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004: Dart Implementation

This RFC proposes adding a Dart implementation of Connect starting with support for clients. This will be similar in scope to Connect-Swift and Connect-Kotlin implementations.

Why​

Dart is a popular programming language powering frameworks like Flutter which is used for cross-platform app development. This project will bring Connect support to the growing Flutter and Dart ecosystem.

Anticipated complications​

Dart runs on several platforms: Web, Android, and iOS to name a few. Most of the platforms have their own HTTP stack with varying levels of support for HTTP specs. It doesn't have a unified abstraction and APIs for common HTTP constructs like headers and cancellation.

We have to create a minimal HTTP abstraction to support various platforms. We can then have concrete implementations using the following:

  • HttpClient from dart:io. This is part of the std library and runs on native platforms. This only supports HTTP/1.
  • package:http2. This is a HTTP/2 implementation maintained by the Dart team. This can be used to support bidirectional streaming and gRPC. Connection management has to be implemented on top of this.
  • dart:js_interop: This is part of the std library and can be used to create bindings for web APIs. We can use the fetch APIs to support Connect and gRPC-Web.

Platforms like Android and iOS have their own HTTP stack, and it is desirable to use them. package:http can be used as a cross platform client to support both. It is maintained by the Dart team. The only blocker is the missing support for aborting requests. The dart:io based client will run on these devices. Depending on the how the issue progresses we may have to provide ancillary packages to support native stacks. This however, need not block v1 as we won't need API changes to support them.

Supported features​

Clients should support all three protocols and four RPC kinds where possible:

  • Connect, gRPC-Web:
    • Should be supported on all platforms.
    • On the web, unary and server stream can be supported.
  • gRPC:
    • Can be supported on native platforms with support for HTTP/2

These should be continuously tested using the conformance suite on both native and web platforms.

Others​

Clients should also support ancillary features, including but not limited to interceptors, compression, mocking support, and a pluggable HTTP stack.

Maintainers​

The project can be maintained by: