update

picker/scripts/boot.js

@@ -0,0 +1,35 @@
+/** The boot contract: one fetch that tells the page how to start.
+ *
+ * Both scripts on the page read it, so it is fetched once and shared. A server
+ * that predates the contract answers 404, and the page starts the way it always
+ * did: a blank questionnaire.
+ */
+
+let bootPromise = null;
+
+export function getBoot() {
+  bootPromise ??= fetch('/boot.json')
+    .then((response) => (response.ok ? response.json() : null))
+    .catch(() => null)
+    .then((data) => ({
+      mode: data?.mode === 'doc' ? 'doc' : 'questionnaire',
+      prior: data?.prior && typeof data.prior === 'object' && !Array.isArray(data.prior) ? data.prior : null,
+      priorSource: data?.priorSource || null,
+      doc: data?.doc || null,
+    }));
+  return bootPromise;
+}
+
+/* Restoring a previous run happens inside the questionnaire, which owns the
+   state being restored. The document waits on this before it renders, so it
+   never reads a form that is still half filled. It resolves either way: a run
+   with nothing to restore is ready immediately. */
+let settle;
+export const hydrationReady = new Promise((resolve) => { settle = resolve; });
+
+/* The attribute is the observable half: it drives the note on the start screen
+   and gives anything watching the page one thing to wait for. */
+export function markHydrated(source) {
+  document.body.dataset.hydrated = source || 'none';
+  settle(source || null);
+}