On Fri, 29 May 2020, Jan U. Hasecke wrote:
One and a half question.
Why did you set \ChapterCounter explizitly and why is \ChapterCounter
part of the datasetvariable?
\dataset... is a high level interface for collecting two-pass data in ConTeXt.
Here is a simple example:
\definedataset[name]
\starttext
The values of the three dataset variables are:
\startlines
\datasetvariable{name}{hash1}{key}
\datasetvariable{name}{hash2}{key}
\datasetvariable{name}{hash3}{key}
\stoplines
Now we set the values.
\setdataset[name][hash1][key=value1]
\setdataset[name][hash2][key=value2]
\setdataset[name][hash3][key=value3]
\stoptext
Note that we are using the values stored in different hash-ids before they are
defined. ConTeXt stores the values in the `.tuc` file and uses them. This is
what is stored in the `.tuc` file:
utilitydata.job.datasets.collected={
["name"]={
["hash1"]={
["key"]="value1",
},
["hash2"]={
["key"]="value2",
},
["hash3"]={
["key"]="value3",
},
},
}
The magic of loading and reusing them happens behind the scenes, so the user
doesn't have to worry about it. See
https://wiki.contextgarden.net/System_Macros/Key_Value_Assignments#Multi-pass_data
for more details.
Now, suppose you wanted to use the image defined as part of `\startchapter` as
the background image of the _current_ page, then you could access the value of
the \structureuservariable. For example,
\startchapter[title=whatever][image=filename]
\structureuservariable{image}
\stopchapter
But you want to use the value of the image _before_ the start of the chapter.
So, Wolfgang is using the two-pass mechanism to store the value of the image in
the database named 'chapter` in the hash-location `\ChapterCounter` and then
retrieving it before it is defined!
Aditya
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