Add sub data processors 

The problem it solves 

Content Block fields hold editor-managed content. Sometimes the JSON needs computed data: a page tree menu, a list of the latest news records, or anything else a TYPO3 data processor can produce. Instead of writing that in headless.php, wire standard data processors via TypoScript.

Wire a sub data processor 

tt_content.vendor_yourcontentblockelement.fields.data.dataProcessing.10 {
    dataProcessing {
        10 = menu
        10 {
            levels = 2
            as = navigation
        }
    }
}

What happens:

• The path addresses the nb-content-blocks-json processor (setup position 10 inside fields.data.dataProcessing) that the Site Set defines for lib.contentBlock.
• Inside it, dataProcessing. works exactly like everywhere else in TypoScript — any registered data processor can be used (menu, database-query, files, your own ...).
• The as name of each sub processor becomes a new key in data (here: data.navigation).

Result 

{
    "data": {
        "header": "My header",
        "navigation": [
            { "title": "Home", "link": "/" },
            { "title": "Products", "link": "/products/" }
        ]
    }
}

The sub-processor results are merged into data after the Content Block fields; the internal keys data and current are stripped from the result first, so they cannot collide with your fields.

Notes 

• Sub data processors run with the block's ContentObjectRenderer context — field: references in the sub processor resolve against the block's record.
• This mechanism is used in production (e.g. dynamic card lists rendered by an additional processor with as = cards).
• Prefer it over headless.php for anything TypoScript already provides.