Only show Save button when settings are different

Before now, the Save button appeared when the voltage and current
widgets changed their value.  If they were then changed back to the
original values, the Save button was still visible, allowing unnecessary
wear on the flash.  (the sarcastic horror!)

Now, the Save button is only shown when the voltage and current are not
both at their saved values.  A minor tweak, but we're closer to the
original design now.

pd-buddy-gtk.py

 
     def __init__(self, builder):
         self.builder = builder
+        self.serial_port = None
+        self.voltage = None
+        self.current = None
 
     def on_pdb_window_realize(self, *args):
         # Get the list
         pdb_send_message(self.serial_port, 'load')
         tmpcfg = pdb_send_message(self.serial_port, 'get_tmpcfg')
 
-        # Get information
+        # Get voltage and current from device and load them into the GUI
         for line in tmpcfg:
             if line.startswith(b'v:'):
                 v = line.split()[1]
                 i = float(line.split()[1])
                 current.set_value(i)
 
-        # Hide the Save button
-        rev = self.builder.get_object("header-sink-save-revealer")
-        rev.set_reveal_child(False)
+        self._store_device_settings()
+        self._set_save_button_visibility()
 
         # Show the Sink page
         hst = self.builder.get_object("header-stack")
         st.set_visible_child(select)
 
     def on_header_sink_save_clicked(self, button):
-        rev = self.builder.get_object("header-sink-save-revealer")
-        rev.set_reveal_child(False)
+        self._store_device_settings()
+        self._set_save_button_visibility()
 
         pdb_send_message(self.serial_port, 'write')
 
-    def on_voltage_combobox_changed(self, combo):
+    def _store_device_settings(self):
+        """Store the settings that were loaded from the device"""
+        # Get voltage and current widgets
+        voltage = self.builder.get_object("voltage-combobox")
+        current = self.builder.get_object("current-spinbutton")
+
+        # Remember the loaded settings
+        self.voltage = voltage.get_active_id()
+        self.current = current.get_value()
+
+    def _set_save_button_visibility(self):
+        """Show the save button if there are new settings to save"""
+        # Get relevant widgets
+        voltage = self.builder.get_object("voltage-combobox")
+        current = self.builder.get_object("current-spinbutton")
         rev = self.builder.get_object("header-sink-save-revealer")
-        rev.set_reveal_child(True)
+
+        # Set visibility
+        rev.set_reveal_child(voltage.get_active_id() != self.voltage
+                             or current.get_value() != self.current)
+
+    def on_voltage_combobox_changed(self, combo):
+        self._set_save_button_visibility()
 
         pdb_send_message(self.serial_port,
                          'set_v {}'.format(int(combo.get_active_text())*1000))
 
     def on_current_spinbutton_changed(self, spin):
-        rev = self.builder.get_object("header-sink-save-revealer")
-        rev.set_reveal_child(True)
+        self._set_save_button_visibility()
 
         pdb_send_message(self.serial_port,
                          'set_i {}'.format(int(spin.get_value()*1000)))