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    def timerEvent(self, ev):                              <IMG
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        self.emit(PYSIGNAL("timeSignal"), (self.dataFunction(),))<IMG
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        </PRE
><DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#D1"><IMG
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></DT
><DD
>The
              <TT
CLASS="CLASSNAME"
>DataSource</TT
> class is a generic
              datasource. We base it on <TT
CLASS="CLASSNAME"
>QObject</TT
>
              so we can emit signals from it.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#D2"><IMG
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><DD
>The constructor of
              <TT
CLASS="CLASSNAME"
>DataSource</TT
> takes a function as
              the first parameter. This is the actual dataproducing
              function. We saw their definitions above. Remember,
              every function is an object in its own right &#8212; you can
              pass them on as arguments, add them to object
              dictionaries, etc.</DD
><DT
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><DD
>Every second (1000
              milliseconds) the timer will generate an event that
              will be caught by the <TT
CLASS="FUNCTION"
>timerEvent</TT
>
              function.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#D4"><IMG
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><DD
>By creating a local name that
              links to the passed function object, we can call this
              function as if it were a plain member function of the
              class.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#D5"><IMG
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><DD
>The
              <TT
CLASS="FUNCTION"
>timerEvent</TT
> is called every second
              because of the events generated by the timer
              object.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#D6"><IMG
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><DD
>A Python signal is emitted, of
              the name "timeSignal" which passes the result of the
              <TT
CLASS="FUNCTION"
>dataFunction</TT
> on.</DD
></DL
></DIV
><PRE
CLASS="PROGRAMLISTING"
>class DataWindow(frmDataSource):                           <IMG
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    def __init__(self, *args):
        apply(frmDataSource.__init__, (self,) + args)

        self.sources = {                                   <IMG
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            "random" : DataSource(randomFunction),
            "time" : DataSource(timeFunction),
            "cookies" : DataSource(cookieFunction)
            }

        self.cmbSource.insertStrList(self.sources.keys())  <IMG
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        self.currentSource=self.sources.keys()[0]          <IMG
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        self.connect(self.sources[self.currentSource],     <IMG
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                     PYSIGNAL("timeSignal"),
                     self.appendData)
                     
    def switchDataSource(self, source):                    <IMG
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        source=str(source)                                 <IMG
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        self.disconnect(self.sources[self.currentSource],  <IMG
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                     PYSIGNAL("timeSignal"),
                     self.appendData)
        self.connect(self.sources[source],                 <IMG
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                     PYSIGNAL("timeSignal"),
                     self.appendData)
        self.currentSource=source
        
    def appendData(self, value):                           <IMG
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        self.mleWindow.insertLine(value)
        self.mleWindow.setCursorPosition(self.mleWindow.numLines(), 0)
        </PRE
><DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW1"><IMG
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></DT
><DD
>The <TT
CLASS="CLASSNAME"
>DataWindow</TT
> class is a
              subclass of the generated form &#8212; class
              <TT
CLASS="CLASSNAME"
>frmDataSource</TT
>.
            </DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW2"><IMG
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></DT
><DD
>We create a Python dictionary, which takes
              <TT
CLASS="CLASSNAME"
>DataSource</TT
> objects (each
              instantiated with a different data generating function)
              and maps them to distinct names.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW3"><IMG
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></DT
><DD
>The <TT
CLASS="VARNAME"
>self.cmbSource</TT
> combobox is
              defined in the generated form. We fill the combobox with
              the set of keys to the dictionary. To do this, we use
              <TT
CLASS="FUNCTION"
>InsertStrList</TT
> and not
              <TT
CLASS="FUNCTION"
>InsertStringList</TT
>. A list of Python
              strings is converted automatically to a
              <TT
CLASS="CLASSNAME"
>QStrList</TT
>, while a
              <TT
CLASS="CLASSNAME"
>QStringList</TT
> object must be constructed
              separately.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW4"><IMG
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></DT
><DD
><TT
CLASS="VARNAME"
>self.currentSource</TT
> is a local
            variable where we keep track of what datasource we're
            looking at.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW5"><IMG
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></DT
><DD
>Simply connect the "timeSignal" Python signal from
            one of the objects in the dictionary of datasources to the
            slot that will display the output.</DD
><DT
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></DT
><DD
>The <TT
CLASS="FUNCTION"
>switchDataSource</TT
> function
              is where interesting things happen. This function is
              a slot that is called whenever the user selects
              something from the combobox. The
              <TT
CLASS="FUNCTION"
>clicked()</TT
> signal of the
              combobox was connected to the
              <TT
CLASS="FUNCTION"
>switchDataSource</TT
> slot
              of the Designer.


            </DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW7"><IMG
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><DD
>The variable passed by the signal
              connected to this slot is of the
              <TT
CLASS="CLASSNAME"
>QString</TT
> type. The index to
              the dictionary of data sources is a Python string. This
              is one instance where we <SPAN
><I
CLASS="EMPHASIS"
>must</I
></SPAN
>
              convert a <TT
CLASS="CLASSNAME"
>QString</TT
> to a Python
              string.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW8"><IMG
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></DT
><DD
>Using the cached current datasource, we disconnect
              the signals it generates from the
              <TT
CLASS="FUNCTION"
>appendData</TT
> function.</DD
><DT
><A href="index.lxp@lxpwrap=x1631_252ehtm.htm#DW9"><IMG
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></DT
><DD
>After the signal is disconnected, we can create a
            new connection.</DD
><DT
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></DT
><DD
>This is the function that shows the data. It simply
            adds every value that is passed on by the signal to the
            multi-line edit widget, and then sets the cursor to the
            last line. If this is not done, the display will not follow
            the added data, and instead stay at the beginning.</DD
></DL
></DIV
><PRE
CLASS="PROGRAMLISTING"
>def main(args):
    a = QApplication(args)
    QObject.connect(a,SIGNAL('lastWindowClosed()'),a,SLOT('quit()'))
    w = DataWindow()
    a.setMainWidget(w)
    w.show()
    a.exec_loop()

if __name__ == '__main__':
    main(sys.argv)
        </PRE
></DIV
><P
>As you can see, connecting and
      disconnecting signals and slots is a natural and intuitive
      technique. Their use is not limited to connecting GUI widgets,
      as signals and slots are also useful for the separation of the
      data model of an application from its interface. In Part III, We
      will investigate an application model based on the strict
      separation of model and interface, using signals and slots to
      tie everything together.</P
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