Python: Initial support for asyncio

The API is minimal at this point: `slint.run_event_loop()` takes an
optional coroutine parameter, that's run. As the loop is run, an
asyncio event loop (as per asyncio.get_event_loop()) is active, which
maps socket operations to smol's Async adapter and polls them within the
Slint event loop.

cc #4137

Author: tronical

.github/workflows/ci.yaml

@@ -177,6 +177,7 @@ jobs:
             CARGO_PROFILE_DEV_DEBUG: 0
             CARGO_INCREMENTAL: false
             RUST_BACKTRACE: full
+            SLINT_BACKEND: winit
         strategy:
             matrix:
                 os: [ubuntu-22.04, macos-14, windows-2022]

CHANGELOG.md

@@ -3,6 +3,10 @@
 # Changelog
 All notable changes to this project are documented in this file.
 
+## [1.14.0] - TBD
+
+ - Python: Added support for asyncio by making the Slint event loop act as asyncio event loop.
+
 ## [1.13.1] - 2025-09-11
 
  - Windows: Fixed flickering when updating the menu bar.

api/python/slint/Cargo.toml

@@ -52,6 +52,7 @@ chrono = "0.4"
 spin_on = { workspace = true }
 css-color-parser2 = { workspace = true }
 pyo3-stub-gen = { version = "0.9.0", default-features = false }
+smol = { version = "2.0.0" }
 
 [package.metadata.maturin]
 python-source = "slint"

api/python/slint/README.md

@@ -22,7 +22,7 @@ in detail.
 
 ## Installation
 
-Slint can be installed with `uv` or `pip` from the [Python Package Index](https://pypi.org):
+Install Slint with `uv` or `pip` from the [Python Package Index](https://pypi.org):
 
 ```bash
 uv add slint
@@ -326,6 +326,22 @@ value = slint.loader.app.MyOption.Variant2
 main_window.data = value
 ```
 
+## Asynchronous I/O
+
+Use Python's [asyncio](https://docs.python.org/3/library/asyncio.html) library to write concurrent Python code with the `async`/`await` syntax.
+
+Slint's event loop is a full-featured [asyncio event loop](https://docs.python.org/3/library/asyncio-eventloop.html#asyncio-event-loop). While
+the event loop is running, [`asyncio.get_event_loop()`](https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.get_running_loop) returns
+a valid loop. To run an async function when starting the loop, pass a coroutine to `slint.run_event_loop()`.
+
+For the common use case of interacting with REST APIs, we recommend the [`aiohttp` library](https://docs.aiohttp.org/en/stable/).
+
+### Known Limitations
+
+- Pipes and sub-processes are only supported on Unix-like platforms.
+- Exceptions thrown in the coroutine passed to `slint.run_event_loop()` don't cause the loop to terminate. This behaviour may
+  change in a future release.
+
 ## Third-Party Licenses
 
 For a list of the third-party licenses of all dependencies, see the separate [Third-Party Licenses page](thirdparty.html).

