Decoupled winforms event loop from AppContext.

changes/750.bugfix.rst

@@ -0,0 +1 @@
+The WinForms event loop was decoupled from the main form, allowing background tasks to run without a main window being present.

winforms/src/toga_winforms/__init__.py

@@ -2,6 +2,18 @@
 
 import toga
 
+# Add a reference to the Winforms assembly
 clr.AddReference("System.Windows.Forms")
 
+# Add a reference to the WindowsBase assembly. This is needed to access
+# System.Windows.Threading.Dispatcher.
+#
+# This assembly isn't exposed as a simple dot-path name; we have to extract it from the
+# Global Assembly Cache (GAC). The version number and public key doesn't appear to
+# change with Windows version or the underlying .NET, and has been available since
+# Windows 7.
+clr.AddReference(
+    "WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
+)
+
 __version__ = toga._package_version(__file__, __name__)

winforms/src/toga_winforms/app.py

@@ -5,12 +5,10 @@
 from ctypes import windll
 
 import System.Windows.Forms as WinForms
-from System import (
-    Environment,
-    Threading,
-)
+from System import Environment, Threading
 from System.Media import SystemSounds
 from System.Net import SecurityProtocolType, ServicePointManager
+from System.Windows.Threading import Dispatcher
 
 import toga
 from toga import Key
@@ -60,6 +58,7 @@ def __init__(self, interface):
     def create(self):
         self.native = WinForms.Application
         self.app_context = WinForms.ApplicationContext()
+        self.app_dispatcher = Dispatcher.CurrentDispatcher
 
         # Check the version of windows and make sure we are setting the DPI mode
         # with the most up to date API
@@ -250,7 +249,7 @@ def run_app(self):
             # in a usable form.
             self.native.ThreadException += self.winforms_thread_exception
 
-            self.loop.run_forever(self.app_context)
+            self.loop.run_forever(self)
         except Exception as e:
             # In case of an unhandled error at the level of the app,
             # preserve the Python stacktrace

winforms/src/toga_winforms/libs/proactor.py

@@ -9,7 +9,7 @@
 
 
 class WinformsProactorEventLoop(asyncio.ProactorEventLoop):
-    def run_forever(self, app_context):
+    def run_forever(self, app):
         """Set up the asyncio event loop, integrate it with the Winforms event loop, and
         start the application.
 
@@ -27,8 +27,8 @@ def run_forever(self, app_context):
         # select call to process.
         self.call_soon(self._loop_self_reading)
 
-        # Remember the application context.
-        self.app_context = app_context
+        # Remember the application.
+        self.app = app
 
         # Set up the Proactor.
         # The code between the following markers should be exactly the same as
@@ -65,23 +65,19 @@ def run_forever(self, app_context):
         self.enqueue_tick()
 
         # Start the Winforms event loop.
-        WinForms.Application.Run(self.app_context)
+        WinForms.Application.Run(self.app.app_context)
 
     def enqueue_tick(self):
         # Queue a call to tick in 5ms.
-        self.task = Action[Task](self.tick)
-        Task.Delay(5).ContinueWith(self.task)
+        if not self.app._is_exiting:
+            self.task = Action[Task](self.tick)
+            Task.Delay(5).ContinueWith(self.task)
 
     def tick(self, *args, **kwargs):
         """Cause a single iteration of the event loop to run on the main GUI thread."""
-        # FIXME: this only works if there is a "main window" registered with the
-        # app (#750).
-        #
-        # If the app context has a main form, invoke run_once_recurring()
-        # on the thread associated with that form.
-        if self.app_context.MainForm:
+        if not self.app._is_exiting:
             action = Action(self.run_once_recurring)
-            self.app_context.MainForm.Invoke(action)
+            self.app.app_dispatcher.Invoke(action)
 
     def run_once_recurring(self):
         """Run one iteration of the event loop, and enqueue the next iteration (if we're

winforms/src/toga_winforms/window.py

@@ -131,18 +131,17 @@ def get_visible(self):
         return self.native.Visible
 
     def winforms_FormClosing(self, sender, event):
-        # If the app is exiting, or a manual close has been requested,
-        # don't get confirmation; just close.
+        # If the app is exiting, or a manual close has been requested, don't get
+        # confirmation; just close.
         if not self.interface.app._impl._is_exiting and not self._is_closing:
             if not self.interface.closeable:
-                # Closeability is implemented by shortcutting the close handler.
+                # Window isn't closable, so any request to close should be cancelled.
                 event.Cancel = True
             elif self.interface.on_close._raw:
-                # If there is an on_close event handler, process it;
-                # but then cancel the close event. If the result of
-                # on_close handling indicates the window should close,
-                # then it will be manually triggered as part of that
-                # result handling.
+                # If there is an on_close event handler, process it; but then cancel
+                # the close event. If the result of on_close handling indicates the
+                # window should close, then it will be manually triggered as part of
+                # that result handling.
                 self.interface.on_close(self)
                 event.Cancel = True