees that, when ASP.NET AJAX is fully started up on the client when the page has fully loaded, your initialization routine will be called. What the initialization routine does, however, is up to you.
How Does an Animation Work?
You're 90% of the way to understanding Animations now. All that's really required is a bit of a filler-explanation for what, specifically, the Animation extender control is doing.
On the server the Animation control will take your XML markup and create an in-memory Animation object instance representing all of the properties and child/parent associations. Since an Animation is an extender control, the ScriptManager will ask it to render. The result of the Render call will be the JSON-formatted version of the Animation object from the server (property values, event-handling code and all). Just like above, the JSON-formatted initialization code will be wrapped around a Sys.Application.add_init() script method so that it will be executed when the page is loaded on the client. This is an example of what the output looks like:
…this is actually the result of a view source on one of the animation examples on www.asp.net. There is a lot of code here, but if you squint carefully you can see it all starts with a Sys.Application.add_init() call. Also if you look carefully you’ll see that it’s actually instantiating, on the client, an object of type AjaxControlToolkit.Animation.AnimationBehavior.
The AnimationBehavior Object
If you recall from earlier discussions about Animations, there are actually many instances where multiple animation effects (a fade out, or a color change) can occur in sequence as the result of a single event (onclick). Each animation effect is actually an Animation object, and the object that surrounds each Animation (to make them happen in sequence) is also an Animation object – so we have Animation objects housing multiple Animation objects. This can be confusing when rendered out to the client, so the ASP.NET AJAX client framework wraps all the animations that need to be rendered to the client for a single Animation extender as an AnimationBehavior object. The AnimationBehavior object is what gets instantiated on the client and has its initialize() method called.
The initialize() method of the AnimaitonBehavior object will inspect which events need to be registered (OnClick, etc) and attach event handlers to the target control on the page (the client-side control which we’re extending). Whenever that event occurs on the client, the event handler for the AnimationBehavior will be called and it will route that event to the proper Animation object. Here is an example of the AnimationBehavior’s OnClick event hander:
…which, after a bit more traversing the internals of the script library, plays the Animation.
How does the Animation object actually play the animation?
It may seem like somewhat of a mystery when you declare XML on your page that then translates into some dynamically updating DHTML object, but if you think carefully about what it take to animate a DHTML object the mystery subsides.
All that it takes to animate something on your page is a timer ticking at a regular interval whose callback method updates some DHTML object property (x- and y-position, for example). And that’s all an Animation object is on the client – just a fancy timer that updates properties with values you specify; invoking the play method above simply starts off the timer, and thus the DHTML effects. Therefore all that’s needed within the JSON data is enough information to identify what property you’re updating on what object, at what interval, and with what values – the existing script of the Animation object will take care of the rest. If you look at some of the JSON-serialized data that is being rehydrated on the client, you actually can see just that information (take a closer look):
..and that’s how Animations work.
In my first post in this series on using ASP.NET AJAX animations I described the basics of what an Animation was and the foundations behind using it. In my second post, I then started showing you how to go about using it (the basics). What remains are a few exceptional Animations that will be used in unusual circumstances (hence them being "exceptional") but which enable you with a great deal of power when you do decide to use them.
The Condition Animation
Why stop a good thing? So I'm going to continue using the (somewhat lame) example from my previous posts where I take a simple ASP.NET button control and do things to it using the Animation framework. In this example I leverage an animation called the Condition Animation; it gives you the ability to programmatically choose one of two (at most) animations to run depending on the boolean result of some script. For example, take the following Animation XML:
By clicking button Button1, the Condition Animation will be executed (if this block doesn't make sense, see my first post and read forward). Take note of the Condition Animation and it's child Animations. The first thing to take note of is the ConditionScript attribute on the Condition element; it's a simple statement: "false;". Although this is just a boolean result (false), it could be an entire JavaScript statement which evaluates to either true or false. For Condition Animations, if the resultant script statement evaluates to true, then the first child Animation is executed (in this sample, the FadeOut animation); if the script statement evalutes to false, then the second child Animation (which is the Color animation) would be executed.
