From: Aditya Mahajan <adityam@umich.edu>
To: mailing list for ConTeXt users <ntg-context@ntg.nl>
Subject: Re: Let chapter start on odd page and make the even page before show an image
Date: Fri, 29 May 2020 12:03:43 -0400 (EDT) [thread overview]
Message-ID: <nycvar.YAK.7.77.849.2005291152530.856860@nqv-guvaxcnq> (raw)
In-Reply-To: <4a348554-556e-df7f-1b89-22b854f36290@mailbox.org>
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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next prev parent reply other threads:[~2020-05-29 16:03 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-29 8:29 juh
2020-05-29 10:45 ` Wolfgang Schuster
2020-05-29 13:10 ` Jan U. Hasecke
2020-05-29 13:35 ` Wolfgang Schuster
2020-05-29 15:42 ` Jan U. Hasecke
2020-05-29 16:03 ` Aditya Mahajan [this message]
2020-05-29 17:27 ` Jan U. Hasecke
2020-05-29 20:13 ` Henning Hraban Ramm
2020-05-29 20:18 ` Aditya Mahajan
2020-05-29 21:20 ` Henning Hraban Ramm
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