Add support for background and session-based apps

android/src/toga_android/app.py

@@ -10,6 +10,7 @@
 from java import dynamic_proxy
 from org.beeware.android import IPythonApp, MainActivity
 
+import toga
 from toga.command import Command, Group, Separator
 from toga.handlers import simple_handler
 
@@ -182,6 +183,9 @@ def onPrepareOptionsMenu(self, menu):
 
 
 class App:
+    # Android apps exit when the last window is closed
+    CLOSE_ON_LAST_WINDOW = True
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
@@ -241,10 +245,13 @@ def set_icon(self, icon):
         pass  # pragma: no cover
 
     def set_main_window(self, window):
-        # The default layout of an Android app includes a titlebar; a simple App then
-        # hides that titlebar. We know what type of app we have when the main window is
-        # set.
-        self.interface.main_window._impl.configure_titlebar()
+        if window is None or window == toga.App.BACKGROUND:
+            raise ValueError("Apps without main windows are not supported on Android")
+        else:
+            # The default layout of an Android app includes a titlebar; a simple App
+            # then hides that titlebar. We know what type of app we have when the main
+            # window is set.
+            self.interface.main_window._impl.configure_titlebar()
 
     ######################################################################
     # App resources

changes/2209.feature.rst

@@ -0,0 +1 @@
+Toga can now define an app whose life cycle isn't tied to a single main window.

changes/2653.bugfix.rst

@@ -0,0 +1 @@
+On Winforms, the window of an application that is set as the main window is no longer shown as a result of assigning the window as ``App.main_window``.

changes/97.feature.rst

@@ -0,0 +1 @@
+Toga can now define apps that persist in the background without having any open windows.

cocoa/src/toga_cocoa/app.py

@@ -1,6 +1,5 @@
 import asyncio
 import inspect
-import os
 import sys
 from pathlib import Path
 from urllib.parse import unquote, urlparse
@@ -28,6 +27,7 @@
     NSAboutPanelOptionApplicationVersion,
     NSAboutPanelOptionVersion,
     NSApplication,
+    NSApplicationActivationPolicyAccessory,
     NSApplicationActivationPolicyRegular,
     NSBeep,
     NSBundle,
@@ -108,6 +108,9 @@ def validateMenuItem_(self, sender) -> bool:
 
 
 class App:
+    # macOS apps persist when there are no windows open
+    CLOSE_ON_LAST_WINDOW = False
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
@@ -117,28 +120,21 @@ def __init__(self, interface):
         asyncio.set_event_loop_policy(EventLoopPolicy())
         self.loop = asyncio.new_event_loop()
 
-        # Stimulate the build of the app
-        self.create()
-
-    def create(self):
         self.native = NSApplication.sharedApplication
-        self.native.setActivationPolicy(NSApplicationActivationPolicyRegular)
 
         # The app icon been set *before* the app instance is created. However, we only
         # need to set the icon on the app if it has been explicitly defined; the default
         # icon is... the default. We can't test this branch in the testbed.
         if self.interface.icon._impl.path:
             self.set_icon(self.interface.icon)  # pragma: no cover
 
-        self.resource_path = os.path.dirname(
-            os.path.dirname(NSBundle.mainBundle.bundlePath)
-        )
+        self.resource_path = Path(NSBundle.mainBundle.bundlePath).parent.parent
 
         self.appDelegate = AppDelegate.alloc().init()
         self.appDelegate.impl = self
         self.appDelegate.interface = self.interface
         self.appDelegate.native = self.native
-        self.native.setDelegate_(self.appDelegate)
+        self.native.setDelegate(self.appDelegate)
 
         # Create the lookup table for menu items
         self._menu_groups = {}
@@ -422,7 +418,10 @@ def set_icon(self, icon):
             self.native.setApplicationIconImage(None)
 
     def set_main_window(self, window):
-        pass
+        if window == toga.App.BACKGROUND:
+            self.native.setActivationPolicy(NSApplicationActivationPolicyAccessory)
+        else:
+            self.native.setActivationPolicy(NSApplicationActivationPolicyRegular)
 
