Expo: Native download

createExpoSupabaseDownload returns the download function of the transfer port, implemented for iOS and Android. An app with an attachment column calls it once, in src/kizunasync.ts, where it replaces the download half of the Storage transfer passed to createSupabaseKizunaSync; nothing else in the app calls it. An app needs this page when it syncs an attachment column on React Native, or to diagnose an error a download raises.

It reads a peer's attachment over a short-lived signed URL with Expo's own fetch, verifies the bytes, and writes them into the file store. The other four transfer methods need no native override, so the app composes this one over createSupabaseTransfer.

Examples#

Basic#

The call sits in src/kizunasync.ts, next to the file store.

// src/kizunasync.ts (excerpt)
import { openExpoFileStore } from 'kizunasync/expo/file-store'
import { createExpoSupabaseDownload } from 'kizunasync/expo/transfer'
import { supabase } from './supabase-client'

const fileStore = openExpoFileStore()
const download = createExpoSupabaseDownload({ client: supabase, fileStore })

Compose the whole transfer port#

This is src/kizunasync.ts from Initializing with attachments added. The transfer option spreads createSupabaseTransfer and replaces its download key with this function.

// src/kizunasync.ts
import { attachment, byOwner, defineConfig } from 'kizunasync'
import { openExpoDriver } from 'kizunasync/expo'
import { openExpoFileStore } from 'kizunasync/expo/file-store'
import { createExpoSupabaseDownload } from 'kizunasync/expo/transfer'
import { createSupabaseKizunaSync, createSupabaseTransfer } from 'kizunasync/supabase'
import { supabase } from './supabase-client'

const config = defineConfig({
  tables: {
    todos: {
      sync: 'read-write',
      bucket: byOwner('user_id'),
      attachments: { image_path: attachment('todo-images') },
    },
  },
})

const fileStore = openExpoFileStore()
const ports = { client: supabase, fileStore }

export const kizunasync = createSupabaseKizunaSync({
  supabase,
  driver: openExpoDriver('todos.db'),
  config,
  fileStore,
  transfer: { ...createSupabaseTransfer(ports), download: createExpoSupabaseDownload(ports) },
})

Parameters#

The returned function takes the arguments the port defines.

Returns#

ITransfer['download'], a function returning Promise<void> that resolves once the bytes are written to the file store. It reports no progress of its own; the attachment queue tracks state per ref, which useAttachment reads.

Errors#

Every failure is thrown, and the attachment queue decides what to do with it from the code the error carries.

Notes#

The override reads the bytes with Expo's fetch, a WHATWG-compliant client with real binary support, so response.arrayBuffer() yields them directly, with no Blob round-trip. React Native forbids building a Blob from an ArrayBuffer, which is what supabase-js download() does. On Expo web the app passes no transfer, and createSupabaseKizunaSync builds the Storage transfer over the browser file store.

The read goes over a signed URL that lasts 60 seconds, which works for a public Storage bucket and a private one alike because the signature is issued under the caller's own session. Supabase documents the request in createSignedUrl and the plain read in Downloading. Kizuna adds the verification and the atomic write: the hash is checked before the file lands, so a partial or wrong body never becomes a cached attachment.

Uploads, confirmation, metadata, and removal stay on createSupabaseTransfer, which is why the second example spreads it and replaces one key. Storage policies on storage.objects decide what the session may read, as Access control describes.

Import this module from kizunasync/expo/transfer rather than the package root.

That import path keeps expo/fetch and the supabase-js peer out of the resolution graph for an app without attachments.

Related reference#

• File store
• Initializing
• JavaScript: Create the Storage transfer
• JavaScript: Resolve a download