Python: Add support for invoke_from_event_loop

Fixes #4203

api/python/slint/lib.rs

@@ -63,6 +63,19 @@ fn set_xdg_app_id(app_id: String) -> Result<(), errors::PyPlatformError> {
     slint_interpreter::set_xdg_app_id(app_id).map_err(|e| e.into())
 }
 
+#[gen_stub_pyfunction]
+#[pyfunction]
+fn invoke_from_event_loop(callable: Py<PyAny>) -> Result<(), errors::PyEventLoopError> {
+    slint_interpreter::invoke_from_event_loop(move || {
+        Python::attach(|py| {
+            if let Err(err) = callable.call0(py) {
+                eprintln!("Error invoking python callable from closure invoked via slint::invoke_from_event_loop: {}", err)
+            }
+        })
+    })
+    .map_err(|e| e.into())
+}
+
 use pyo3::prelude::*;
 
 #[pymodule]
@@ -90,6 +103,7 @@ fn slint(_py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
     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)?)?;
+    m.add_function(wrap_pyfunction!(invoke_from_event_loop, m)?)?;
 
     Ok(())
 }

api/python/slint/slint/slint.pyi

@@ -143,6 +143,7 @@ class Timer:
     def restart(self) -> None: ...
 
 def set_xdg_app_id(app_id: str) -> None: ...
+def invoke_from_event_loop(callable: typing.Callable[[], None]) -> None: ...
 def run_event_loop() -> None: ...
 def quit_event_loop() -> None: ...
 

api/python/slint/tests/test_invoke_from_event_loop.py

@@ -0,0 +1,32 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+from slint import slint as native
+import threading
+from datetime import timedelta
+
+was_here = False
+
+
+def test_threads() -> None:
+    global was_here
+    was_here = False
+
+    def invoked_from_event_loop() -> None:
+        global was_here
+        was_here = True
+        native.quit_event_loop()
+
+    def quit() -> None:
+        native.invoke_from_event_loop(invoked_from_event_loop)
+
+    thr = threading.Thread(target=quit)
+    native.Timer.single_shot(timedelta(milliseconds=10), lambda: thr.start())
+    fallback_timer = native.Timer()
+    fallback_timer.start(
+        native.TimerMode.Repeated, timedelta(milliseconds=100), native.quit_event_loop
+    )
+    native.run_event_loop()
+    thr.join()
+    fallback_timer.stop()
+    assert was_here