[wp-hackers] Applying a filter to the main loop only

John Blackbourn johnbillion+wp at gmail.com
Mon Jul 26 19:38:38 UTC 2010

On Mon, Jul 26, 2010 at 7:24 PM, Mike Schinkel
<mikeschinkel at newclarity.net> wrote:
> Whether John uses pre_get_posts or parse_request/request he still has the same problem; how to tell which is the "main" loop?  It's a great "Best Practices" question, actually.
> As we use WordPress for a lot more patterns besides just blogging there are emerging lots of use-cases that the global nature of some of these hooks makes difficult. I've struggled with figuring out to write robust code because it seems most of the time I'm just inspecting "artifacts" (i.e. testing some object property for some value that I discover it has in my use-cases but unsure or unaware if it will always be true.) Coming up with a set of "well known solutions" for these main hooks would be rather helpful.

Spot on, Mike. I find myself grepping through WordPress all the time
to find the hooks and functions that I need and coming to my own
conclusions about which hooks and filters are most appropriate. An
archive of best practices aimed at plugin and theme developers (as
mentioned in another thread) would save people a lot of time (and I'd
be very willing to contribute to it).

> For example, one solution that comes to mind for John is to use a static (or global) "flag" variable initially set to false or unset and then change it to flag that you've made it through the main query's loop.  But are there ever queries that may run before the main loop, and if so it will break this logic.

Yeah your second point was my main problem; There are potentially
loops that run both before and after the main loop so it's not just a
case of assuming the first loop is the main loop as using a flag.

> Back to John's question; how to determine if your hook is in the "main" loop; what artifacts does one inspect?

I've determined that the 'request' filter is the best fit here as it's
a filter on the main request parameters (ie. the results of parsing
the query string) and therefore only affects the main loop. The
'parse_request' filter is similar but includes additional parameters
that I didn't need in my scenario.


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