Set home position required.

[hamishwillee]

#646 adds a home position listener that sets our home position attribute when the appropriate message is received (this will be sent by autopilot whenever the home position is changed). The same dk-python attribute is also set when waypoint_0 is read (the "legacy" home position location).

We need to make a few additional changes:

• Add attribute listener for home message (done in adds notification whenever home location is changed #650)
• Create a setter to set the value rather than using attribute (as discussed in Create setter functions rather than setting attributes directly #566).

  • The setter currently uses MAV_CMD_DO_SET_HOME rather than SET_HOME_POSITION. The former adds ability to set the home position as current position (which we don't use) but seems to be very slightly lossy in "round trip" - ie what you set using the MAV_CMD is not exactly what you get back from the HOME_POSITION.

  • Propose a setter than uses both methods, and can set the value to home position. Something like:

    def set_home_position(self,location=None):
      if not isinstance(pos, LocationGlobal):
      raise Exception('Excepting home_location to be set to a LocationGlobal.')
    
      # Set cached home location.
      self._home_location = copy.copy(pos)
    
      if location:
          msg = this.message_factory.set_home_position_encode(
              0,       # System ID maybe
              location.lat * 1.0e7,       # lat - int32_t Latitude (WGS84), in degrees \* 1E7
              location.lon * 1.0e7,       # lon - int32_t Longitude (WGS84), in degrees \* 1E7
              location.alt * 1000,       # alt - int32_t  Altitude (AMSL), in meters \* 1000 (positive for up)
              0, 0, 0, [1,1,1,1], 0, 0, 0 #Remaining parameters not supported by ardupilot.
              )    
          # send command to vehicle
          this.send_mavlink(msg)
      else:
          #use current location as home location
          self.send_mavlink(self.message_factory.command_long_encode(
          0, 0, mavutil.mavlink.MAV_CMD_DO_SET_HOME, 0, 1, 0, 0, 0, 0, 0, 0))
    

• Add code to request the home position on DK connection (otherwise we won't receive it if the home position has already been set).
• Add code to disable setting/getting the value from waypoint_0 if the message is supported. The best way to do this is to read the capabilities - but not yet supported: Need capabilities for HOME_POSITION support (all messages) ArduPilot/ardupilot#4503

[hamishwillee]

@peterbarker

1. What is best message to trigger requesting the home_position on? Ie we could trigger it "first have GPS fix"?
2. Does the setter above make sense to you?