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")?;

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.
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).
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.