How to vertically center content of page without makeup in ConTeXt?

Question

I need to vertically center the content of page, content is forced to be on a separate page (delimeted with \page command).

previous page content

\page 

this content must be verticaly centered on it's page.

\page 

next page

placing \vfill after first \page does nothing. I need to center content without using makeup, because makeup removes page numbering, but I need to typeset this page as usual.

Answer 1

When entering a page in normal typesetting mode, Context

1. is in vertical mode, and
2. will gobble all skips that follow immediately so as to align the top of the first line of the text body appropriately.

For this reason, inserting a vertical skip has no effect at the top of a page.

To work around this behavior you need to force TeX into horizontal mode, which you can do by

1. inserting horizontal material, for instance a strut, or
2. terminating vertical mode explicitly, which is usually done by means of the \dontleavehmode directive (the Context synonym for the \quitvmode primitive), or alternatively the lesser known \leavevmode [0].

The idiomatic solution would look like this:

\starttext
  \dontleavehmode \vfill
  foo
  \vfill
\stoptext

Thus, if you intend to vertically center a page without using makeups, you could define an environment as follows (thanks to /u/Aditya for the suggestion):

\definestartstop [centeredpage] [
  before=\dontleavehmode \vfill,
  after=\vfill,
]

\starttext
  \startcenteredpage
    foo
  \stopcenteredpage
\stoptext

On a related note, what you are asking for can actually be achieved conveniently by tweaking the options to \setupmakeup. The key settings are pagestate whose value is passed to an implicit \setuppagenumber, as well as headerstate and footerstate that govern the behavior of running heads and footers, respectively. To make headers and footers appear on the makeup, and to prevent the page numbering from ignoring that page, these options need to be set to start (they default to stop). For example:

\definemakeup [centeredpage]
\setupmakeup [centeredpage] [
  pagestate=start,
  top=\vfill,
  bottom=\vfill,
  headerstate=start,
  footerstate=start,
  page=yes,
]

\setupheadertexts [dummy header]
\setupfootertexts [dummy footer] [{\getnumber [userpage]}]

\starttext
  text on page before
  \startcenteredpagemakeup
    \input ward
  \stopcenteredpagemakeup
  text on page after
\stoptext

[0] For a discussion of \leavevmode vs. \dontleavehmode / \quitvmode see this post by Taco on the Pdftex list.