added preview method

studio/utils/preview.ts

@@ -0,0 +1,20 @@
+import {
+  PortableTextBlock,
+  isPortableTextSpan,
+  isPortableTextTextBlock,
+} from "sanity";
+
+// This help method convert rich text content to plaintext so it could be used as preview in sanity studio
+// (inspired by https://www.sanity.io/docs/previewing-block-content)
+export function richTextPreview(
+  richText: PortableTextBlock[],
+): string | undefined {
+  const block = richText.find((block) => block._type === "block");
+  if (!isPortableTextTextBlock(block)) {
+    return undefined;
+  }
+  return block.children
+    .filter((child) => isPortableTextSpan(child))
+    .map((span) => span.text)
+    .join("");
+}