api/python/slint/async_adapter.rs

@@ -0,0 +1,148 @@
+// Copyright © SixtyFPS GmbH <[email protected]>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+use std::rc::Rc;
+
+use pyo3::prelude::*;
+use pyo3_stub_gen::{derive::gen_stub_pyclass, derive::gen_stub_pymethods};
+
+#[cfg(unix)]
+struct PyFdWrapper(std::os::fd::RawFd);
+
+#[cfg(unix)]
+impl std::os::fd::AsFd for PyFdWrapper {
+    fn as_fd(&self) -> std::os::fd::BorrowedFd<'_> {
+        unsafe { std::os::fd::BorrowedFd::borrow_raw(self.0) }
+    }
+}
+
+#[cfg(windows)]
+struct PyFdWrapper(#[cfg(windows)] std::os::windows::io::RawSocket);
+
+#[cfg(windows)]
+impl std::os::windows::io::AsSocket for PyFdWrapper {
+    fn as_socket(&self) -> std::os::windows::io::BorrowedSocket<'_> {
+        unsafe { std::os::windows::io::BorrowedSocket::borrow_raw(self.0) }
+    }
+}
+
+struct AdapterInner {
+    adapter: smol::Async<PyFdWrapper>,
+    readable_callback: Option<Py<PyAny>>,
+    writable_callback: Option<Py<PyAny>>,
+}
+
+#[gen_stub_pyclass]
+#[pyclass(unsendable)]
+pub struct AsyncAdapter {
+    inner: Option<Rc<AdapterInner>>,
+    task: Option<slint_interpreter::JoinHandle<()>>,
+}
+
+#[gen_stub_pymethods]
+#[pymethods]
+impl AsyncAdapter {
+    #[new]
+    fn py_new(fd: i32) -> Self {
+        #[cfg(windows)]
+        let fd = u64::try_from(fd).unwrap();
+        AsyncAdapter {
+            inner: Some(Rc::new(AdapterInner {
+                adapter: smol::Async::new(PyFdWrapper(fd)).unwrap(),
+                readable_callback: Default::default(),
+                writable_callback: Default::default(),
+            })),
+            task: None,
+        }
+    }
+
+    fn wait_for_readable(&mut self, callback: Py<PyAny>) {
+        self.restart_after_mut_inner_access(|inner| {
+            inner.readable_callback.replace(callback);
+        });
+    }
+
+    fn wait_for_writable(&mut self, callback: Py<PyAny>) {
+        self.restart_after_mut_inner_access(|inner| {
+            inner.writable_callback.replace(callback);
+        });
+    }
+}
+
+impl AsyncAdapter {
+    fn restart_after_mut_inner_access(&mut self, callback: impl FnOnce(&mut AdapterInner)) {
+        if let Some(task) = self.task.take() {
+            task.abort();
+        }
+
+        // This detaches and basically makes any existing future that might get woke up fail when
+        // trying to upgrade the weak.
+        let mut inner = Rc::into_inner(self.inner.take().unwrap()).unwrap();
+
+        callback(&mut inner);
+
+        let inner = Rc::new(inner);
+        let inner_weak = Rc::downgrade(&inner);
+        self.inner = Some(inner);
+        self.task = Some(
+            slint_interpreter::spawn_local(std::future::poll_fn(move |cx| loop {
+                let Some(inner) = inner_weak.upgrade() else {
+                    return std::task::Poll::Ready(());
+                };
+
+                let readable_poll_status: Option<std::task::Poll<Py<PyAny>>> =
+                    inner.readable_callback.as_ref().map(|callback| {
+                        if inner.adapter.poll_readable(cx).is_ready() {
+                            std::task::Poll::Ready(Python::attach(|py| callback.clone_ref(py)))
+                        } else {
+                            std::task::Poll::Pending
+                        }
+                    });
+
+                let writable_poll_status: Option<std::task::Poll<Py<PyAny>>> =
+                    inner.writable_callback.as_ref().map(|callback| {
+                        if inner.adapter.poll_writable(cx).is_ready() {
+                            std::task::Poll::Ready(Python::attach(|py| callback.clone_ref(py)))
+                        } else {
+                            std::task::Poll::Pending
+                        }
+                    });
+
+                let fd = inner.adapter.get_ref().0;
+
+                drop(inner);
+
+                if let Some(std::task::Poll::Ready(callback)) = &readable_poll_status {
+                    Python::attach(|py| {
+                        callback.call1(py, (fd,)).expect(
+                            "unexpected failure running python async readable adapter callback",
+                        );
+                    });
+                }
+
+                if let Some(std::task::Poll::Ready(callback)) = &writable_poll_status {
+                    Python::attach(|py| {
+                        callback.call1(py, (fd,)).expect(
+                            "unexpected failure running python async writable adapter callback",
+                        );
+                    });
+                }
+
+                match &readable_poll_status {
+                    Some(std::task::Poll::Ready(..)) => continue, // poll again and then probably return in the next iteration
+                    Some(std::task::Poll::Pending) => return std::task::Poll::Pending, // waker registered, come back later
+                    None => {} // Nothing to poll
+                }
+
+                match &writable_poll_status {
+                    Some(std::task::Poll::Ready(..)) => continue, // poll again and then probably return in the next iteration
+                    Some(std::task::Poll::Pending) => return std::task::Poll::Pending, // waker registered, come back later
+                    None => {} // Nothing to poll
+                }
+
+                return std::task::Poll::Ready(());
+            }))
+            .unwrap(),
+        );
+    }
+}

api/python/slint/interpreter.rs

@@ -439,10 +439,6 @@ impl ComponentInstance {
         Ok(self.instance.hide()?)
     }
 
-    fn run(&self) -> Result<(), PyPlatformError> {
-        Ok(self.instance.run()?)
-    }
-
     fn __traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError> {
         self.callbacks.__traverse__(&visit)?;
         for global_callbacks in self.global_callbacks.values() {

api/python/slint/lib.rs

@@ -11,6 +11,7 @@ use interpreter::{
     CompilationResult, Compiler, ComponentDefinition, ComponentInstance, PyDiagnostic,
     PyDiagnosticLevel, PyValueType,
 };
+mod async_adapter;
 mod brush;
 mod errors;
 mod models;
@@ -42,10 +43,11 @@ thread_local! {
 
