index.lxp@lxpwrap=c1267_252ehtm.htm

GUI Programming with Python

>Example 7-3. A central registry of connected widgets</B
></P
><PRE
CLASS="PROGRAMLISTING"
>#
# registry.py &#8212; a central registry of connected widgets
#
class Registry:

    def __init__(self):
        self.connections={}                                <IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
>

    def add(self, occasion, function):                     <IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
>
        if self.connections.has_key(occasion) == 0:
            self.connections[occasion]=[function]
        else:
            self.connections[occasion].append(function)

    def remove(self, occasion, function):                  <IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
>
        if self.connections.has_key(occasion):
            self.connections[occasion].remove(function)

    def execute(self, occasion):
        if self.connections.has_key(occasion):             <IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
>
            for function in self.connections[occasion]:
                apply(function)

registry=Registry()
                                                           <IMG
SRC="images/callouts/5.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(5)"></A
>
class Button:
                                                           <IMG
SRC="images/callouts/6.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(6)"></A
>
    def clicked(self):
        registry.execute("clicked")

class Application:

    def __init__(self):
        self.button=Button()
        registry.add("clicked", self.doAppSpecificFunction)
        registry.add("clicked", self.doSecondFunction)     <IMG
SRC="images/callouts/7.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(7)"></A
>

    def doAppSpecificFunction(self):
        print "Function called"

    def doSecondFunction(self):
        print "A second function is called."

