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<div class="section" title="Getting Your Location">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="gps"></a>Getting Your Location</h2></div></div></div>
<p>
As the most useful thing a GPS can do is tell you its location, the most
common interface implemented by the GPS object is
<span class="interface">Position</span>. This interface has a method called
<code class="methodname">GetPosition</code> which will return the current
position, and a signal called <code class="methodname">PositionChanged</code>
which is emitted when the position has changed. Note that although the
GPS device itself will report a new position every second, Gypsy will only
emit <code class="methodname">PositionChanged</code> signals if the position has
actually changed since the last emission.
</p>
<p>
However, before you can use the GPS object for anything useful, you must
tell Gypsy to activate the GPS. This is done via the
<code class="methodname">Start</code> method on the <span class="interface">Device</span>
interface.
</p>
<pre class="programlisting">device = dbus.Interface(gps, dbus_interface="org.freedesktop.Gypsy.Device")
device.Start()</pre>
<p>
The <code class="methodname">GetPosition</code> method and
<code class="methodname">PositionChanged</code> signal both have the same
arguments. The first argument is a bitset of fields which are valid.
This is very important because often the GPS will only be able to get a
longitude/latitude fix, and not be able to determine the altitude. Next
is a Unix-style timestamp, so the application can know when the location
was obtained. Next are the longitude and latitude, in fractional degrees.
Finally the altitude, in meters.
</p>
<pre class="programlisting"># Some constants we'll need later
POSITION_FIELDS_NONE = 0
POSITION_FIELDS_LATITUDE = 1 << 0
POSITION_FIELDS_LONGITUDE = 1 << 1
POSITION_FIELDS_ALTITUDE = 1 << 2
# Get a proxy to the Position interface, and listen for position changed signals
position = dbus.Interface(gps, dbus_interface="org.freedesktop.Gypsy.Position")
def position_changed(fields_set, timestamp, latitude, longitude, altitude):
print "%+2f, %+2f (%1fm)" % (
(fields_set & POSITION_FIELDS_LATITUDE) and latitude or -1.0,
(fields_set & POSITION_FIELDS_LONGITUDE) and longitude or -1.0,
(fields_set & POSITION_FIELDS_ALTITUDE) and altitude or -1.0)
position.connect_to_signal("PositionChanged", position_changed)</pre>
<p>
Finally, we can enter the main loop. As the location changes we'll get
signals, which result in the <code class="methodname">position_changed</code>
callback being called, which prints out the current position.
</p>
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