Given that you now have programmatic control over what Animation is executed, you can take conditions on the page, evaluate them through the script, and then decide which Animation to run.
The Case Animation
Being limited to one of two Animations is, well limiting. In the event that you need to pick from a wider spectrum of Animations, but still do so using the result of a script statement, then you would leverage the Case Animation. Take the following XML Animation block:
The result of the SelectScript must be a 0-based index into the array of child Animations (0 is the first Animation or the "FadeOut" animation, 1 is the second animation or the "Color" animation in our example, and so forth). In my example I simply return "1" but, like the Condition Animation above, you could have an entire script statement as the SelectScript value. So long as your script statement evaluates to a valid index into the array of child Animations beneath your Case animation it will behave properly.
In the above example, the Color animation will be executed.
The Script Animation
To flat out execute script in the Animation you would leverage the Script Animation. Take the following example:
What we have here is a Sequence Animation housing a FadeOut and then a ScriptAction animation. Given what we already know about a Sequence animation, the child nodes (or child Animations) will be executed sequentially from top to bottom. Therefore, when Button1 is clicked, the Button will fade out over half a second and then a script modal window will pop up saying "All done". Of course, much more advanced script could be executed from the ScriptAction, but you get the basic idea.
What's Next?
Okay, next we are going to dive into the pipeline to see just how this magic is done. Stick around...
In my previous post I gave a brief overview of what an Animation is in ASP.NET AJAX, and now it's time to understand how you go about actually using them. By the end of this post you should be able to look at a block of Animation XML and understand what it's doing and you should be able to write a block of Animation XML without needing to lookback on a sample block of code (too much).
Let's Recap for a Moment
Before diving, let's recap from my previous post: an Animation in the ASP.NET AJAX framework is any DHTML effect that leverages the Animation framework provided by the ASP.NET AJAX toolkit (which can be found here). The problem found in playing with DHTML in the past has been that you've needed JavaScript (and sometimes pretty hairy JavaScript) to do anything meaningful with it. As programmers we're "resourceful" (or lazy) and therefore don't want to do more work than we need. With the Animation framework in ASP.NET AJAX, you only need to worry about XML markup specific to the animation you want.
In short, ASP.NET AJAX Animations are the easy way of adding cool DHTML animations and effects to your page - it achieves this by letting you define and play with your animations declaratively through an XML syntax.
Show Me an Example
Let's go back to an earlier example that I did (because of its utter simplicity) and work out from there. In my previous post I showed an example effect on a button where, once you clicked the button, it's text color changed red and then faded out:
to....
You achived this effect by using declarative XML syntax. The syntax let you play with the properties (the starting color and ending color in this example) without needing to write any code; thereby making things easier and more manageable for everyone. Let's break the syntax I showed you earlier down one element at a time so that you can see why a particular element exists and what it does.
Understanding the ASP.NET Animation XML Syntax
As I said earlier, to write an Animation in ASP.NET AJAX, you use an XML syntax. The syntax that resulted in the above button color change looks like this:
First thing to take away is that this is a control extender - or an object in the ASP.NET AJAX framework that let's you apply AJAX'ish properties to an already existing, standard ASP.NET control (you can learn more about extenders here). What this means, practically, is that the animation needs to know to what standard ASP.NET control the animation will be applied. If you take a look at the first line of XML, you'll see the TargetControlID property pointing to Button1. You should know that this Animation will, then, be applied to Button1.
Second is the Animations element which will house all of the animations you will apply to Button1 (that's right, you can apply more than one, even in parallel). This may seem somewhat redundant, but it is required.
Third we have an event name. In our example, OnClick, means that the Animations will be executed during the OnClick event of Button1 (bear in mind that this is the client onclick event, not the server one). Therefore when someone clicks Button1, the animations specified below the OnClick event node will be executed.