     ######################################################################
     # App resources

cocoa/tests_backend/app.py

@@ -229,3 +229,11 @@ def keystroke(self, combination):
             keyCode=key_code,
         )
         return toga_key(event)
+
+    async def restore_standard_app(self):
+        # If the tesbed app has been made a background app, it will no longer be
+        # active, which affects whether it can receive focus and input events.
+        # Make it active again. This won't happen immediately; allow for a short
+        # delay.
+        self.app._impl.native.activateIgnoringOtherApps(True)
+        await self.redraw("Restore to standard app", delay=0.1)

core/src/toga/app.py

@@ -2,6 +2,7 @@
 
 import asyncio
 import importlib.metadata
+import inspect
 import signal
 import sys
 import warnings
@@ -205,6 +206,13 @@ class App:
     _impl: Any
     _camera: Camera
     _location: Location
+    _main_window: Window | str | None
+
+    #: A constant that can be used as the main window to indicate that an app will
+    #: run in the background without a main window.
+    BACKGROUND: str = "background app"
+
+    _UNDEFINED: str = "<main window not assigned>"
 
     def __init__(
         self,
@@ -377,7 +385,7 @@ def __init__(
 
         self._startup_method = startup
 
-        self._main_window: MainWindow | None = None
+        self._main_window = App._UNDEFINED
         self._windows = WindowSet(self)
 
         self._full_screen_windows: tuple[Window, ...] | None = None
@@ -515,35 +523,61 @@ def main_loop(self) -> None:
         self._impl.main_loop()
 
     @property
-    def main_window(self) -> MainWindow | None:
-        """The main window for the app."""
+    def main_window(self) -> Window | str | None:
+        """The main window for the app.
+
+        See :ref:`the documentation on assigning a main window <assigning-main-window>`
+        for values that can be used for this attribute.
+        """
+        if self._main_window is App._UNDEFINED:
+            raise ValueError("Application has not set a main window.")
+
         return self._main_window
 
     @main_window.setter
-    def main_window(self, window: MainWindow | None) -> None:
-        # The main window must be closable
-        if isinstance(window, Window) and not window.closable:
-            raise ValueError("The window used as the main window must be closable.")
+    def main_window(self, window: MainWindow | str | None) -> None:
+        if window is None or window is App.BACKGROUND or isinstance(window, Window):
+            # The main window must be closable
+            if isinstance(window, Window) and not window.closable:
+                raise ValueError("The window used as the main window must be closable.")
+
+            old_window = self._main_window
+            self._main_window = window
+            try:
+                self._impl.set_main_window(window)
+            except Exception as e:
+                # If the main window could not be changed, revert to the previous value
+                # then reraise the exception
+                if old_window is not App._UNDEFINED:
+                    self._main_window = old_window
+                raise e
+        else:
+            raise ValueError(f"Don't know how to use {window!r} as a main window.")
 
-        self._main_window = window
-        self._impl.set_main_window(window)
+    def _create_initial_windows(self):
+        # TODO: Create the initial windows for the app.
 
-    def _verify_startup(self) -> None:
-        if not isinstance(self.main_window, Window):
-            raise ValueError(
-                "Application does not have a main window. "
-                "Does your startup() method assign a value to self.main_window?"
-            )
+        # Safety check: Do we have at least one window?
+        if len(self.app.windows) == 0 and self.main_window is None:
+            # macOS document-based apps are allowed to have no open windows.
+            if self.app._impl.CLOSE_ON_LAST_WINDOW:
+                raise ValueError("App doesn't define any initial windows.")
 
     def _startup(self) -> None:
         # Install the platform-specific app commands. This is done *before* startup so
         # the user's code has the opporuntity to remove/change the default commands.
         self._impl.create_app_commands()
 
