dart_bridge

dart_bridge is a Python C extension that enables bi-directional communication between embedded Python and a Dart application (such as a Flutter app) via Dart's FFI (Foreign Function Interface) and ports.

It is designed for use cases where you want to embed Python inside a Dart app, send messages between Python and Dart, and run long-lived, message-driven Python scripts that cooperate with Dart logic.

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🚀 Features

• ✅ Send messages (bytes) from Python to Dart
• ✅ Receive messages from Dart into Python via FFI
• ✅ Cross-thread safe using PyGILState_Ensure
• ✅ Fully compatible with Dart's dart_api_dl.h via FFI
• ✅ Portable across macOS, Linux, Windows, Android, and iOS
• ✅ Compatible with Python 3.12+ and the abi3 stable ABI

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💡 Usage Scenarios

• Embedding Python in Flutter for dynamic scripting, ML, or automation
• Integrating Python data pipelines or AI models into mobile/desktop apps
• Building hybrid apps where UI is Flutter but business logic is Python
• Real-time event or message processing in an embedded Python runtime

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🔄 Communication Overview

Dart → Python:
- Dart calls native function: `DartBridge_EnqueueMessage(...)`
- Python handler receives bytes via a registered Python function

Python → Dart:
- Python calls `dart_bridge.send_bytes(bytes)`
- Dart receives data via a `ReceivePort` attached to FFI `SendPort`

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Python API

dart_bridge.send_bytes(data: bytes) -> bool

Sends bytes to Dart via a previously registered Dart port.
Returns True if successful.
Raises RuntimeError if the port is not set or Dart_PostCObject_DL fails.

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dart_bridge.set_enqueue_handler_func(func: Callable[[bytes], None])

Registers a Python function that will be called when Dart sends a message using FFI.
The function should accept a single bytes argument.

This enables Dart → Python messaging.

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Dart FFI Requirements

On the Dart side, the following native functions are available via FFI:

// Initialize Dart dynamic linking (must be called once)
intptr_t DartBridge_InitDartApiDL(void* data);

// Register a Dart SendPort for messages from Python
void DartBridge_SetSendPort(int64_t port);

// Send message from Dart into Python (calls the Python handler)
void DartBridge_EnqueueMessage(const char* data, size_t len);

These are typically loaded in Dart using DynamicLibrary.lookupFunction.

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Installation

Install from PyPI:

pip install dart_bridge

Or build from source with:

pip install .

Make sure to include dart_api_dl.h from the Dart SDK in your compiler's include path.

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Example Usage

Dart side

final dylib = DynamicLibrary.open('dart_bridge.so');

final initApi = dylib.lookupFunction<
  IntPtr Function(Pointer<Void>),
  int Function(Pointer<Void>)
>('DartBridge_InitDartApiDL');

final setPort = dylib.lookupFunction<
  Void Function(Int64),
  void Function(int)
>('DartBridge_SetSendPort');

final enqueue = dylib.lookupFunction<
  Void Function(Pointer<Char>, IntPtr),
  void Function(Pointer<Char>, int)
>('DartBridge_EnqueueMessage');

initApi(NativeApi.initializeApiDLData);
setPort(sendPort.nativePort);

final msg = 'hello from Dart';
final ptr = msg.toNativeUtf8().cast<Char>();
enqueue(ptr, msg.length);
calloc.free(ptr);

Python side

import asyncio
import dart_bridge

queue = asyncio.Queue()

def enqueue_from_dart(data: bytes):
    print("Received:", data)
    queue.put_nowait(data)

dart_bridge.set_enqueue_handler_func(enqueue_from_dart)

async def main():
    while True:
        msg = await queue.get()
        dart_bridge.send_bytes(b"ECHO: " + msg)

asyncio.run(main())

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Thread Safety

• DartBridge_EnqueueMessage is safe to call from Dart threads or isolates.
• It uses PyGILState_Ensure() internally.
• Your Python handler will run in a background thread. Use loop.call_soon_threadsafe() if needed.

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License

MIT

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Contributing

Pull requests are welcome! This project helps bridge Dart and Python through portable, async-safe FFI communication.