The events OnClick, OnLoad, OnMouseOver, OnMouseOut, OnHoverOver and OnHoverOut are valid events; meaning you don't have to just use OnClick, you could also use OnHover. It's interesting to note that you can have all of these event handlers under the same Animations element if you wanted to - therefore, you could have one Animation registered for OnClick, and a totally different one registered for OnMouseOver, thereby creating a very dynamic control!
Fourth we have a sequence element; but - I have to admit to something here, this is actually not necessary in our example. I added it because I wanted to make a point about something. The point I want to make is a subtle one: you can only have one Animation registered for a particular event type (OnClick, OnMouseOver). The reason this isn't a limitation, however, is because there are two Animations called Sequence and Parallel which are not only valid Animations themselves, but allow nesting of other animations. Therefore, Sequence is an Animation itself that will call each child Animation one after another (in "Sequence"), and Parallel is an Animation that will call each child Animation at the same time (in "Parallel"). This is a subtle, yet powerful, thing to realize.
So..the following XML is actually valid (note the missing Sequence element):
But, if you wanted to change the color of the button and THEN fade it away completely, for example, you would have to nest the Color Animation and FadeOut animation within a Sequence Animation (not doing so will result in a runtime exception). The XML must look like this:
The result from the above XML declaration is to fade the button's color from FF0000 to 666666 and then completely fade it out of existence in .5 seconds. What's really cool is if you wanted to do both at the same time, all you have to do is change the Sequence element to a Parallel element:
Cool, huh? Now the color change and the fade-to-nothing will happen at the same time or in Parallel.
The lowest level element of the structure is always the actual Animation itself (Color for color change, FadeOut for fading out the element, etc).
Does it Get Tougher?
Yes, somewhat because you can actually tie into the XML script to be executed when certain events occur and conditional statements. We'll explore that, however, next time...
In two previous articles here and here I did a fairly high-level description of JSON and why it's important in ASP.NET AJAX. Hopefully you either read those or you already have a fairly decent understanding of the topic. The point behind writing those posts was to give you the knowledge required to understand the internals of the Animation framework in ASP.NET AJAX. I'm going to begin the dive now, but before doing so actually give a brief overview of, exactly, what Animations are in ASP.NET AJAX. I'll attempt to answer the questions why are they important, and why should you - as a .NET developer - care.
What are Animations in ASP.NET AJAX?
Animations are not built-in to ASP.NET AJAX, they are an addition to it brought about by the ASP.NET control toolkit, an open-source project going on over here. What they provide is an easy-to-use method of adding fancy DHTML effects (such as call-out windows, flashing buttons, rotating objects, etc) to your page. You will find a feature such as this handy when you need to make a web-site that stands out above the rest and is a bit more interactive than, say, your typical web application. For example, you could make a window pop-out at the user when he/she clicks a button, like this:
What makes the ASP.NET Animation framework compelling, however, is not that you can do things as above (after all, you have been able to do shtick like this for years in JavaScript), but that all that's required is a bit of XML markup in your ASPX page:
<ajaxToolkit:AnimationExtender id="MyExtender"
runat="server" TargetControlID="MyPanel">
<Animations>
<OnClick>
<FadeOut Duration=".5" Fps="20" />
</OnClick>
</Animations>
</ajaxToolkit:AnimationExtender>
...therein lies the cool factor.
Cool, how do I do it?
I'll show you, but I'll do it with a rather simple example first so that we don't spend a great deal of time (and space) with the inconsequential.
First, you create a new ASP.NET AJAX web application using Visual Studio 2005. I won't go into the details of doing this as you already know how to do this.
Second, you add a reference to the control toolkit's main assembly to your project (if you do not have this, then you can download it and all the supporting stuff from here).
Third, register the assembly and it's namespace in your ASPX page:
Fourth, slap a button on your ASPX page:
Fifth, flip over to the code view and add the following code:
...note two things: first, I added OnClientClick event handler to the button to prevent it fom posting back to the server; second, I added a block of XML markup as an AnimationExtender (the ajaxToolkit:AnimationExtender block). Also note how I assigned the TargetControlID property of the AnimationExtender to the Button I just added moments ago.
Now, if you F5 and run the application, you'll see that - when you click the button - the button text will turn red:
...and then fade out:
What else can Animations do?
Practically anything you want. It's a framework, mind you, so it's very extensible and maleable; but the nice folks that built the framework have supplied a great deal of pre-made animations for your consumption, and you can find the list here (which includes everything from fadeouts to movement).
After following through the list of pre-made animations, you can't help but get a bit excited about the kinds of things you can do. In the next section I'll explain the nature of Animations (the mindset you need when working with them), and then in the section after that I'll explain how they work (given that we now understand JSON).
....stay tuned.
Okay, in my last post on this topic I gave a brief overview of what JSON is and how it comes into play with your ASP.NET AJAX solution. Now I'm going to stretch out a bit and show you a few examples doing something practical with JSON data in ASP.NET AJAX. I'll be careful to explain what I'm doing along the way. By understanding this, you'll be ready to move into how Animations work in ASP.NET AJAX, and therefore be able to leverage them in your applications.
You know that JSON is a "stringified" version of an object, and you know that this stringified version is used to preserve the object across boundaries where raw, binary data transmission is not practical (or possible). However, one thing I didn't really touch on, and so I'll now explain, is exactly why someone would transmit an object to some other environment anyway. Objects are nice ways to not only pack functionality, but also state - so in the case of browser-based programming, for example, you could easily see yourself creating an object that represents the behavior of a button element, setting some state (such as color, what happens when the button is clicked, etc) and sending it down to the client. So, the reason you'd do such a thing as send an object down to the client is because that object represents state and functionality specified by you on the server; once downloaded to the client browser it can then be used to reflect that state to elements within the browser in a nice, packaged form.
Let's step back a bit and make up a quick example.
Say you have a DIV tag on your page and this DIV tag is to do something when you click it. You could set the div tag's onclick handler to run some kind of javascript:
<div id='mydiv' onclick='doSomething();"/>
We've all seen this before and it's nothing new; it's just JavaScript. However, what if you wanted some sort of variation in what the method doSomething() actually does? Perhaps it displays something a bit different depending on the user logged in, or perhaps you want something special done because it's Halloween - whatever. Now you're in a pinch. In ASP.NET, you might go ahead and start doing more dynamic things to it through inline ASPX or through Response.Write(); basically modifying what the browser gets through the output stream (such as tweaking arguments to doSomething()). After a while, however, this gets really, really ugly to manage. What would be nicer is if an object basically represented the functionality of the DIV tag, and so on the server all you had to do was set up a couple properties (such as the text displayed to the user when he/she clicks the DIV tag) and have those properties get reflected in the client browser. In this scenario, an object would exist on the server where you can programmatically tweak properties, and then it gets sent down to the client where it actually "behaves" as script.
As you can imagine, the way the object would be sent down to the client is as a JSON-seralized object.
We saw earlier that the modern browser has capabilities, through the eval() method, to rehydrate an object out of JSON-formatted data. We also saw earlier how the ASP.NET AJAX framework provides a number of ways to wrap the eval() method to make it more robust and secure. All that's needed, then, is a way to take an object on the server and create a JSON-formatted string of data out of it so that the browser can create a version of it for itself. There is - it's called the JavaScriptSerializer.
If you had an instance of the JavaScriptSerializer in your class like this:
Then you could serialize an object with it like this:
...you can see here that I'm taking some instance of an Animation type and serializing it as a JSON string. Note that the Animation type isn't JavaScript, it's a full-blown .NET type (a real managed typed). The string returned simply represents the properties and events of the type.
If my animation had properties (like, say, the text that is displayed when I click something), then the JSON string will represent the property and the value of that property. Anything that will rehydrate the object from that JSON string will set the property to the original value. Therefore, the object's state gets preserved as it crosses from the server to the client.
There are a number of ways to rehydrate the object using the client framework of ASP.NET AJAX so it gets reborn in the browser. A handy shortcut method for doing this is $create(). The create method will take the type it is to create as the first argument, and then a initialize properties and events on the type through JSON-formatted data. The $create() method, then, creates objects out of JSON. Once done, the newly created object will have it's initalize() method called by $create() (giving you the opportunity to attach event handlers and the like).