 #[gen_stub_pyfunction]
 #[pyfunction]
-fn run_event_loop() -> Result<(), PyErr> {
+fn run_event_loop(py: Python<'_>) -> Result<(), PyErr> {
     EVENT_LOOP_EXCEPTION.replace(None);
     EVENT_LOOP_RUNNING.set(true);
-    let result = slint_interpreter::run_event_loop();
+    // Release the GIL while running the event loop, so that other Python threads can run.
+    let result = py.detach(|| slint_interpreter::run_event_loop());
     EVENT_LOOP_RUNNING.set(false);
     result.map_err(|e| errors::PyPlatformError::from(e))?;
     EVENT_LOOP_EXCEPTION.take().map_or(Ok(()), |err| Err(err))
@@ -100,6 +102,7 @@ fn slint(_py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<brush::PyBrush>()?;
     m.add_class::<models::PyModelBase>()?;
     m.add_class::<value::PyStruct>()?;
+    m.add_class::<async_adapter::AsyncAdapter>()?;
     m.add_function(wrap_pyfunction!(run_event_loop, m)?)?;
     m.add_function(wrap_pyfunction!(quit_event_loop, m)?)?;
     m.add_function(wrap_pyfunction!(set_xdg_app_id, m)?)?;

api/python/slint/noxfile.py

@@ -8,4 +8,4 @@
 def python(session: nox.Session):
     session.env["MATURIN_PEP517_ARGS"] = "--profile=dev"
     session.install(".[dev]")
-    session.run("pytest", "-s")
+    session.run("pytest", "-s", "-v")

api/python/slint/pyproject.toml

@@ -37,7 +37,7 @@ Changelog = "https://github.com/slint-ui/slint/blob/master/CHANGELOG.md"
 Tracker = "https://github.com/slint-ui/slint/issues"
 
 [project.optional-dependencies]
-dev = ["pytest", "numpy>=2.3.2", "pillow>=11.3.0"]
+dev = ["pytest", "numpy>=2.3.2", "pillow>=11.3.0", "aiohttp>=3.12.15"]
 
 [dependency-groups]
 dev = [
@@ -48,13 +48,14 @@ dev = [
   "ruff>=0.9.6",
   "pillow>=11.3.0",
   "numpy>=2.3.2",
+  "aiohttp>=3.12.15",
 ]
 
 [tool.uv]
 # Rebuild package when any rust files change
 cache-keys = [{ file = "pyproject.toml" }, { file = "Cargo.toml" }, { file = "**/*.rs" }]
 # Uncomment to build rust code in development mode
-# config-settings = { build-args = '--profile=dev' }
+config-settings = { build-args = '--profile=dev' }
 
 [tool.mypy]
 strict = true

api/python/slint/slint/__init__.py

@@ -16,7 +16,10 @@
 import pathlib
 from .models import ListModel, Model
 from .slint import Image, Color, Brush, Timer, TimerMode
+from .loop import SlintEventLoop
 from pathlib import Path
+from collections.abc import Coroutine
+import asyncio
 
 Struct = native.PyStruct
 
@@ -52,7 +55,9 @@ def hide(self) -> None:
 
     def run(self) -> None:
         """Shows the window, runs the event loop, hides it when the loop is quit, and returns."""
-        self.__instance__.run()
+        self.show()
+        run_event_loop()
+        self.hide()
 
 
 def _normalize_prop(name: str) -> str:
@@ -426,6 +431,56 @@ def set_xdg_app_id(app_id: str) -> None:
     native.set_xdg_app_id(app_id)
 
 
+quit_event = asyncio.Event()
+
+
+def run_event_loop(
+    main_coro: typing.Optional[Coroutine[None, None, None]] = None,
+) -> None:
+    """Runs the main Slint event loop. If specified, the coroutine `main_coro` is run in parallel. The event loop doesn't
+    terminate when the coroutine finishes, it terminates when calling `quit_event_loop()`.
+
+    Example:
+    ```python
+    import slint
+
+    ...
+    image_model: slint.ListModel[slint.Image] = slint.ListModel()
+    ...
+
+    async def main_receiver(image_model: slint.ListModel) -> None:
+        async with aiohttp.ClientSession() as session:
+            async with session.get("http://some.server/svg-image") as response:
+                svg = await response.read()
+                image = slint.Image.from_svg_data(svg)
+                image_model.append(image)
+
+    ...
+    slint.run_event_loop(main_receiver(image_model))
+    ```
+
+    """
+
+    async def run_inner() -> None:
+        global quit_event
+        loop = typing.cast(SlintEventLoop, asyncio.get_event_loop())
+        if main_coro:
+            loop.create_task(main_coro)
+
+        await quit_event.wait()
+
+    global quit_event
+    quit_event = asyncio.Event()
+    asyncio.run(run_inner(), debug=False, loop_factory=SlintEventLoop)
+
+
+def quit_event_loop() -> None:
+    """Quits the running event loop in the next event processing cycle. This will make an earlier call to `run_event_loop()`
+    return."""
+    global quit_event
+    quit_event.set()
+
+
 __all__ = [
     "CompileError",
     "Component",
@@ -440,4 +495,6 @@ def set_xdg_app_id(app_id: str) -> None:
     "TimerMode",
     "set_xdg_app_id",
     "callback",
+    "run_event_loop",
+    "quit_event_loop",
 ]