base: update the battery state to unknown after 5 minutes

If we don't get battery updates for too long, we don't really know anymore
what the battery is like.

Fixes #47

README.md

@@ -157,7 +157,11 @@ org.freedesktop.tuhi1.Device
 
   Property: BatteryPercent (u)
       The last known battery charge level in percent. This charge level is
-      only valid when the BatteryState is other than Unknown.
+      only accurate when the BatteryState is other than Unknown.
+
+      When the BatteryState is Unknown and BatteryPercent is nonzero, the
+      value is the last known percentage value.
+
       Read-only
 
   Property: BatteryState (u)

tuhi/base.py

@@ -15,6 +15,7 @@ import argparse
 import enum
 import logging
 import sys
+import time
 from gi.repository import GObject
 
 from tuhi.dbusserver import TuhiDBusServer
@@ -48,6 +49,8 @@ class TuhiDevice(GObject.Object):
             (GObject.SIGNAL_RUN_FIRST, None, (GObject.TYPE_PYOBJECT,)),
     }
 
+    BATTERY_UPDATE_MIN_INTERVAL = 300
+
     def __init__(self, bluez_device, config, uuid=None, paired=True):
         GObject.Object.__init__(self)
         self.config = config
@@ -58,6 +61,8 @@ class TuhiDevice(GObject.Object):
         self._uuid = uuid
         self._battery_state = TuhiDevice.BatteryState.UNKNOWN
         self._battery_percent = 0
+        self._last_battery_update_time = 0
+        self._battery_timer_source = None
 
         bluez_device.connect('connected', self._on_bluez_device_connected)
         bluez_device.connect('disconnected', self._on_bluez_device_disconnected)
@@ -171,6 +176,21 @@ class TuhiDevice(GObject.Object):
             self.battery_state = TuhiDevice.BatteryState.DISCHARGING
         self.battery_percent = percent
 
+        # If we don't get battery updates for a while, switch the state
+        # to unknown
+        if self._battery_timer_source is not None:
+            GObject.source_remove(self._battery_timer_source)
+        self._battery_timer_source = \
+            GObject.timeout_add_seconds(self.BATTERY_UPDATE_MIN_INTERVAL,
+                                        self._on_battery_timeout)
+        self._last_battery_update_time = time.time()
+
+    def _on_battery_timeout(self):
+        if self._last_battery_update_time < time.time() - self.BATTERY_UPDATE_MIN_INTERVAL:
+            self.battery_state = TuhiDevice.BatteryState.UNKNOWN
+        self._battery_timer_source = None  # gets auto-destroyed
+        return False
+
 
 class Tuhi(GObject.Object):
     __gsignals__ = {