- add background callback register function and hook into ios backgro…

…und notifcation functions

core/pen/include/os.h

@@ -55,6 +55,7 @@ namespace pen
     bool             os_set_keychain_item(const Str& identifier, const Str& key, const Str& value);
     Str              os_get_keychain_item(const Str& identifier, const Str& key);
     bool             os_is_backgrounded();
+    void             os_register_background_callback(void (*callback)(bool));
 
     // music
     struct music_item

core/pen/source/ios/os.mm

@@ -83,6 +83,7 @@ -(MPRemoteCommandHandlerStatus)like;
         pen_text_field*          text_field = nullptr;
         bool                     show_on_screen_keyboard = false;
         pen::music_player_remote music_remote;
+        void                    (*background_callback)(bool) = nullptr;
     };
     os_context s_context;
 
@@ -172,6 +173,24 @@ - (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(N
     }
 }
 
+- (void)applicationWillResignActive:(UIApplication *)app
+{
+    // pairs with applicationDidBecomeActive
+    if(s_context.background_callback)
+    {
+        s_context.background_callback(true);
+    }
+}
+
+- (void)applicationDidBecomeActive:(UIApplication *)app
+{
+    // pairs with applicationWillResignActive
+    if(s_context.background_callback)
+    {
+        s_context.background_callback(true);
+    }
+}
+
 -(MPRemoteCommandHandlerStatus)play {
     if(s_context.music_remote.pause)
     {
@@ -799,4 +818,9 @@ void music_set_now_playing_time_info(u32 position_ms, u32 duration_ms)
     bool os_is_backgrounded() {
         return [[UIApplication sharedApplication] applicationState] == UIApplicationStateBackground;
     }
+
+    void os_register_background_callback(void (*callback)(bool))
+    {
+        s_context.background_callback = callback;
+    }
 }