pdfcraft/crates/xfa

pdfcraft-xfa

XFA forms (XFA 3.3, the template and layout parts). Layer L3; depends on pdfcraft-cos and pdfcraft-fonts only.

A dynamic XFA form is a PDF shell around an XML template (/AcroForm /XFA): one placeholder page ("requires Adobe Reader") and /NeedsRendering true. Viewers without an XFA engine show the placeholder. This crate reads the template, lays it out, and writes ordinary pages and AcroForm fields into the document, so everything else in PdfCraft (rendering, filling, saving, printing, the automation tools) works on it unchanged. The engine does this when it opens such a form.

What it does

• Packets (read_packets): the XDP from one stream or the (name, stream) array; UTF-8 and UTF-16; capped at 64 MB.

• Template (parse): subforms, areas, fields, draws, exclusion groups, page sets with page areas, content areas and media; measurements in mm, cm, in, pt and px; fonts, paragraphs, margins, borders (edge order rules), captions with reserves, check-button items, pictures, rich text (exData XHTML: paragraphs, br, bold/italic/underline/size spans, embedded fields), inline JPEG images, lines, rectangles, occurrences, breakBefore/breakAfter, overflow leaders, columnWidths and colSpan.

• Layout (layout): positioned, tb, lr-tb/rl-tb, table and row layouts; a container without a width is as wide as its content; flow across content areas and pages with page masters chosen by occurrence; a table's header row repeats after a break; presence hidden takes no space, invisible takes space; repeating subforms get their initial instances; fields that compute the page number or count on layout show the numbers.

• PDF (pdf::write_form): pages with content streams in the standard 14 fonts (Arial and friends → Helvetica, Courier New → Courier, serif faces → Times; nothing is embedded), JPEG XObjects, and widgets: text (multi-line, max length, alignment), date (Acrobat's AFDate format and keystroke actions from the picture clause), check boxes and radio groups (from exclGroup), push buttons with Reset / Print / Save As / URI actions recognised from their scripts. Appearance streams are left empty for the forms layer to generate. Each field keeps its SOM path in /PCSom; the AcroForm gets /PCXfaLayout so a saved form is not laid out twice. The XFA packets stay, so Adobe's viewers keep rendering the form from them.

• Data (data): the datasets packet. On layout, a field takes its value from the data node at its SOM path (dates in ISO form, check and radio states by their on values), and a repeating subform or row gets as many instances as the data has. write_datasets merges the AcroForm fields' values (by /PCSom, or by the Designer field names of a static form) into the existing xfa:data: only the bound nodes' text changes and missing nodes are added, so unbound data, other namespaces, attributes and comments stay byte for byte. Values it can't write (a node holding structured content, absurd SOM indices, a packet that is neither UTF-8 nor UTF-16) come back as warnings; UTF-16 packets are written back as UTF-16. read_values goes the other way, so a form filled here shows its values in Adobe's viewers and a form filled there shows them here. The engine does both: values on open (recorded in the document's warnings), the packet after every form edit.

Layout measures each container once per (node, width): width-less containers would otherwise be measured exponentially often in their nesting. A measurement budget backs this up.

• Scripts (script): the live form as scripts see it (form_tree: the instance tree with values, presence and access, and the ScriptEvents in document order, built within MAX_FORM_NODES objects and cut short, with truncated set, past that; has_scripts says whether a template has any, and the engine skips all of this for forms without), the presence and access overrides scripts make, rows added and removed in the data (DataOps, applied to the in-memory DataNode while an event runs and written by write_data_ops as one rewrite of the datasets packet per event), and rerender, which lays the form out again from template, data and overrides. The scripts themselves run in the js crate's XFA object model; the engine runs initialize and calculate on open, change, exit, validate and calculate after a field changes, and click for buttons.

  Private keys PdfCraft writes (other viewers ignore them):

  Key	Where	Holds
  /PCSom	generated fields	the field's SOM path, so values go back to the data
  /PCXfaLayout	AcroForm	what laying out produced, so a saved form is not laid out again
  /PCXfaOverrides	AcroForm	<< /Presence << /som (hidden) … >> /Access << /som (readOnly) … >> >>: presence and access scripts set, by SOM path (instance 0 of a repeating subform stands for all); applied to the template before every layout, so undo, save and reopen keep them. Adobe's viewers draw from the XFA packets and don't read it; at most 10 000 entries are read or written
  /PCXfaClick	generated push buttons	true: the template has a click script for this button. Such buttons carry no /A: XFA JavaScript is not Acrobat JavaScript, so it is never written as a /S /JavaScript action other viewers would run. PdfCraft finds the script by the field's /PCSom

API sketch

if pdfcraft_xfa::is_dynamic(&doc) {
    let report = pdfcraft_xfa::render_into(&mut doc)?;   // pages, fields, warnings
    for f in pdfcraft_forms::fields(&doc) { pdfcraft_forms::redraw_field(&mut doc, &f.name)?; }
}
let form = pdfcraft_xfa::layout_xml(template_xml)?;       // pages of items, for tests

Deliberately not done (yet)

• Data binding is the default one only: explicit bind ref expressions, global binding and data descriptions are not followed, and no standalone XML or XDP data file is imported or exported.
• Scripting. JavaScript only: FormCalc scripts (the default language of older forms) are detected and reported, not run. The object model covers what forms commonly use (xfa.form navigation and SOM resolution, rawValue, presence, access, instance managers, xfa.host messages, reset, print, focus and URLs, xfa.layout page numbers, xfa.event); not xfa.template, data descriptions, xfa.connectionSet, border/font/ui properties (reads give a sink that accepts writes), execEvent, or the change event per keystroke. Validate failures show their message and keep the value, as Acrobat does for scripts. A hidden instance of a repeating subform hides every instance (overrides apply to the template). A repeating subform shows what its data holds once the data holds any instance, else its initial count, and never fewer than one (occur min="0" still shows one row). Each script runs in a fresh engine on its own thread; a form with hundreds of calculate scripts pays that on every field change (a shared engine per event is the obvious next step).
• Script limits. Forms run their scripts on open without being asked, so the engine bounds each event: a script's effects (values set again on the same object merge, the last wins; 10 000 effects, 100 message boxes, 1 000 console lines per script and per event), its loop iterations (100 000 per call frame for initialize, calculate and validate, 1 000 000 for click and change), 2 000 scripts and 50 relayouts per event, and 3 s (open, changes) or 5 s (click) of scripts per event, after which the rest are skipped and reported. Only a click may print, save, open a link or move the focus; other events asking are noted in the console. What open-time scripts change is noted in the document's XFA warnings. Known gaps: the engine's loop limit is per call frame, so loops inside nested calls can run far longer; such a script is abandoned after 1 s (3 s for a click) on its thread, which keeps running until the engine's own limits end it, and the document's scripts are turned off. Memory a script allocates is not capped (the same holds for AcroForm JavaScript).
• Static XFA forms (/NeedsRendering absent, AcroForm fields present) keep their AcroForm; only their data is read and written.
• Choice lists are text fields; signature, image, barcode and password fields are left blank; PNG and GIF images, keep constraints, subformSet relations, rl-tb is mirrored lr-tb, font metrics are the approximate Helvetica ones, and line heights are 1.15 × size.
• Acrobat cannot be an oracle here (clean-room rules): layout follows the specification, and fidelity has been checked by eye on real government forms, not pixel-compared.