Direct browser → Dropbox uploads for Phoenix.LiveView.

Magpie.LiveView.UploadWriter keeps the bytes off your disk but still routes them through your server. presign_upload/4 removes the server from the path entirely: it mints a one-time upload link with Magpie.Files.get_temporary_upload_link/3 and hands it to the browser, which posts the file straight to Dropbox.

def mount(_params, _session, socket) do
  {:ok, allow_upload(socket, :avatar, accept: ~w(.jpg .png), external: &presign/2)}
end

defp presign(entry, socket) do
  Magpie.LiveView.presign_upload(MyApp.dropbox_client(), entry, socket,
    path: "/Avatars/" <> entry.client_name
  )
end

The client-side half lives in priv/static/magpie_uploader.js. Register it on your LiveSocket under the name Magpie returns in the metadata:

import Uploaders from "../../deps/magpie/priv/static/magpie_uploader"

let liveSocket = new LiveSocket("/live", Socket, {
  uploaders: Uploaders,
  params: {_csrf_token: csrfToken}
})

Once the entry completes, consume_uploaded_entries/3 receives the metadata returned below — the file is already in Dropbox:

consume_uploaded_entries(socket, :avatar, fn %{path: path}, _entry ->
  {:ok, path}
end)

Trade-offs

The destination is baked into the link when it is minted, so the browser cannot redirect the upload somewhere else — but it also cannot be changed after the fact, and the link is single-use.

Dropbox caps a temporary upload link at 150 MB, the same limit as a single-request upload. presign_upload/4 rejects larger entries up front rather than handing out a link that is guaranteed to fail; for those, fall back to Magpie.LiveView.UploadWriter, which chunks through an upload session.

Magpie does not depend on :phoenix_live_view — entry is only read for client_name and client_size, and socket is passed through untouched.