Handles back button, goes back to screen history

And eventually quits the application, fixes #95

src/main.py

@@ -13,6 +13,7 @@
 from kivy.app import App
 from kivy.clock import Clock, mainthread
 from kivy.core.clipboard import Clipboard
+from kivy.core.window import Window
 from kivy.logger import LOG_LEVELS, Logger
 from kivy.metrics import dp
 from kivy.properties import NumericProperty, ObjectProperty, StringProperty
@@ -1033,11 +1034,44 @@ def __init__(self, **kwargs):
         self.screen_history = []
         self.register_event_type('on_alias_updated')
         Clock.schedule_once(lambda dt: self.load_landing_page())
+        Window.bind(on_keyboard=self.on_keyboard)
+
+    def on_keyboard(self, window, key, *args):
+        """
+        Handles the back button (Android) and ESC key.
+        Goes back to the previous screen or quite the application
+        if there's none left.
+        """
+        if key == 27:
+            screen_manager = self.screen_manager
+            # if we already are in the overview screen, also move back to
+            # the overview subtab of the overview screen
+            if screen_manager.current == 'overview':
+                overview_bnavigation = self.overview_bnavigation
+                tab_manager = overview_bnavigation.ids['tab_manager']
+                if tab_manager.current != 'overview':
+                    tab_manager.current = 'overview'
+                    return True
+                else:
+                    # if we were already in the overview:overview subtab,
+                    # then propagates the key which in this case will exit
+                    # the application
+                    return False
+            self.screen_manager_previous()
+            # stops the propagation
+            return True
+        return False
 
     @property
-    def overview(self):
-        overview_screen = self.ids.overview_screen_id
+    def overview_bnavigation(self):
+        screen_manager = self.screen_manager
+        overview_screen = screen_manager.get_screen('overview')
         overview_bnavigation = overview_screen.ids.overview_bnavigation_id
+        return overview_bnavigation
+
+    @property
+    def overview(self):
+        overview_bnavigation = self.overview_bnavigation
         return overview_bnavigation.ids.overview_id
 
     @property
@@ -1053,10 +1087,8 @@ def switch_account(self):
 
     @property
     def send(self):
-        screen_manager = self.screen_manager
-        overview_screen = screen_manager.get_screen('overview')
-        overview_bnavigation_id = overview_screen.ids.overview_bnavigation_id
-        return overview_bnavigation_id.ids.send_id
+        overview_bnavigation = self.overview_bnavigation
+        return overview_bnavigation.ids.send_id
 
     @property
     def manage_keystores(self):
@@ -1099,7 +1131,7 @@ def unbind_current_account_balance(self):
         """
         self.unbind(current_account_balance=self.update_toolbar_title_balance)
 
-    def screen_manager_current(self, current, direction=None):
+    def screen_manager_current(self, current, direction=None, history=True):
         screens = {
             'overview': OverviewScreen,
             'switch_account': SwitchAccountScreen,
@@ -1108,20 +1140,31 @@ def screen_manager_current(self, current, direction=None):
             'about': AboutScreen,
         }
         screen_manager = self.screen_manager
+        # creates the Screen object if it doesn't exist
         if not screen_manager.has_screen(current):
             screen = screens[current](name=current)
             screen_manager.add_widget(screen)
         if direction is not None:
             screen_manager.transition.direction = direction
         screen_manager.current = current
-        self.screen_history.append(current)
+        if history:
+            # do not update history if it's the same screen because we do not
+            # want the go back button to behave like it was doing nothing
+            if not self.screen_history or self.screen_history[-1] != current:
+                self.screen_history.append(current)
+        # in this case let's reset since the overview is the root screen
+        # because we never want the back button to bring us from overview
+        # to another screen
+        if current == 'overview':
+            self.screen_history = []
 
     def screen_manager_previous(self):
         try:
-            previous_screen = self.screen_history[-2]
+            previous_screen = self.screen_history.pop(-2)
         except IndexError:
             previous_screen = 'overview'
-        self.screen_manager_current(previous_screen, direction='right')
+        self.screen_manager_current(
+            previous_screen, direction='right', history=False)
 
     @staticmethod
     def show_invalid_form_dialog():