Add datestamps to message widgets when day changes

When creating a message widget, show the date if the previous message
wasn't from today.

Fixes tdryer#27.

hangups/ui/__main__.py

@@ -14,6 +14,8 @@
 
 
 LOG_FORMAT = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
+MESSAGE_TIME_FORMAT = '%I:%M:%S %p'
+MESSAGE_DATETIME_FORMAT = '%y-%m-%d %I:%M:%S %p'
 COL_SCHEMES = {
     # Very basic scheme with no colour
     'default': {
@@ -260,11 +262,12 @@ class MessageWidget(urwid.WidgetWrap):
 
     """Widget for displaying a single message in a conversation."""
 
-    def __init__(self, timestamp, text, user=None):
+    def __init__(self, timestamp, text, user=None, show_date=False):
         # Save the timestamp as an attribute for sorting.
         self.timestamp = timestamp
         text = [
-            ('msg_date', '(' + self._get_date_str(timestamp) + ') '),
+            ('msg_date', '(' + self._get_date_str(timestamp,
+                                                  show_date=show_date) + ') '),
             ('msg_text', text)
         ]
         if user is not None:
@@ -273,31 +276,41 @@ def __init__(self, timestamp, text, user=None):
         super().__init__(self._widget)
 
     @staticmethod
-    def _get_date_str(timestamp):
+    def _get_date_str(timestamp, show_date=False):
         """Convert UTC datetime into user interface string."""
-        return timestamp.astimezone(tz=None).strftime('%I:%M:%S %p')
+        fmt = MESSAGE_DATETIME_FORMAT if show_date else MESSAGE_TIME_FORMAT
+        return timestamp.astimezone(tz=None).strftime(fmt)
 
     def __lt__(self, other):
         return self.timestamp < other.timestamp
 
     @staticmethod
-    def from_conversation_event(conversation, conv_event):
+    def from_conversation_event(conversation, conv_event, prev_conv_event):
         """Return MessageWidget representing a ConversationEvent.
 
         Returns None if the ConversationEvent does not have a widget
         representation.
         """
         user = conversation.get_user(conv_event.user_id)
+        # Check whether the previous event occurred on the same day as this
+        # event.
+        if prev_conv_event is not None:
+            is_new_day = (conv_event.timestamp.astimezone(tz=None).date() !=
+                          prev_conv_event.timestamp.astimezone(tz=None).date())
+        else:
+            is_new_day = False
         if isinstance(conv_event, hangups.ChatMessageEvent):
-            return MessageWidget(conv_event.timestamp, conv_event.text, user)
+            return MessageWidget(conv_event.timestamp, conv_event.text, user,
+                                 show_date=is_new_day)
         elif isinstance(conv_event, hangups.RenameEvent):
             if conv_event.new_name == '':
                 text = ('{} cleared the conversation name'
                         .format(user.first_name))
             else:
                 text = ('{} renamed the conversation to {}'
                         .format(user.first_name, conv_event.new_name))
-            return MessageWidget(conv_event.timestamp, text)
+            return MessageWidget(conv_event.timestamp, text,
+                                 show_date=is_new_day)
         elif isinstance(conv_event, hangups.MembershipChangeEvent):
             event_users = [conversation.get_user(user_id) for user_id
                            in conv_event.participant_ids]
@@ -307,7 +320,8 @@ def from_conversation_event(conversation, conv_event):
                         .format(user.first_name, names))
             else:  # LEAVE
                 text = ('{} left the conversation'.format(names))
-            return MessageWidget(conv_event.timestamp, text)
+            return MessageWidget(conv_event.timestamp, text,
+                                 show_date=is_new_day)
         else:
             return None
 
@@ -327,15 +341,19 @@ def __init__(self, conversation):
         self._first_loaded = False  # Whether the first event is loaded
 
         # Focus position is the first displayable event ID, or None.
-        is_displayable = lambda ev: (
-            MessageWidget.from_conversation_event(conversation, ev) is not None
-        )
-        self._focus_position = next((ev.id_ for ev in
-                                     reversed(conversation.events)
-                                     if is_displayable(ev)), None)
+        self._focus_position = next((
+            ev.id_ for ev in reversed(conversation.events)
+            if self._is_event_displayable(ev)
+        ), None)
 
         self._conversation.on_event.add_observer(self._handle_event)
 
+    def _is_event_displayable(self, conv_event):
+        """Return True if the ConversationWidget is displayable."""
+        widget = MessageWidget.from_conversation_event(self._conversation,
+                                                       conv_event, None)
+        return widget is not None
+
     def _handle_event(self, conv_event):
         """Handle updating and scrolling when a new event is added.
 
@@ -384,14 +402,26 @@ def __getitem__(self, position):
         """Return widget at position or raise IndexError."""
         if position == self.POSITION_LOADING:
             if self._first_loaded:
+                # TODO: Show the full date the conversation was created.
                 return urwid.Text('No more messages', align='center')
             future = asyncio.async(self._load())
             future.add_done_callback(lambda future: future.result())
             return urwid.Text('Loading...', align='center')
         # May return None if the event doesn't have a widget representation.
         try:
+            # Get the previous displayable event, or None if it isn't loaded or
+            # doesn't exist.
+            prev_position = self._get_position(position, prev=True)
+            if prev_position == self.POSITION_LOADING:
+                prev_event = None
+            else:
+                prev_event = self._conversation.get_event(prev_position)
+
+            # When creating the widget, also pass the previous event so a
+            # timestamp can be shown if this event occurred on a different day.
             widget = MessageWidget.from_conversation_event(
-                self._conversation, self._conversation.get_event(position)
+                self._conversation, self._conversation.get_event(position),
+                prev_event
             )
         except KeyError:
             raise IndexError('Invalid position: {}'.format(position))
@@ -415,12 +445,10 @@ def _get_position(self, position, prev=False):
                     raise IndexError('Reached first position')
             # Skip events that aren't represented by a widget and try the next
             # one.
-            try:
-                self[ev.id_]
-            except IndexError:
-                position = ev.id_
-            else:
+            if self._is_event_displayable(ev):
                 return ev.id_
+            else:
+                position = ev.id_
 
     def next_position(self, position):
         """Return the position below position or raise IndexError."""