feat(functions_client): Add SSE support to invoke method

[dshukertjr]

What kind of change does this PR introduce?

Adds support for streaming response of an edge function that returns Server Sent Events.

Fixes #894

What is the current behavior?

There is no support for SSE, and developers have to use other libraries to listen to SSE.

What is the new behavior?

Users can listen to SSE like the following:

final response = await supabase.functions.invoke('sse-function');
response.data
    .transform(const Utf8Decoder())
    .listen((val) {
      print(val);
    });

Additional Context

On the web, you would need to pass a custom HTTP client like this:

final fetchClient = FetchClient(mode: RequestMode.cors);
await Supabase.initialize(
  url: supabaseUrl,
  anonKey: supabaseKey,
  httpClient: fetchClient,
);

[dshukertjr]

Here, I thought about adding .transform(const Utf8Decoder()) so that the developers don't have to add them by themselves, but Utf8Decoder() has a allowMalformed option, and I wanted to give the developers full control of what to do with the option, so I'm just returning raw ByteStream.

      data = response.stream.transform(const Utf8Decoder());

[Vinzent03]

The documentation for data in the FunctionResponse should be updated as well.

[Vinzent03]

From the documentation it says, that all headers are converted to lowercase. So the first check should never succeed.

[dshukertjr]

Hmm, I deleted the response.headers['Content-Type'] ?? and ran the test, but it seems like the tests are failing when the mock server returns Content-Type header in upper case, which suggests that the header keys are preserved the way it was sent from the server.

[Vinzent03]

I still think the documentation is correct, but we just skip the translation when mocking the response object directly. So just keep it.

[Vinzent03]

(I accidentally approved)

[dshukertjr]

Added this, because I thought people might want to type cast the response.data as ByteStream.

[Vinzent03]

What about adding the missing support for the stream in web to the doc as well? I already see an issue coming in here...

[nietsmmar]

Additional Context

On Flutter Web, the stream emits data only when the server is done sending the entire events. See the following issue for more info dart-lang/http#337

Thanks for your effort!

But is there no possibility for it to work in web? I managed to make it work in web (with older gotrue/supabase-flutter version) using flutter_client_sse and especially fetch_client. Could there be an option to use fetch_client as alternate http client as darts HttpClient does not support it yet.

Maybe like it is done in here: pratikbaid3/flutter_client_sse#19 (comment)

There is the option to provide an httpClientProvider so I can use fetch_client instead of dart-lang http for web.

This would make it possible to support streaming in web too.

[dshukertjr]

@nietsmmar Actually, just by passing fetch client, I was able to stream SSE on the web!

final client = FetchClient(mode: RequestMode.cors);
await Supabase.initialize(
  url: supabaseUrl,
  anonKey: supabaseKey,
  debug: false,
  httpClient: kIsWeb ? client : null,
);