On 2022-12-20 14:31, Pawel Urbanski via ntg-context wrote:
> Hi,
> Let me clarify my question more...
> I need to attach some value to a chapter in that key. That custom 
> prefix + the value in the key is ment to be in the header or a footer 
> as a matter of fact. The case can apply to both headers and footers.
> I tried:
> \chapter[title={A sample title}][custom_random={custom_value}]
> \setupheadertexts[{Prefix: \structureuservariable{custom_random}}]
> ...
> \stopchapter
> I tried this code when trying to figure out the mechanism. The prefix 
> string in placed in the header, but hte custom value of the attached 
> key is not expanded..
> Evey page will contain just one section started by a \startchapter and 
> \stop chapter. I would like to put this custom attached value in the 
> right slot in the document header.
>
> I hope that htis time around htigs are more clear.
>
> On Tue, Dec 20, 2022 at 8:13 PM Henning Hraban Ramm via ntg-context 
> <ntg-context@ntg.nl> wrote:
>
>     Am 20.12.22 um 19:44 schrieb Pawel Urbanski via ntg-context:
>     > Dear Friends,
>     > I s-- pent some time searching and coding simple documents, but
>     failed and
>     > therefore I'm asking for help...
>     > A section such as chapter cn have custom keys with values. One
>     uses the:
>     > \structureuservariable command to access such values.
>     > How can I use such a value to create a custom header in a format:
>     > Prefix: \structureuservariable{foo}
>     >
>     > I am asking for the only thing that worked when I tried was
>     calling the:
>     > \setupheadertexts command inside a chapter environment block
>     with some
>     > prefix. Unfortunately the value from the custom field was not
>     expanded.
>     > It was correctly inserted in a regular paragraph.
>     > For example:
>     > \startchapter
>     > [title={A sample title}][custom_something={random_value}]
>     > ...
>     > \stopchapter
>     >
>     > How can I put the value of: custom_something in the header with
>     some prefix?
>
>     \define[2]\MyChapter{No.#1 – Prefix
>     \structureuservariable{custom_something}: #2}
>
>     \setuphead[chapter][
>       command=\MyChapter,
>     ]
>
>     Is this what you meant?
>
>     Hraban
>

Perhaps:

    \setuppagenumbering[alternative=doublesided,location=bottom]
    \startsetups chapter:headings
       \doifsomethingelse{\structureuservariable{custom}}
                         {\setupheadertexts[RI]
                                           [Prefix:
    \structureuservariable{custom}]
                                           [Prefix:
    \structureuservariable{custom}]
                                           [VO]}
                         {\setupheadertexts[RI][RO][VO][VI]}
    \stopsetups
    \setuphead[chapter]
               [style=\tfa,
                after={\blank\setups{chapter:headings}},
               ]
    \starttext
    \startchapter[title=Nothing special]
    \dorecurse{10}{\input knuth\par}
    \stopchapter
    \startchapter[title=Something special][custom={Hi there!}]
    \dorecurse{10}{\input knuth\par}
    \stopchapter
    \startchapter[title=More nothing special]
    \dorecurse{10}{\input knuth\par}
    \stopchapter
    \startchapter[title=More something special][custom={Bye Bye!}]
    \dorecurse{10}{\input knuth\par}
    \stopchapter
    \stoptext

-- 
Rik