Callbacks

htmlbag exposes two callback hooks for embedders that want to observe or react to the layout pipeline.

ElementCallback

type ElementEvent struct {
    TagName     string
    TextContent string
    VList       *node.VList
}

type ElementCallbackFunc func(event ElementEvent)

Set CSSBuilder.ElementCallback to a function that runs after each block-level element’s VList is built. Used for collecting headings, detecting figure captions, building a table of contents, or debugging. Fires before page-output, so the Page field of any heading reference will not yet be valid here.

PageInitCallback

type PageInitCallbackFunc func()

Set CSSBuilder.PageInitCallback to a function that runs once per new page, just after the page is initialised but before any content is placed. Use it to draw page-level chrome (running headers, footers, page numbers, watermarks). Read the active page geometry from cb.frontend.Doc.CurrentPage.Userdata[PageDimensionsKey].

Headings

Independent of the callback, CSSBuilder.Headings accumulates a []HeadingEntry for every <h1>–<h6> encountered. The Page field is filled at output time, so wait until OutputPages* returns before reading the slice for TOC generation.