[NTG-context] Let chapter start on odd page and make the even page before show an image

[Aditya Mahajan]

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