all clocks: Add a preview() method ready for a watch face chooser

One benefit from wasp-os' dirt simple drawing model is that it is very
easy for apps to provide full screen previews of themselves. Add this
for all clocks ready for us to add a watch face chooser application.

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

wasp/apps/chrono.py

@@ -50,6 +50,11 @@ class ChronoApp():
         """Periodic callback to update the display."""
         self._draw()
 
+    def preview(self):
+        """Provide a preview for the watch face selection."""
+        wasp.system.bar.clock = False
+        self._draw(True)
+
     def _draw(self, redraw=False):
         """Draw or lazily update the display.
 

wasp/apps/clock.py

@@ -59,6 +59,11 @@ class ClockApp():
         """Periodic callback to update the display."""
         self._draw()
 
+    def preview(self):
+        """Provide a preview for the watch face selection."""
+        wasp.system.bar.clock = False
+        self._draw(True)
+
     def _draw(self, redraw=False):
         """Draw or lazily update the display.
 

wasp/apps/dual_clock.py

@@ -82,6 +82,11 @@ class DualClockApp():
         """Periodic callback to update the display."""
         self._draw()
 
+    def preview(self):
+        """Provide a preview for the watch face selection."""
+        wasp.system.bar.clock = False
+        self._draw(True)
+
     def _draw(self, redraw=False):
         """Draw or lazily update the display.
 

wasp/apps/fibonacci_clock.py

@@ -76,6 +76,11 @@ class FibonacciClockApp():
     def tick(self, ticks):
         self._draw()
 
+    def preview(self):
+        """Provide a preview for the watch face selection."""
+        wasp.system.bar.clock = False
+        self._draw(True)
+
     def _draw(self, redraw=False):
         """Draw or lazily update the display."""
         draw = wasp.watch.drawable

wasp/apps/template.py

@@ -63,6 +63,18 @@ class TemplateApp():
         """Notify the application the device is waking up."""
         pass
 
+    def preview(self):
+        """Provide a preview for the watch face selection.
+
+        preview() must be implemented by watch face applications because it
+        is called by the watch face selector. When called the application should
+        redraw the screen as through it was the foreground() application. The
+        application will not be active after the preview.
+
+        Other applications should not implement this entry point.
+        """
+        pass
+
     def press(self, button, state):
         """Notify the application of a button-press event."""
         draw = wasp.watch.drawable

wasp/apps/word_clock.py

@@ -55,6 +55,11 @@ class WordClockApp():
         """Periodic callback to update the display."""
         self._draw()
 
+    def preview(self):
+        """Provide a preview for the watch face selection."""
+        wasp.system.bar.clock = False
+        self._draw(True)
+
     def _draw(self, redraw=False):
         """Draw or lazily update the display.