form_foundation

Schema-agnostic building blocks for automatic form generators: typed field widgets, editor groups, list scaffolding, and a shared theme — the presentation layer a generator drives, with no schema logic of its own.

This package is the foundation under json_schema_form_builder (forms generated from JSON Schema). It is published separately so that other generators — a different schema dialect, a server-driven form engine, a hand-rolled admin panel — can reuse the same widgets and get the same look without pulling in any JSON Schema machinery.

Design

• The app (or the generator) owns the data. Every widget is props-in/callbacks-out: it renders a plain Dart value and reports the edited value back. No form state objects, no controllers to manage.
• The app owns the validation. Widgets display the error strings they are given; they never decide what is valid.
• One theme for everything. FormTheme / FormThemeData configure paddings, group frames, and code text style for every widget below them, the way Material component themes do.
• Generator-side contracts are minimal. FieldHandler, FieldHandlerRegistry, and NodeState define just enough for a generator to map nodes of its model to editors; nothing in them is specific to any schema language.

What's inside

Typed field widgets — a consistent Material look for the common scalar types:

Structure and scaffolding:

• EditorGroup — the framed/expanded container with a title, help text, and an error panel; the visual unit of nesting (EditorGroupView selects the presentation).
• ListEditor + RemovableRow + AddButton — rows with remove buttons, reordering, and a typed "add" menu, for collection editors.
• NodeEditor — renders a node through a FieldHandlerRegistry, the bridge a generator plugs its handlers into.
• CollapsedValueField — the compact "summary + edit button" representation a responsive editor collapses into (WidthThresholdBuilder decides when).
• FormDialog, ErrorsPanel, FieldLabel — the dialog wrapper, the error list, and the label with the required marker used by all of the above.

Usage

The widgets compose like any other Flutter widgets; the app keeps the values and passes errors in:

FormTheme(
  data: const FormThemeData(
    fieldPadding: EdgeInsets.symmetric(horizontal: 16, vertical: 8),
  ),
  child: Column(
    children: [
      StringField(
        title: 'Name',
        required: true,
        value: name,
        error: name.trim().isEmpty ? 'Name is required' : null,
        onChanged: (value) => setState(() => name = value),
      ),
      EditorGroup(
        title: 'Phones',
        content: Column(
          children: [
            for (final (i, phone) in phones.indexed)
              RemovableRow(
                field: StringField(
                  title: 'Phone ${i + 1}',
                  value: phone,
                  onChanged: (value) => setState(() => phones[i] = value),
                ),
                onRemove: () => setState(() => phones.removeAt(i)),
              ),
          ],
        ),
      ),
    ],
  ),
)

The example/ app is a complete hand-written contact form built this way — no generator involved.

When to use what

• You want forms generated from JSON Schema — use json_schema_form_builder; it drives these widgets for you.
• You are building a form generator for another schema language or a server-driven UI — depend on this package and implement your handlers against FieldHandler / FieldHandlerRegistry.
• You are writing a custom field handler for json_schema_form_builder and want it to match the built-in editors — build its UI from these widgets.