Add backend implementations for background and session 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
@@ -245,10 +249,15 @@ 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:
+            raise RuntimeError("Session-based apps are not supported on Android")
+        elif window == toga.App.BACKGROUND:
+            raise RuntimeError("Background apps 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

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

gtk/src/toga_gtk/app.py

@@ -17,16 +17,16 @@
 
 
 class App:
+    # GTK apps exit when the last window is closed
+    CLOSE_ON_LAST_WINDOW = True
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
 
         gbulb.install(gtk=True)
         self.loop = asyncio.new_event_loop()
 
-        self.create()
-
-    def create(self):
         # Stimulate the build of the app
         self.native = Gtk.Application(
             application_id=self.interface.app_id,
@@ -36,6 +36,7 @@ def create(self):
 
         # Connect the GTK signal that will cause app startup to occur
         self.native.connect("startup", self.gtk_startup)
+        # Activate is a no-op, but GTK complains if you don't implement it
         self.native.connect("activate", self.gtk_activate)
 
         self.actions = None
@@ -46,14 +47,6 @@ def gtk_activate(self, data=None):
     def gtk_startup(self, data=None):
         self.interface._startup()
 
-        # Now that we have menus, make the app take responsibility for
-        # showing the menubar.
-        # This is required because of inconsistencies in how the Gnome
-        # shell operates on different windowing environments;
-        # see #872 for details.
-        settings = Gtk.Settings.get_default()
-        settings.set_property("gtk-shell-shows-menubar", False)
-
         # Set any custom styles
         css_provider = Gtk.CssProvider()
         css_provider.load_from_data(TOGA_DEFAULT_STYLES)
@@ -178,6 +171,14 @@ def create_menus(self):
         # Set the menu for the app.
         self.native.set_menubar(menubar)
 
+        # Now that we have menus, make the app take responsibility for
+        # showing the menubar.
+        # This is required because of inconsistencies in how the Gnome
+        # shell operates on different windowing environments;
+        # see #872 for details.
+        settings = Gtk.Settings.get_default()
+        settings.set_property("gtk-shell-shows-menubar", False)
+
     ######################################################################
     # App lifecycle
     ######################################################################
@@ -190,14 +191,21 @@ def main_loop(self):
         # Modify signal handlers to make sure Ctrl-C is caught and handled.
         signal.signal(signal.SIGINT, signal.SIG_DFL)
 
+        # Retain a reference to the app so that no-window apps can exist
+        self.native.hold()
+
         self.loop.run_forever(application=self.native)
 
+        # Release the reference to the app
+        self.native.release()
+
     def set_icon(self, icon):
         for window in self.interface.windows:
             window._impl.native.set_icon(icon._impl.native(72))
 
     def set_main_window(self, window):
-        pass
+        if isinstance(window, toga.Window):
+            window._impl.native.set_role("MainWindow")
 
     ######################################################################
     # App resources

iOS/src/toga_iOS/app.py

@@ -4,6 +4,7 @@
 from rubicon.objc import objc_method
 from rubicon.objc.eventloop import EventLoopPolicy, iOSLifecycle
 
+import toga
 from toga.command import Command
 from toga.handlers import simple_handler
 from toga_iOS.libs import UIResponder, UIScreen, av_foundation
@@ -51,6 +52,9 @@ def application_didChangeStatusBarOrientation_(
 
 
 class App:
+    # iOS apps exit when the last window is closed
+    CLOSE_ON_LAST_WINDOW = True
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
@@ -110,7 +114,10 @@ def set_icon(self, icon):
         pass  # pragma: no cover
 
     def set_main_window(self, window):
-        pass
+        if window is None:
+            raise RuntimeError("Session-based apps are not supported on Android")
+        elif window == toga.App.BACKGROUND:
+            raise RuntimeError("Background apps are not supported on Android")
 
     ######################################################################
     # App resources

textual/src/toga_textual/app.py

@@ -1,5 +1,6 @@
 import asyncio
 
+import toga
 from textual.app import App as TextualApp
 
 from .screens import Screen as ScreenImpl
@@ -16,6 +17,9 @@ def on_mount(self) -> None:
 
 
 class App:
+    # Textual apps exit when the last window is closed
+    CLOSE_ON_LAST_WINDOW = True
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
@@ -54,7 +58,12 @@ def set_icon(self, icon):
         pass
 
     def set_main_window(self, window):
-        self.native.push_screen(self.interface.main_window.id)
+        if window is None:
+            raise RuntimeError("Session-based apps are not supported on Textual")
+        elif window == toga.App.BACKGROUND:
+            raise RuntimeError("Background apps are not supported on Textual")
+        else:
+            self.native.push_screen(self.interface.main_window.id)
 
     ######################################################################
     # App resources

web/src/toga_web/app.py

@@ -1,3 +1,4 @@
+import toga
 from toga.app import overridden
 from toga.command import Command, Group
 from toga.handlers import simple_handler
@@ -7,13 +8,16 @@
 
 
 class App:
+    # Web apps exit when the last window is closed
+    CLOSE_ON_LAST_WINDOW = True
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
 
     def create(self):
-        # self.resource_path = os.path.dirname(os.path.dirname(NSBundle.mainBundle.bundlePath))
         self.native = js.document.getElementById("app-placeholder")
+        # self.resource_path = ???
 
         # Call user code to populate the main window
         self.interface._startup()
@@ -70,7 +74,10 @@ def set_icon(self, icon):
         pass
 
     def set_main_window(self, window):
-        pass
+        if window is None:
+            raise RuntimeError("Session-based apps are not supported on Web")
+        elif window == toga.App.BACKGROUND:
+            raise RuntimeError("Background apps are not supported on Web")
 
     ######################################################################
     # App resources

winforms/src/toga_winforms/app.py

@@ -10,7 +10,7 @@
 from System.Net import SecurityProtocolType, ServicePointManager
 from System.Windows.Threading import Dispatcher
 
-from toga import Key
+import toga
 from toga.app import overridden
 from toga.command import Command, Group
 from toga.handlers import simple_handler
@@ -55,6 +55,9 @@ def print_stack_trace(stack_trace_line):  # pragma: no cover
 
 
 class App:
+    # Winforms apps exit when the last window is closed
+    CLOSE_ON_LAST_WINDOW = True
+
     def __init__(self, interface):
         self.interface = interface
         self.interface._impl = self
@@ -126,8 +129,6 @@ def create(self):
         # Call user code to populate the main window
         self.interface._startup()
 
-        self.interface.main_window._impl.set_app(self)
-
     ######################################################################
     # Commands and menus
     ######################################################################
@@ -154,7 +155,7 @@ def create_standard_app_commands(self):
             Command(
                 self.interface.on_exit,
                 "Exit",
-                shortcut=Key.MOD_1 + "q",
+                shortcut=toga.Key.MOD_1 + "q",
                 group=Group.FILE,
                 section=sys.maxsize,
                 id=Command.EXIT,
@@ -233,7 +234,10 @@ def set_icon(self, icon):
             window._impl.native.Icon = icon._impl.native
 
     def set_main_window(self, window):
-        self.app_context.MainForm = window._impl.native
+        if isinstance(window, toga.Window):
+            self.app_context.MainForm = window._impl.native
+        else:
+            self.app_context.MainForm = None
 
     ######################################################################
     # App resources
@@ -319,7 +323,7 @@ def create_app_commands(self):
             Command(
                 lambda w: self.open_file,
                 text="Open...",
-                shortcut=Key.MOD_1 + "o",
+                shortcut=toga.Key.MOD_1 + "o",
                 group=Group.FILE,
                 section=0,
             ),