pdfcraft/crates/forms/README.md

# pdfcraft-forms

Interactive forms (AcroForm, ISO 32000-2 §12.7). Layer L3; depends on `pdfcraft-cos` and
`pdfcraft-fonts`.

## API

```rust
let all: Vec<Field> = fields(&doc);                         // terminal fields with widgets, options, flags, /DA
let snapshot = fields_with_adopted(&doc);                     // fields and page-only adoption count in one pass
let all = snapshot.items;
let adopted = snapshot.adopted_page_fields;
set_value(&mut doc, "name", &FieldValue::Text("Ada".into()))?;
set_value(&mut doc, "agree", &FieldValue::Check(true))?;
set_value(&mut doc, "size", &FieldValue::Radio(Some("L".into())))?;
set_value(&mut doc, "toppings", &FieldValue::Choice(vec!["Ham".into()]))?;
reset(&mut doc, None)?;                                      // Clear form (back to /DV)

// Prepare a form
let name = add_field(&mut doc, 0, [72.0, 700.0, 216.0, 722.0], &NewField::Text { multiline: false }, None)?; // "Text1"
set_props(&mut doc, &name, &FieldProps { name: Some("email".into()), required: Some(true), ..Default::default() })?;
// A partial appearance edit keeps each widget's other styles and custom /DA font.
set_props(&mut doc, "email", &FieldProps {
    appearance: Some(LookPatch { width: Some(3.0), fill: Some(None), ..Default::default() }),
    ..Default::default()
})?;
delete_field(&mut doc, "email")?;

// A table of text fields (view space, y down), laid out and named before any of it is added:
// refused whole for a cell under MIN_FIELD_SIDE or a name with a period, repeated or taken.
let names = all.iter().map(|f| f.name.as_str());
let base = table::free_base(names.clone());                    // "Table1", or the next one free
let mut grid = table::FieldTable::new([40.0, 240.0, 240.0, 360.0], &base);
grid.set_columns(2, &base);
let cells: Vec<table::Cell> = grid.cells_for(names)?;      // "Table1_1", "Table1_c2_1", …
```

## Behaviour

- Inheritable attributes (`/FT`, `/Ff`, `/V`, `/DV`, `/DA`, `/Q`, `/MaxLen`) are resolved down the
  field tree; the form's `/DA` and `/Q` are the last fallback.
- Text and choice fields get new appearance streams (`/Tx BMC … EMC`): auto-size (`0 Tf`), comb,
  multiline wrapping, password masking, quadding, `/MK` background and border, list-box selection.
  The `/DA` font is used when `/DR` has it as a simple font or a composite font with a predefined
  Unicode CMap; otherwise Helvetica (WinAnsi). A value or button caption the font can't hold
  (Chinese in a Helvetica field, a character beyond U+FFFF in a UCS-2 CMap) is kept, as Acrobat
  keeps it: `/V` is stored as typed, the appearance draws `?` for those characters, the widget is
  marked with the first one (`pdfcraft_fonts::UNDRAWN_MARK`, cleared once all of it can be drawn)
  and the form sets `/NeedAppearances`, so other viewers draw the field from its value with fonts
  that have them. Not in a document that declares PDF/A, which forbids that flag (`declares_pdfa`).
  `undrawn_fields` lists the marked fields; the engine reports each edit that draws one, and
  flattening refuses to make that `?` permanent.
- Displaying a form that leaves its appearances to the viewer (`/NeedAppearances`, or a text or
  choice widget with no `/AP /N`): `with_field_appearances` returns a copy with those widgets
  drawn from their values, as typing them would draw them; the engine shows and prints that copy
  and saves the file as it was (#3470). `with_field_appearances_cached` keeps each widget's
  drawing while the objects it was drawn from are the same (`FieldAppearanceCache`), so typing in
  one field redraws that field, not the form.
- Check boxes and radio buttons switch `/V` and each widget's `/AS`. Widgets with no appearances
  get PdfCraft's own (a drawn check mark or dot; no symbol font needed).
- Imported `/MaxLen 0` is treated as unlimited (also clearing an inherited limit), while the
  original entry is preserved. Positive limits still apply.
- Values are validated (options, `MaxLen`, single vs multi-select, NoToggleToOff, read-only) and a
  failed call changes nothing.
- `FieldProps::appearance` applies a `LookPatch` independently to each widget. `None` keeps
  a property; `Some(None)` removes a border/fill colour. Unchanged `/DA`, custom font resources,
  widget-specific `/DA` overrides, indirect `/MK` and `/BS` contents, dash arrays and unknown
  dictionary keys are preserved. It is mutually exclusive with the complete `look` replacement.
  Use the engine's `Edit::Batch` to change several fields atomically in one undo step.
- `/RV` (rich text) is removed when a value is set so it can't contradict `/V`.
- New fields follow Acrobat: names Text1, Check Box1, Group1 (radio groups), Dropdown1, List Box1,
  Button1, Date1, Signature1; `/Helv` is added to `/DR`; appearances are drawn at once. Radio
  buttons joining a group become kid widgets with their own export state. Renaming changes only
  the last part of a dotted name.
- Renaming a field renames it in every Calculate script that reads it
  (`af::rename_in_calculation`): `AFSimple_Calculate` field lists, simplified field notation
  (the `BVCALC` comment and its translation), and `getField` calls whose argument is the name as
  one string literal. The rest of each script is kept as written. Names built at run time, a
  script stream that isn't UTF-8, other events' scripts and a name another field still has are
  left alone. `calculations_changed` compares two readings of a form to say which formulas an
  edit rewrote.
- The calculation order (`/CO`, §12.7.3): `calculation_order` lists the calculated fields in the
  order `recalculate` runs them, with the fields each formula reads (`Calculate::reads`,
  `af::calculation_reads`; names built at run time aren't known) and what is wrong with the
  order: a field reading one that calculates later, a loop, a field `/CO` leaves out, and `/CO`
  entries that calculate nothing. `CalculationOrder::fixed` is Fix order: a stable topological
  sort that keeps the current order wherever it can and leaves loops as they are.
  `set_calculation_order` writes `/CO` afresh. A field that gets a calculation joins the end of
  the order; one whose calculation changes keeps its place.
- `Widget::caption` reads a push button's normal label (`/MK /CA`, including indirect values).
  `FieldProps { caption: Some((widget_index, text)), ..Default::default() }` changes or clears
  that label, independently of the field name. The index is 0-based. A label-only edit redraws
  just that widget, copying shared/indirect `/MK` and appearance dictionaries before editing.
  Other widgets, actions, icons, rotation and unknown entries are preserved. Existing icon-only
  layouts keep hiding the caption; caption rendering retains the existing Helvetica/WinAnsi
  limits and reports unavailable characters as above.

## Not yet

Rich-text values and exact appearance metrics/layout remain incomplete. JavaScript
execution and FDF/XFDF exchange are integrated through the engine; this crate provides
field actions and native AF rules. `/MK /R` rotation (0, 90, 180, 270) is drawn into the
appearance.