Therefore, say we had some JavaScript class called AnimationBehavior and it has a series of properties. On the server you set those properties and then create a JSON string out of the object. $create() can take that string, initilialize a JavaScript version of the type and then call the new AnimationBehavior object's initalize() method. From there you can attach event handlers based on the properties and events you set through JSON.
So....
- You have a fully-qualified .NET object.
- You can set properties on that object and have that object serialized as a JSON string.
- The ASP.NET AJAX client framework can take that string and create a version of the managed type in JavaScript for it's own use through JSON and the $create() method.
- The $create() method, once finished initializing the new type, will call the type's initialize() method, giving you the opportunity to do something meaningful (such as initialize your object).
Note that eval(), normally, creates an opaque object out of JSON data. eval(), used alone, will not create anything typed strongly. $create(), on the other hand, expects an already existing type in JavaScript to instantiate and initialize through the JSON data. If you're creating a valid ASP.NET AJAX client type, it will have an initialize() member method, thereby working perfectly fine with $create().
In the next section I'll walk you through exactly what an Animation is, and then we'll see how they use all of what you just learned.
I have an interest into going into detail over the serialization framework provided by ASP.NET AJAX. The reason - ultimately - is that I would like to start explaining how the Animation framework works in ASP.NET AJAX. However, as with any sort of understanding, there are earlier pieces that must be grasped first before you can truly understand the later pieces: the "crawl before you walk" problem. So, let's start off with the basics and, over the next couple days, we'll move our way through the serialization framework and into the Animation framework of ASP.NET AJAX. Doing so, you'll be able to really start cooking with some fascinating animations in your browser-side programming. My goal, also, is to aid you in understanding how it all works under the hood (as usual).
So, let's explore JSON.
JSON - the JavaScript Object Notation - is a "stringified" version of an object; or an object represented as human-readable text. It is valuable because the web, primarily, doesn't do well with raw, binary data....as you know, the data must be encoded so that it can be properly transmitted via the web medium (learn more). However, it is more space-savvy then various alternatives (such as XML), thereby making it more appealing to transmit large, detailed blocks of data (such as objects).
The basic idea behind JSON is that when you need to transmit some object over the web boundary, you serialize it in the JSON wire-format, make the transmission, and then deserialize it on the receiving end into the original object. The result is a clean hand-off of the object from one universe to another across the finicky web. The actual data format isn't important; though if you are interested Wikipedia has a nice overview of it here. But, suffice it to say that in JSON, you're taking some opaque object and representing it as a block of ASCII-based text.
Why is it important?
In ASP.NET AJAX, especially, JSON is VERY important. Why? Because ASP.NET AJAX is striving, very hard, to be as strongly-typed as it can at all times for objects written in ECMA script. Objects are flittering and flying about all over the ASP.NET AJAX world and, therefore, a consistent wire-format for transmitting and working with those objects is necessary. As we will see when we start encountering the Animation framework of ASP.NET AJAX, serialization via JSON is used extensively in converting the animation XML into maleable, server-side objects that we may program against; likewise, those same objects on the server can be serialized into meaningful JavaScript objects for the client-side framework. All of this is done using JSON formatting in ASP.NET AJAX.
How does it work?
I mentioned earlier that the formatting rules of JSON isn't important - and for us, as ASP.NET AJAX programmers - it isn't. The reason is due to the fact that we rarely ever need to hand-tweak anything because formatters for JSON exist and are quite mature already, on both the client AND the server-sides. However, there is a powerful framework at work here that deserves a bit of understanding, regardless of the actual formatting rules.
At the core, the most important element required for JSON serialization within the browser itself is the eval() method. The eval() method takes a block of data, which can be either script or JSON data, and evaluates it. The "evaluation" process will do something to the text; if it's script, then it will execute it, otherwise it will go through and parse out the JSON data and return a deseralized object back to the script. There are security implications involved in this sort of process, and so you should be very cautious to leverage eval() on your own. Instead, you should use Sys.Serialization.JavaScriptSerializer as an ASP.NET AJAX developer. This type exposes two, obvious methods: serialize() and deserialize(). Under the hood, this JavaScriptSerializer object leverages eval(), but more cautiously.
So, JSON works by leveraging API frameworks that convert the formatted data to objects and back again for us.
Conclusion
In the next part of this topic I'll go over the actual process of taking objects represented in XML and creating ECMA script from them (and back again); a process that is used extensively in the Animation framework of ASP.NET AJAX to work with the animation objects declaratively.
http://msdn2.microsoft.com/en-us/library/ms364069(vs.80).aspxWriting code that is fully interoperable with two different runtimes can blow; 'nuff said. The single-most valuable thing to remember when doing COM interop (in my opinion) is to add your damn .NET module to the GAC. Why? The reason lies in the internals of COM, and if you've ever had the distinct pleasure of pounding your head against the desk while working on COM you'll quickly understand what I'm trying to say. COM was built with the ideals of interoperability in mind: so long as the two, distinct components stuck to a particular contract-based communcation (through "interfaces") then everything worked nicely. In COM to get at a particular bit of functionality (which is exposed via an "interface") you must get at the Com Class (coclass) which implements the interface. A particular COM server houses the Com Class (coclass) which implements the interface. Basically, to get at functionality, then, you must locate the physical module, then ask the physical module to activate a particular "object" and then ask the "object" for an interface pointer to the functionality; the rest is done through vtable pointer arithmetic (this is all if you're using "IUnknown" semantics, otherwise the game is slighty different). .NET interops with COM by having the mscorlib.dll module be the server - it is responsible for loading the necessary DLL files that implement your .NET functionality and doing all the proper dirty work of interoperating with COM. Say you have a COM plugin, then, for some application - but the plugin is written in VB.NET. The main application attempts attempts to create a com instance (through CoCreateInstance()) of your .NET module - this routes the COM runtime to Mscorlib.dll which attempts to locate your DLL; however, how does Mscorlib.dll find it? Well, if you haven't placed it directly next to (in the same directory as) the main application or in the GAC, MScorlib.dll will not be able to find your module and you're, effectively, "dot screwed". ....by placing the module in the GAC you'll be able to guarantee mscorlib.dll will find your module.
Say you have a situation where you want to catch all unhandled web errors in your global.asax file. You reimplement part of your site in ASP.Net AJAX and suddenly some exceptions that were being caught are no longer caught, the question is, what's happening?
The answer lies within the PageRequestManager: the server-side "brains" of ASP.Net AJAX. This object is a control just like any other when used on an ASP.Net page and registers for errors on the page within it's OnInit() method:
if (_owner.IsInAsyncPostBack) {
_owner.IPage.Error += OnPageError;
}
The IPage reference is a reference to a page wrapper that ASP.Net AJAX uses internally; suffice it to say that it's simply registering for the event fired when an unhandled exception occurs on a page - that part is nothing new to AJAX.
What happens, though is that the following block of code gets executed within the OnPageError() handler if you do not have custom errors redirect enabled:
if (showAsyncErrorMessage) {
_owner.IPage.Response.Clear();
EncodeString(_owner.IPage.Response.Output, ErrorToken, httpCode.ToString(CultureInfo.InvariantCulture), errorMessage);
_owner.IPage.Response.End();
}
The page response gets cleared, a special blob of text gets inserted, and then the response stream ends. Doing this effectively wipes the page response (including any error information in it) away - thereby short-circuiting the ability for your handler method in Global.asax to get called.
Run Fiddler on your machine to see the kind of stuff that gets pushed back to the browser when an error is handled by this OnPageError() method, it's a simple '|' seperated string.
If you wish to get your handler to work in the Global.asax file again, you must add some kind of <customErrors> redirect page - this drives the execution flow differently through the PageRequestManager object and leaves the error information together for your global error handler(s).
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