examples: Add asyncio versions of async examples

The minimal + eratosthenes examples for async were previously based on
the Trio async package, now there are also versions based on the asyncio
package with minimal changes.

Task-number: PYSIDE-769
Change-Id: I5c1220e1c8ebeefb9a530745e52d8f907e08bb2c
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
(cherry picked from commit 3d036e0aab8fe1f835e18136573ffa8ad0359ff1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 118 insertions, 0 deletions

diff --git a/examples/async/minimal/minimal_asyncio.py b/examples/async/minimal/minimal_asyncio.py
new file mode 100644
index 000000000..337a2558d
--- /dev/null
+++ b/examples/async/minimal/minimal_asyncio.py
@@ -0,0 +1,118 @@
+# Copyright (C) 2022 The Qt Company Ltd.
+# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
+
+from PySide6.QtCore import (Qt, QEvent, QObject, Signal, Slot)
+from PySide6.QtWidgets import (QApplication, QLabel, QMainWindow, QPushButton, QVBoxLayout, QWidget)
+
+import asyncio
+import outcome
+import signal
+import sys
+import traceback
+
+
+class MainWindow(QMainWindow):
+
+    def __init__(self, async_signal):
+        super().__init__()
+
+        self.async_signal = async_signal
+
+        widget = QWidget()
+        self.setCentralWidget(widget)
+
+        layout = QVBoxLayout(widget)
+
+        self.text = QLabel("The answer is 42.")
+        layout.addWidget(self.text, alignment=Qt.AlignmentFlag.AlignCenter)
+
+        async_trigger = QPushButton(text="What is the question?")
+        async_trigger.clicked.connect(self.async_start)
+        layout.addWidget(async_trigger, alignment=Qt.AlignmentFlag.AlignCenter)
+
+    @Slot()
+    def async_start(self):
+        self.async_signal.emit()
+
+    async def set_text(self):
+        await asyncio.sleep(1)
+        self.text.setText("What do you get if you multiply six by nine?")
+
+
+class AsyncHelper(QObject):
+
+    trigger_signal = Signal()
+
+    class ReenterQtObject(QObject):
+        """ This is a QObject to which an event will be posted, allowing
+            Trio to resume when the event is handled. event.fn() is the
+            next entry point of the Trio event loop. """
+        def event(self, event):
+            if event.type() == QEvent.User + 1:
+                event.fn()
+                return True
+            return False
+
+    class ReenterQtEvent(QEvent):
+        """ This is the QEvent that will be handled by the ReenterQtObject.
+            self.fn is the next entry point of the Trio event loop. """
+        def __init__(self, fn):
+            super().__init__(QEvent.Type(QEvent.User + 1))
+            self.fn = fn
+
+    def __init__(self, entry=None):
+        super().__init__()
+        self.reenter_qt = self.ReenterQtObject()
+        self.entry = entry
+        self.loop = asyncio.new_event_loop()
+
+    def set_entry(self, entry):
+        self.entry = entry
+
+    @Slot()
+    def launch_guest_run(self):
+        """ To use asyncio and Qt together, one must run the asyncio
+            event loop as a "guest" inside the Qt "host" event loop. """
+        if not self.entry:
+            raise Exception("No entry point for the asyncio event loop was set.")
+        asyncio.set_event_loop(self.loop)
+        self.loop.create_task(self.entry())
+        self.loop.call_soon(self.next_guest_run_schedule)
+        self.loop.run_forever()
+
+    def continue_loop(self):
+        """ This function is called by an event posted to the Qt event
+            loop to restart the asyncio event loop. """
+        self.loop.call_soon(self.next_guest_run_schedule)
+        self.loop.run_forever()
+
+    def next_guest_run_schedule(self):
+        """ This function serves to pause and re-schedule the guest
+            (asyncio) event loop inside the host (Qt) event loop. It is
+            registered in asyncio as a callback to be called at the next
+            iteration of the event loop. When this function runs, it
+            first stops the asyncio event loop, then by posting an event
+            on the Qt event loop, it both relinquishes to Qt's event
+            loop and also schedules the asyncio event loop to run again.
+            Upon handling this event, a function will be called that
+            resumes the asyncio event loop. """
+        self.loop.stop()
+        QApplication.postEvent(self.reenter_qt, self.ReenterQtEvent(self.continue_loop))
+
+
+if __name__ == "__main__":
+    app = QApplication(sys.argv)
+    async_helper = AsyncHelper()
+    main_window = MainWindow(async_helper.trigger_signal)
+    async_helper.set_entry(main_window.set_text)
+
+    # This establishes the entry point for the Trio guest run. It varies
+    # depending on how and when its event loop is to be triggered, e.g.,
+    # at a specific moment like a button press (as here) or rather from
+    # the beginning.
+    async_helper.trigger_signal.connect(async_helper.launch_guest_run)
+
+    main_window.show()
+
+    signal.signal(signal.SIGINT, signal.SIG_DFL)
+    app.exec()