apps: clock: Simplify and increase commenting

The main simplification is to adopt the status bar (with the status bar
clock disabled) and to restructure the way draw and redraw are handled.
Since the clock application is one of the most popular to customize it
has also had extensive commenting added to describe how it works.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

wasp/apps/clock.py

@@ -13,16 +13,9 @@ import icons
 import fonts.clock as digits
 
 DIGITS = (
-        digits.clock_0,
-        digits.clock_1,
-        digits.clock_2,
-        digits.clock_3,
-        digits.clock_4,
-        digits.clock_5,
-        digits.clock_6,
-        digits.clock_7,
-        digits.clock_8,
-        digits.clock_9
+        digits.clock_0, digits.clock_1, digits.clock_2, digits.clock_3,
+        digits.clock_4, digits.clock_5, digits.clock_6, digits.clock_7,
+        digits.clock_8, digits.clock_9
 )
 
 MONTH = 'JanFebMarAprMayJunJulAugSepOctNovDec'
@@ -39,60 +32,78 @@ class ClockApp():
     ICON = icons.clock
 
     def __init__(self):
-        self.meter = wasp.widgets.BatteryMeter()
-        self.notifier = wasp.widgets.NotificationBar()
+        self._bar = wasp.widgets.StatusBar()
 
     def foreground(self):
-        """Activate the application."""
-        self.on_screen = ( -1, -1, -1, -1, -1, -1 )
-        self.draw()
+        """Activate the application.
+
+        Configure the status bar, redraw the display and request a periodic
+        tick callback every second.
+        """
+        self._bar.clock = False
+        self._draw(True)
         wasp.system.request_tick(1000)
 
     def sleep(self):
+        """Prepare to enter the low power mode.
+
+        :returns: True, which tells the system manager not to automatically
+                  switch to the default application before sleeping.
+        """
         return True
 
     def wake(self):
-        self.update()
+        """Return from low power mode.
+
+        Time will have changes whilst we have been asleep so we must
+        udpate the display (but there is no need for a full redraw because
+        the display RAM is preserved during a sleep.
+        """
+        self._draw()
 
     def tick(self, ticks):
-        self.update()
+        """Periodic callback to update the display."""
+        self._draw()
 
-    def draw(self):
-        """Redraw the display from scratch."""
+    def _draw(self, redraw=False):
+        """Draw or lazily update the display.
+
+        The updates are as lazy by default and avoid spending time redrawing
+        if the time on display has not changed. However if redraw is set to
+        True then a full redraw is be performed.
+        """
         draw = wasp.watch.drawable
 
+        if redraw:
+            now = wasp.watch.rtc.get_localtime()
+
+            # Clear the display and draw that static parts of the watch face
             draw.fill()
             draw.rleblit(digits.clock_colon, pos=(2*48, 80), fg=0xb5b6)
-        self.on_screen = ( -1, -1, -1, -1, -1, -1 )
-        self.update()
-        self.meter.draw()
 
-    def update(self):
-        """Update the display (if needed).
+            # Redraw the status bar
+            self._bar.draw()
+        else:
+            # The update is doubly lazy... we update the status bar and if
+            # the status bus update reports a change in the time of day 
+            # then we compare the minute on display to make sure we 
+            # only update the main clock once per minute.
+            now = self._bar.update()
+            if not now or self._min == now[4]:
+                # Skip the update
+                return
 
-        The updates are a lazy as possible and rely on an prior call to
-        draw() to ensure the screen is suitably prepared.
-        """
-        now = wasp.watch.rtc.get_localtime()
-        if now[3] == self.on_screen[3] and now[4] == self.on_screen[4]:
-            if now[5] != self.on_screen[5]:
-                self.meter.update()
-                self.notifier.update()
-                self.on_screen = now
-            return False
+        # Format the month as text
+        month = now[1] - 1
+        month = MONTH[month*3:(month+1)*3]
 
-        draw = wasp.watch.drawable
+        # Draw the changeable parts of the watch face
         draw.rleblit(DIGITS[now[4]  % 10], pos=(4*48, 80))
         draw.rleblit(DIGITS[now[4] // 10], pos=(3*48, 80), fg=0xbdb6)
         draw.rleblit(DIGITS[now[3]  % 10], pos=(1*48, 80))
         draw.rleblit(DIGITS[now[3] // 10], pos=(0*48, 80), fg=0xbdb6)
-        self.on_screen = now
-
-        month = now[1] - 1
-        month = MONTH[month*3:(month+1)*3]
         draw.string('{} {} {}'.format(now[2], month, now[0]),
                 0, 180, width=240)
 
-        self.meter.update()
-        self.notifier.update()
-        return True
+        # Record the minute that is currently being displayed
+        self._min = now[4]