source: trunk/doc/src/examples/trafficinfo.qdoc@ 137

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/*!
    \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{Qt Examples#Widgets}{Widgets examples}.
*/