App: Fix pausing without app instance

[Fak3]

On android, pause event can come in early, before the app instance is
created.

fixes #6400

[matham]

I'm not very familiar with how we handle pausing, but does App know how to deal with starting while it is paused? Because the previous code always terminated the app if it wasn't started yet, but with the changes the app will be able to start while it is paused.

[Fak3]

@matham I don't know much about it either, but it seems to me that when paused, the whole kivy app is stuck in this loop: https://github.com/Fak3/kivy/blob/074e14ed4b4a9b7dccf3d28996c47b875cd5b51a/kivy/core/window/window_sdl2.py#L485 so the app instance will not be created until it is unpaused.

[Sirfanas]

Having same issue, could you put your code as PEP8 so Travis stop fail ?
My App close itself sometimes for this reason...

[TechnoLenzer]

I have both an on_pause and on_resume function in my app class which work fine normally, so why is this issue still persisting? Should I basically restart everything (as in reschedule, reload screen, reset variables) within my on_resume function?

[matham]

It'd also be nice to add a unit test for this - if you can.

[p3g4asus]

Thank you Fak3!!!!!!! This bug was driving me crazy. My app closed without any error and I did not have any Idea on how to fix. I applyed this patch and now it works. It is unreasonable to not have fixed this more than a year after this `patch.

--- a/kivy/core/window/window_sdl2.py	2020-08-06 15:43:26.000000000 +0200
+++ b/kivy/core/window/window_sdl2.py	2020-10-04 18:25:42.000000000 +0200
@@ -10,13 +10,13 @@
     - support scrolling
     - clean code
     - manage correctly all sdl events
 
 '''
 
-__all__ = ('WindowSDL', )
+__all__ = ('WindowSDL2', )
 
 from os.path import join
 import sys
 from kivy import kivy_data_dir
 from kivy.logger import Logger
 from kivy.base import EventLoop
@@ -226,17 +226,15 @@
             self.dispatch('on_memorywarning')
 
         elif action == 'app_willenterbackground':
             from kivy.base import stopTouchApp
             app = App.get_running_app()
             if not app:
-                Logger.info('WindowSDL: No running App found, exit.')
-                stopTouchApp()
-                return 0
+                Logger.info('WindowSDL: No running App found, pause.')
 
-            if not app.dispatch('on_pause'):
+            elif not app.dispatch('on_pause'):
                 Logger.info(
                     'WindowSDL: App doesn\'t support pause mode, stop.')
                 stopTouchApp()
                 return 0
 
             self._pause_loop = True
@@ -245,13 +243,14 @@
             # on iOS, the did enter foreground is launched at the start
             # of the application. in our case, we want it only when the app
             # is resumed
             if self._pause_loop:
                 self._pause_loop = False
                 app = App.get_running_app()
-                app.dispatch('on_resume')
+                if app:
+                    app.dispatch('on_resume')
 
         elif action == 'windowresized':
             self._size = largs
             self._win.resize_window(*self._size)
             # Force kivy to render the frame now, so that the canvas is drawn.
             EventLoop.idle()
@@ -704,17 +703,14 @@
     def do_pause(self):
         # should go to app pause mode (desktop style)
         from kivy.app import App
         from kivy.base import stopTouchApp
         app = App.get_running_app()
         if not app:
-            Logger.info('WindowSDL: No running App found, exit.')
-            stopTouchApp()
-            return
-
-        if not app.dispatch('on_pause'):
+            Logger.info('WindowSDL: No running App found, pause.')
+        elif not app.dispatch('on_pause'):
             Logger.info('WindowSDL: App doesn\'t support pause mode, stop.')
             stopTouchApp()
             return
 
         # XXX FIXME wait for sdl resume
         while True:
@@ -729,14 +725,15 @@
                 EventLoop.quit = True
                 break
             elif action == 'app_willenterforeground':
                 break
             elif action == 'windowrestored':
                 break
-
-        app.dispatch('on_resume')
+            
+        if app:
+            app.dispatch('on_resume')
 
     def _update_modifiers(self, mods=None, key=None):
         if mods is None and key is None:
             return
         modifiers = set()
 

[p3g4asus]

This change is necessary because, otherwise, if the app on_pause gets called before the app is available (and this indeed CAN happen), when the app becomes available, it simply silently closes itself generating no error message.

[matham]

Thanks for confirming this works