feat: add postComposerActionMessage helper

src/next/index.ts

@@ -4,3 +4,7 @@
 export { getFrameMetadata } from './getFrameMetadata.js'
 export { handle } from '../vercel/index.js'
 export { isFrameRequest } from './isFrameRequest.js'
+export {
+  postComposerActionMessage,
+  postComposerCreateCastActionMessage,
+} from './postComposerActionMessage.js'

src/next/postComposerActionMessage.ts

@@ -0,0 +1,38 @@
+export type ComposerActionMessage = {
+  type: 'createCast'
+  data: {
+    cast: {
+      parent?: string | undefined
+      text: string
+      embeds: string[]
+    }
+  }
+}
+
+/**
+ * Posts Composer Action Message to `window.parent`.
+ */
+export function postComposerActionMessage(message: ComposerActionMessage) {
+  if (typeof window === 'undefined')
+    throw new Error(
+      '`postComposerActionMessage` must be called in the Client Component.',
+    )
+
+  window.parent.postMessage(message, '*')
+}
+
+/**
+ * Posts Composer Create Cast Action Message to `window.parent`.
+ *
+ * This is a convinience method and it calls `postComposerActionMessage` under the hood.
+ */
+export function postComposerCreateCastActionMessage(
+  message: ComposerActionMessage['data']['cast'],
+) {
+  return postComposerActionMessage({
+    type: 'createCast',
+    data: {
+      cast: message,
+    },
+  })
+}