Merge pull request #2835 from freakboy3742/async-startup

Modify startup on Windows and macOS so event loop is running when startup is called

changes/2834.bugfix.rst

@@ -0,0 +1 @@
+The event loop is now guaranteed to be running when your app's ``startup()`` method is invoked. This wasn't previously the case on macOS and Windows.

cocoa/src/toga_cocoa/app.py

@@ -107,8 +107,8 @@ def __init__(self, interface):
         # Create the lookup table for commands and menu items
         self._menu_items = {}
 
-        # Call user code to populate the main window
-        self.interface._startup()
+        # Populate the main window as soon as the event loop is running.
+        self.loop.call_soon_threadsafe(self.interface._startup)
 
     ######################################################################
     # Commands and menus

examples/handlers/handlers/app.py

@@ -44,7 +44,7 @@ async def do_async(self, widget, **kwargs):
         self.async_label.text = "Ready."
         widget.enabled = True
 
-    async def do_background_task(self, widget, **kwargs):
+    async def do_background_task(self):
         """A background task."""
         # This task runs in the background, without blocking the main event loop
         while True:
@@ -64,7 +64,7 @@ def startup(self):
 
         # Add a background task.
         self.counter = 0
-        self.add_background_task(self.do_background_task)
+        asyncio.create_task(self.do_background_task())
 
         # Buttons
         btn_style = Pack(flex=1)

winforms/src/toga_winforms/app.py

@@ -123,8 +123,8 @@ def create(self):
                 "You may experience difficulties accessing some web server content."
             )
 
-        # Call user code to populate the main window
-        self.interface._startup()
+        # Populate the main window as soon as the event loop is running.
+        self.loop.call_soon_threadsafe(self.interface._startup)
 
     ######################################################################
     # Commands and menus