Form Patterns

Theme: keep cascading selections (pick A → derive B → write form) on a single, predictable path.

Accessing the form: form vs form.current

This is not absolute — it depends on the call site:

• Inside the form component (forms/index.tsx), or anywhere holding a Form.useForm() instance → call it directly: form.setFieldsValue(...).
• In the outer Drawer/Modal wrapper that opens the form and holds it via ref={form} (const form = useRef(null)), driven by an open prop → go through the ref: form.current?.setFieldsValue(...).

The reference template below is written for the Drawer-wrapper scenario (it reacts to open and owns the shared selection state), so it uses form.current? throughout. If you lift this logic into the form body with a useForm() instance, drop the .current.

1. No fallback for derived selection

When "pick A then auto-pick B", match by rule and return undefined if no match — let the form field stay empty. Do not silently fall back to list[0]; a fallback hides data issues and fakes a valid selection.

const findB = (key, list) =>
  key ? list.find((x) => x.key === key) : undefined;

For form fields, clear with undefined, not ''. In Ant Design undefined restores the placeholder; '' is treated as a real value.

2. Async race protection

For fetches triggered by a lifecycle entry (e.g. modal open), tag each invocation with a session ref. Discard stale results if the session rotated (modal closed and re-opened) before the response arrives.

const sessionRef = useRef(0);

useEffect(() => {
  if (!open) {
    sessionRef.current += 1;
    return;
  }
  const session = ++sessionRef.current;
  Promise.all([fetchA(), fetchB()]).then(([as, bs]) => {
    if (sessionRef.current !== session) return;
    applySelection(as.items[0], findB(as.items[0].key, bs.items));
  });
}, [open]);

3. Reference template

Two cascading selectors backed by a single shared state, with a single atomic write (state + form together):

type Selection = { a?: string; b?: number };

const [selection, setSelection] = useState<Selection>({});
const sessionRef = useRef(0);
const form = useRef<any>(null); // wrapper holds the form via <Form ref={form} /> — see "Accessing the form" above

const findB = (key, list) =>
  key ? list.find((x) => x.key === key) : undefined;

// Single atomic write: state + form together.
const applySelection = (a, b) => {
  setSelection({ a: a.name, b: b?.id });
  form.current?.setFieldsValue({
    field: b?.field,
    spec: { ...currentSpec, ...b?.spec }
  });
};

// Trigger 1: modal opened
useEffect(() => {
  if (!open) {
    sessionRef.current++;
    setSelection({});
    return;
  }
  const session = ++sessionRef.current;
  Promise.all([fetchA(), fetchB()]).then(([as, bs]) => {
    if (sessionRef.current !== session) return;
    const first = as.items[0];
    applySelection(first, findB(first.key, bs.items));
  });
}, [open]);

// Trigger 2: user picks A
const handleAChange = (a) => {
  applySelection(a, findB(a.key, listB));
};

// Trigger 3: user picks B
const handleBChange = (b) => {
  setSelection((prev) => ({ ...prev, b: b.id }));
  form.current?.setFieldsValue({ ...b.fields });
};

4. Controlled input with derived fields

When a controlled field's value comes from both user input and a programmatic default (e.g. a percentage picked on a slider, and a default seeded on select / mode-switch), funnel both through one commit function — don't duplicate "write field + recompute derived" per call site.

• The Form.Item-bound input's onChange(value) forwards the value to the commit fn (the field is antd-bound, but pass the value explicitly so the default path can reuse the same fn instead of reading the store).
• Seed defaults by calling the same commit fn with the computed value.
• Separate the commit action (write field + recompute dependents) from the render-derive (read the field → recompute dependents). Keeping the derive standalone lets it re-run on reload/edit where there's no user event.

// commit action — slider onChange AND default both call this
const commitRatio = (value: number) => {
  form.setFieldsValue({ spec: { resources: { ratio: value, cores: 100 } } });
  rescaleDerived(); // reads ratio from the form, sets the disabled cpu/ram
};

// render-derive — also called from the edit/reload effect
const rescaleDerived = () => {
  const ratio = form.getFieldValue(['spec', 'resources', 'ratio']);
  form.setFieldsValue({
    spec: {
      resources: {
        cpu: floorScale(unit.cpu, ratio),
        ram: floorScale(unit.ram, ratio)
      }
    }
  });
};

// default seeding reuses the commit fn — one path, not a second copy
const applyDefaults = (item) => commitRatio(Math.min(10, item.maxRatio) || 10);

Related

• Module/file structure for forms lives in the create-crud-page skill (section 3).
• Required-field validation: use getRuleMessage.