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/****************************************************************************
**
** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the Technology Preview License Agreement accompanying
** this package.
**
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of this
** file.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\example xmlpatterns/trafficinfo
\title TrafficInfo Example
Shows how XQuery can be used extract information from WML documents provided by a WAP service.
\section1 Overview
The WAP service used in this example is \l{Trafikanten}{wap.trafikanten.no}
that is run by the Norwegian governmental agency for public transport in
Oslo. The service provides real time information about the departure of
busses, trams and undergrounds for every station in the city area.
This example application displays the departure information for a specific
station and provides the feature to filter for a special bus or tram line.
\image trafficinfo-example.png
\section1 Retrieving the Data
Without the knowledge of XQuery, one would use QNetworkAccessManager to
query the WML document from the WAP service and then using the QDom
classes or QXmlStreamReader classes to iterate over the document and
extract the needed information.
However this approach results in a lot of glue code and consumes valuable
developer time, so we are looking for something that can access XML
documents locally or over the network and extract data according to given
filter rules. That's the point where XQuery enters the stage!
If we want to know when the underground number 6 in direction
\Aring\c{}sjordet is passing the underground station in Nydalen on November
14th 2008 after 1pm, we use the following URL:
\c{http://wap.trafikanten.no/F.asp?f=03012130&t=13&m=00&d=14.11.2008&start=1}
The parameters have the following meanings:
\list
\o \e{f} The unique station ID of Nydalen.
\o \e{t} The hour in 0-23 format.
\o \e{m} The minute in 0-59 format.
\o \e{d} The date in dd.mm.yyyy format.
\o \e{start} Not interesting for our use but should be passed.
\endlist
As a result we get the following document:
\quotefile examples/xmlpatterns/trafficinfo/time_example.wml
So for every departure we have a \c <a> tag that contains the time as a
text element, and the following text element contains the line number
and direction.
To encapsulate the XQuery code in the example application, we create a
custom \c TimeQuery class. This provides the \c queryInternal() function
that takes a station ID and date/time as input and returns the list of
times and directions:
\snippet examples/xmlpatterns/trafficinfo/timequery.cpp 1
The first lines of this function synthesize the XQuery strings that fetch
the document and extract the data.
For better readability, two separated queries are used here: the first one
fetches the times and the second fetches the line numbers and directions.
The \c doc() XQuery method opens a local or remote XML document and returns
it, so the \c{/wml/card/p/small/} statement behind it selects all XML nodes
that can be reached by the path, \c wml \rarrow \c card \rarrow \c p \rarrow
\c small.
Now we are on the node that contains all the XML nodes we are interested in.
In the first query we select all \c a nodes that have a \c href attribute
starting with the string "Rute" and return the text of these nodes.
In the second query we select all text nodes that are children of the
\c small node which start with a number.
These two queries are passed to the QXmlQuery instance and are evaluated
to string lists. After some sanity checking, we have collected all the
information we need.
In the section above we have seen that an unique station ID must be passed
as an argument to the URL for retrieving the time, so how to find out which
is the right station ID to use? The WAP service provides a page for that
as well, so the URL
\c{http://wap.trafikanten.no/FromLink1.asp?fra=Nydalen}
will return the following document:
\snippet examples/xmlpatterns/trafficinfo/station_example.wml 0
The names of the available stations are listed as separate text elements
and the station ID is part of the \c href attribute of the parent \c a
(anchor) element. In our example, the \c StationQuery class encapsulates
the action of querying the stations that match the given name pattern with
the following code:
\snippet examples/xmlpatterns/trafficinfo/stationquery.cpp 0
Just as in the \c TimeQuery implementation, the first step is to
synthesize the XQuery strings for selecting the station names and the
station IDs. As the station name that we pass in the URL will be input
from the user, we should protect the XQuery from code injection by using
the QXmlQuery::bindVariable() method to do proper quoting of the variable
content for us instead of concatenating the two strings manually.
So, we define a XQuery \c $station variable that is bound to the user
input. This variable is concatenated inside the XQuery code with the
\c concat method. To extract the station IDs, we select all \c a elements
that have an \c title attribute with the content "Velg", and from these
elements we take the substring of the \c href attribute that starts at the
18th character.
The station name can be extracted a bit more easily by just taking the
text elements of the selected \a elements.
After some sanity checks we have all the station IDs and the corresponding
names available.
The rest of the code in this example is just for representing the time and
station information to the user, and uses techniques described in the
\l{Widget Examples}.
*/
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