How to define items appearing in a list, which appears somewhere else in the document in ConTeXt?

Question

I have a document that will display a summary at the front of each chapter. The summary will list important points appearing later throughout the passages.

For example:

This chapter will talk about:
\printtopics

Section 1
Building a bridge is very easy. First start with some stones. Then,
\definetopic{How to build a temporary bridge from stones.}
find a narrow area in a river. Throw the stones into the river
until the river has completely stopped.
...

This would produce:

This chapter will talk about:
1. How to build a temporary bridge from stones.
2. How to hunt for rabbits.
3. How to find firewood.

Section 1
Building a bridge is very easy. First start with some stones. Then,
find a narrow area in a river. Throw the stones into the river
until the river has completely stopped.
...

The use of \definetopic{How to build a temporary bridge from stones.} means that those words are added to the list, and appear in the summary at the start of the chapter.

How can I define items to be added to a list, appearing at the start of a chapter, but define them within the text of the document, in ConTeXt?

Answer 1

You can define a chapter-local list for this purpose.

\definelist
  [topics]
  [criterium=chapter,
   headnumber=always,
   pagenumber=no]

\definecounter
  [topic]
  [way=bychapter]

\define[1]\definetopic{%
  \incrementcounter[topic]%
  \writetolist[topics]{\rawcountervalue[topic]}{#1}}

\starttext

\startchapter[title=Foo]
  This chapter will talk about:
  \placelist[topics]

  \startsection[title=Bar]
    \definetopic{How to build a temporary bridge from stones.}%
    \input knuth
    \definetopic{How to hunt for rabbits.}%
    \input knuth
    \definetopic{How to find firewood.}%
    \input knuth
  \stopsection
\stopchapter

\startchapter[title=Foo]
  This chapter will talk about:
  \placelist[topics]

  \startsection[title=Bar]
    \definetopic{How to hunt for rabbits.}%
    \input knuth
  \stopsection
\stopchapter

\stoptext