app=Application()
app.button.clicked()
        </PRE
><DIV
CLASS="CALLOUTLIST"
><DL
COMPACT="COMPACT"
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYCONNECTIONS"><IMG
SRC="images/callouts/1.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(1)"></A
></DT
><DD
>The actual registry is a Python dictionary with
                the name <TT
CLASS="VARNAME"
>connections</TT
>. Here,
                each <TT
CLASS="VARNAME"
>occasion</TT
> is used as a key to
                find the actual function object that should be called.</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYADD"><IMG
SRC="images/callouts/2.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(2)"></A
></DT
><DD
>If the &#8216;occasion' is already registered, we simply
                add a new entry to the list; otherwise a new entry
                is created in the registry.</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYREMOVE"><IMG
SRC="images/callouts/3.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(3)"></A
></DT
><DD
>If the &#8216;occasion' exists, then we remove the
                relevant function entry from its list of
                functions.</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYEXECUTE"><IMG
SRC="images/callouts/4.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(4)"></A
></DT
><DD
>We loop over all functions that belong to this
                &#8216;occasion' and simply execute them by calling
                <TT
CLASS="FUNCTION"
>apply()</TT
> on them.</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYCREATE"><IMG
SRC="images/callouts/5.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(5)"></A
></DT
><DD
>A registry is a unique object to an application:
                there should only be one, so we create it globally.</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYBUTTON"><IMG
SRC="images/callouts/6.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(6)"></A
></DT
><DD
>This is the &#8216;button' class.
                Whenever the button is &#8216;clicked', it calls the
                <TT
CLASS="FUNCTION"
>execute()</TT
> function in the
                registry with the &#8216;clicked' occasion.</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYAPPLICATION"><IMG
SRC="images/callouts/7.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(7)"></A
></DT
><DD
>The application creates one button and binds two
                of its functions to the button. This looks a
                lot like the way connections are made in Qt!</DD
><DT
><A href="index.lxp@lxpwrap=c1267_252ehtm.htm#CH6REGISTRYSIMULATE"><IMG
SRC="images/callouts/8.gif"
HSPACE="0"
VSPACE="0"
BORDER="0"
ALT="(8)"></A
></DT
><DD
>Here we simulate a button click by directly
                calling the <TT
CLASS="FUNCTION"
>clicked()</TT
> function on
                the button.</DD
></DL
></DIV
></DIV
><P
>This is one step up from the previous example, which was
        an extremely crude implementation of the well known Observer
        design pattern, in that there is now a &#8216;neutral' object that
        mediates between the button and the application. However, it
        is still not particularly sophisticated. It certainly wouldn't do
        for a real application &#8212; where there might be many
        objects with the same &#8216;occasion'.</P
><P
>It is quite possible to implement a solution like this in
        pure Python, especially with the weak references module that
        debuted in Python 2.1. Bernhard Herzog has done so in his fine
        Python application <SPAN
CLASS="APPLICATION"
>Sketch</SPAN
>
        (http://sketch.sourceforge.net). He had to do it himself
        &#8212; because he was working in PyGTK, not PyQt.
        Fortunately, PyQt has already solved the whole problem for
        us.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
>Signals and slots</A
></H2
><P
>We've just outlined the problem which the
        developers of Qt at Trolltech have solved in a unique and
        flexible manner. They created the concept of signals and
        slots. <SPAN
><I
CLASS="EMPHASIS"
>signals</I
></SPAN
> are sent by an object that
        wants to tell the world something interesting has happened,
        and by <SPAN
><I
CLASS="EMPHASIS"
>connecting</I
></SPAN
> the "signals to the
        slots", those signals arrive at the <SPAN
><I
CLASS="EMPHASIS"
>slots</I
></SPAN
>
        of the objects that are interested.</P
><P
>On the whole the concept is really neat
        and clean and the implementation well-executed. What's more,
        the concept is even better suited to Python than it is to C++.
        If you want to use signals and slots in C++, you have to work
        with a preprocessor, called <B
CLASS="COMMAND"
>moc</B
>, and
        indicate with special macros which function can be called as a
        slot, and which function is a signal. All that is completely
        unnecessary in Python, where a signal is a string, and any
        function can be a slot.</P
><P
>Signals and slots are not magic, of
        course. As with our simple Python registry, there has to be a
        registry of objects that are interested in signals. This
        registry is updated by the <TT
CLASS="FUNCTION"
>connect</TT
> and
        <TT
CLASS="FUNCTION"
>disconnect</TT
> functions; both are member
        functions of the <TT
CLASS="CLASSNAME"
>QObject</TT
> class. The
        registry, as far as Python signals and slots is concerned, is
        kept by the sip library. Signals and slots that are defined in
        the underlying C++ library are maintained by the
        <TT
CLASS="CLASSNAME"
>QObject</TT
> class itself.</P
><P
>In a nutshell, signals and slots are the
        solution Qt provides for situations in which you want two
        objects to interact, while keeping that fact hidden from
        them.</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Signals, messages, events: </B
>This is one area where there is a
          perfect Babel of tongues. Even really knowledgeable people
          like Dr Dobbs' Al Stevens get confused when confronted with
          terms like &#8216;message', &#8216;event' or
          &#8216;signal'.</P
><P
>In PyQt programming, the term
          '&#8216;message' is quite irrelevant &#8212; it is used in
          Windows programming to indicate function calls made from
          your application to the Windows GUI libraries.</P
><P
>Events and signals, on the other hand,
          are central to PyQt. Signals and slots are used to connect
          one object to another. An example is the perennial
          pushbutton, whose <TT
CLASS="FUNCTION"
>clicked()</TT
> signal gets
          connected to the <TT
CLASS="FUNCTION"
>accept()</TT
> slot function
          of a dialog box. Signals are used to connect entities
          internal to the application.</P
><P
>Events are more often generated
          directly by user input, such as moving or clicking with the
          mouse, or typing at the keyboard. As such, they don't
          connect two class instances, but rather a physical object,
          such as a keyboard, with an application. Events are
          encapsulated by the <TT
CLASS="CLASSNAME"
>QEvent</TT
> class, and
          are mostly delivered to <TT
CLASS="CLASSNAME"
>QWidget</TT
> and
          its descendants. Events are used to communication with
          external entities.</P
></BLOCKQUOTE
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