-        # This is a wrapper around the user's startup method that performs any
-        # post-setup validation.
+        # Invoke the user's startup method (or the default implementation)
         self.startup()
-        self._verify_startup()
+
+        # Validate that the startup requirements have been met.
+        # Accessing the main window attribute will raise an exception if the app hasn't
+        # defined a main window.
+        _ = self.main_window
+
+        # Create any initial windows
+        self._create_initial_windows()
 
         # Manifest the initial state of the menus. This will cascade down to all
         # open windows if the platform has window-based menus. Then install the
@@ -559,6 +593,17 @@ def _startup(self) -> None:
                 window._impl.create_toolbar()
                 window.toolbar.on_change = window._impl.create_toolbar
 
+        # Queue a task to run as soon as the event loop starts.
+        if inspect.iscoroutinefunction(self.running):
+            # running is a co-routine; create a sync wrapper
+            def on_app_running():
+                asyncio.ensure_future(self.running())
+
+        else:
+            on_app_running = self.running
+
+        self.loop.call_soon_threadsafe(on_app_running)
+
     def startup(self) -> None:
         """Create and show the main window for the application.
 
@@ -573,6 +618,15 @@ def startup(self) -> None:
 
         self.main_window.show()
 
+    def running(self) -> None:
+        """Logic to execute as soon as the main event loop is running.
+
+        Override this method to add any logic you want to run as soon as the app's event
+        loop is running.
+
+        If necessary, the overridden method can be defined as as an ``async`` coroutine.
+        """
+
     ######################################################################
     # App resources
     ######################################################################
@@ -829,10 +883,6 @@ def __init__(
     def _create_impl(self) -> None:
         self.factory.DocumentApp(interface=self)
 
-    def _verify_startup(self) -> None:
-        # No post-startup validation required for DocumentApps
-        pass
-
     @property
     def document_types(self) -> dict[str, type[Document]]:
         """The document types this app can manage.

core/src/toga/documents.py

@@ -35,7 +35,8 @@ def __init__(
         # Create a platform specific implementation of the Document
         self._impl = app.factory.Document(interface=self)
 
-    def can_close(self) -> bool:
+    # TODO: This will be covered when the document API is finalized
+    def can_close(self) -> bool:  # pragma: no cover
         """Is the main document window allowed to close?
 
         The default implementation always returns ``True``; subclasses can override this
@@ -46,7 +47,10 @@ def can_close(self) -> bool:
         """
         return True
 
-    async def handle_close(self, window: Window, **kwargs: object) -> bool:
+    # TODO: This will be covered when the document API is finalized
+    async def handle_close(
+        self, window: Window, **kwargs: object
+    ) -> bool:  # pragma: no cover
         """An ``on-close`` handler for the main window of this document that implements
         platform-specific document close behavior.
 

core/src/toga/window.py

@@ -300,16 +300,31 @@ def close(self) -> None:
         undefined, except for :attr:`closed` which can be used to check if the window
         was closed.
         """
+        close_window = True
         if self.app.main_window == self:
             # Closing the window marked as the main window is a request to exit.
             # Trigger on_exit handling, which may cause the window to close.
             self.app.on_exit()
-        else:
-            if self.content:
-                self.content.window = None
-            self.app.windows.discard(self)
-            self._impl.close()
-            self._closed = True
+            close_window = False
+        elif self.app.main_window is None:
+            # If this is an app without a main window, this is the last window in the
+            # app, and the platform exits on last window close, trigger an exit.
+            if len(self.app.windows) == 1 and self.app._impl.CLOSE_ON_LAST_WINDOW:
+                self.app.on_exit()
+                close_window = False
+
+        if close_window:
+            self._close()
+
+    def _close(self):
+        # The actual logic for closing a window. This is abstracted so that the testbed
+        # can monkeypatch this method, recording the close request without actually
+        # closing the app.
+        if self.content:
+            self.content.window = None
+        self.app.windows.discard(self)
+        self._impl.close()
+        self._closed = True
 
     @property
     def closed(self) -> bool: