I'm working with ASP.NET AJAX and want to understand the difference between these two snippets:
function pageLoad(sender, eventArgs) { }
and
window.onload = function() { }
A couple things to note first. MS invented a sort of "clientside runtime object" called Sys.Application
. It handles raising init
, load
, and unload
events throughout the [clientside] lifespan of the page, as follows:
Sys.Application.initialize()
begins the init
part of the life cycle. This initialize()
s all clientside AJAX controls, after which they're ready to be interacted with programaticallySys.Application
begins the load
part of the life cycle, calling all handlers that have subscribed to this eventpageLoad
(if one is defined)Step 2) and 3) are repeated for every partial (ie AJAX + UpdatePanel) postback.
So finally the answer: pageLoad
is just a handy shortcut to Sys.Application.add_load()
.
With regards to its relationship to window.onload
however, things start to get interesting. Essentially, MS needed window.onload
to fire only after the init
phase was complete. But you can't control when the browser will fire onload
, as it's tied to "content loaded". This is known as "the window.onload
problem":
onload event fires after all page content has loaded (including images and other binary content). If your page includes lots of images then you may see a noticeable lag before the page becomes active.
So, they just invented their own "special" function to fire at just the right time in their event life cycle and called it "pageLoad"
. And the trick that they used to kickoff this custom event life cycle was to place the call to Sys.Application.initialize()
just before the closing </form>
tag. The serverside runtime does this. Astute readers will note that this trick allowed MS to solve the window.onload
problem, since any code you put into pageLoad
will fire independent of binary content (w/ one rare catch for IE).
> Do they act the same?
Conceptually yes, in practice not at all due to said window.onload
problem. The only rule is that you should put code that interacts with your AJAX controls in pageLoad
only, since window.onload
follows its own event trajectory.
> Or is one called before the other?
They are completely, 100% independent.
> Or will one be called automatically and the another not?
They will both be called if you have them defined.