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<HTML><HEAD><TITLE>Using python to create Macintosh applications, part one</TITLE></HEAD>
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<H1>Using python to create Macintosh applications, part one</H1>
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<HR>
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This document will show you how to create a simple mac-style
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application using Python. We will glance at how to use dialogs and
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resources. <p>
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The example application we look at will be a simple program with a
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dialog that allows you to control and monitor InterSLIP, a device
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driver that connects your mac to the Internet via a modem connection.
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<A HREF="example1/InterslipControl-1.py">Source</A> and resource file
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(in binary and <A
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HREF="example1/InterslipControl-1.rsrc.hqx">BinHex</A> form for
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downloading) for this application are available in the <A
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HREF="example1">example1</A> folder (which you will have to download
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if you are reading this document over the net and if you want to look
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at the resources). <p>
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We will use a C extension module module "interslip" that allows a
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Python program to control and monitor the behaviour of the low-level
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driver, and we will create the user interface around that. If you want
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to actually run the code, you will obvously need InterSLIP and the
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interslip module. The latter is available as a dynamically loadable
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extension for PowerPC macs, and may be compiled in your Python
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interpreter for 68K macs. As of this writing there is still a slight
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problem with the Python interslip module causing it to say "file not
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found" if the driver is not loaded yet. The workaround is to load the
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driver by starting InterSLIP Control and quitting it. <p>
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<CITE>
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If you are interested in building your own extensions to python you
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should check out the companion document <A
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HREF="plugins.html">Creating Macintosh Python C extensions</A>,
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which tells you how to build your own C extension. Not completely
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coincidental this document uses the interslip module that we will use
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here as an example. <p>
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</CITE>
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<H2><A NAME="dialog-resources">Creating dialog resources</A></H2>
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Let us start with the creative bit: building the dialogs and creating
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an icon for our program. For this you need ResEdit, and a reasonable
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working knowledge of how to use it. "Inside Mac" or various books on
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macintosh programming will help here. <p>
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There is one fine point that deserves to be mentioned here: <A
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NAME="resource-numbering">resource numbering</A>. Because often your
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resources will be combined with those that the Python interpreter and
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various standard modules need you should give your DLOG and DITL
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resources numbers above 512. 128 and below are reserved for Apple,
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128-255 for the Python interpreter and 256-511 for standard
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modules. If you are writing a module that you will be distributing for
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inclusion in other people's programs you may want to register a number
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in the 256-511 range, contact Guido or myself or whoever you think is
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"in charge" of Python for the Macintosh at the moment. Even though the
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application we are writing at the moment will keep its resources in a
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separate resource file it is still a good idea to make sure that no
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conflicts arise: once you have opened your resource file any attempt
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by the interpreter to open a dialog will also search your resource
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file. <p>
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Okay, let's have a look at InterslipControl-1.rsrc, our resource file.
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The DLOG and accompanying DITL resource both have number 512. Since
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ResEdit creates both with default ID=128 you should take care to
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change the number on both. The dialog itself is pretty basic: four
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buttons (connect, disconnect, update status and quit), two labels and
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two status fields. <p>
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<H2><A NAME="modal-dialog">An application with a modal dialog</A></H2>
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Next, we will have to write the actual application. For this example,
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we will use a modal dialog. This means that we will put up the dialog
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and go into a loop asking the dialog manager for events (buttons
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pushed). We handle the actions requested by the user until the quit
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button is pressed, upon which we exit our loop (and the program). This
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way of structuring your program is actually rather antisocial, since
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you force the user to do whatever you, the application writer, happen
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to want. A modal dialog leaves no way of escape whatsoever (except
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command-option-escape), and is usually not a good way to structure
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anything but the most simple questions. Even then: how often have you
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been confronted with a dialog asking a question that you could not
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answer because the data you needed was obscured by the dialog itself?
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In the next example we will look at an application that does pretty
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much the same as this one but in a more user-friendly way. <p>
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On to the code itself, in file <A
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HREF="example1/InterslipControl-1.py"> InterslipControl-1.py</A>. Have
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a copy handy before you read on. The file starts off with a
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textstring giving a short description. Not many tools do anything with
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this as yet, but at some point in the future we <EM>will</EM> have all
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sorts of nifty class browser that will display this string, so just
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include it. Just put a short description at the start of each module,
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class, method and function. After the initial description and some
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comments, we import the modules we need. <p>
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<A NAME="easydialogs"><CODE>EasyDialogs</CODE></A> is a handy standard
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module that provides you with routines that put up common text-only
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modal dialogs:
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<UL>
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<LI> <CODE>Message(str)</CODE>
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displays the message "str" and an OK button,
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<LI> <CODE>AskString(prompt, default)</CODE>
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asks for a string, displays OK and Cancel buttons,
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<LI> <CODE>AskYesNoCancel(question, default)</CODE>
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displays a question and Yes, No and Cancel buttons.
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</UL>
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<A NAME="res"><CODE>Res</CODE></A> is a pretty complete interface to
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the MacOS Resource Manager, described fully in Inside Mac. There is
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currently no documentation of it, but the Apple documentation (or
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Think Ref) will help you on your way if you remember two points:
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<UL>
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<LI> Resources are implemented as Python objects, and each routine
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with a resource first argument is implemented as a python method.
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<LI> When in doubt about the arguments examine the routines docstring,
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as in <CODE>print Res.OpenResFile.__doc__</CODE>
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</UL>
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Similarly, <A NAME="dlg"><CODE>Dlg</CODE></A> is an interface to the
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Dialog manager (with Dialogs being implemented as python objects and
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routines with Dialog arguments being methods). The sys module you
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know, I hope. <A NAME="interslip"><CODE>Interslip</CODE></A>,
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finally, is the module with the interface to the InterSLIP driver. We
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use four calls from it:
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<UL>
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<LI> <CODE>open()</CODE>
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opens the driver
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<LI> <CODE>connect()</CODE>
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asks it to initiate a connection procedure (without waiting)
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<LI> <CODE>disconnect()</CODE>
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asks it to initiate a disconnection procedure (without waiting)
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<LI> <CODE>status()</CODE>
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returns the current connection status in the form of an integer state,
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an integer "message sequence number" and a message string.
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</UL>
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Next in the source file we get definitions for our dialog resource
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number and for the item numbers in our dialog. These should match the
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situation in our resource file InterslipControl-1.rsrc,
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obviously. Then we get an array converting numeric state codes
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returned by <CODE>interslip.status()</CODE> to textual messages. <p>
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On to the main program. We start off with opening our resource file,
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which should live in the same folder as the python source. If we
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cannot open it we use <CODE>EasyDialogs</CODE> to print a message and
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exit. You can try it: just move the resource file somewhere else for a
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moment. Then, we try to open the interslip driver, again catching an
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error. All modules that raise <A NAME="macos-errors">MacOS error
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exceptions</A> will pass a 2-tuple to the exception handler with the
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first item being the numeric <CODE>OSErr</CODE> code and the second
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one being an informative message. If no informative message is
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available it will be the rather uninformative <CODE>"MacOS Error
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-12345"</CODE>, but at least the second item will always be a
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printable string. Finally we call do_dialog() to do the real work. <p>
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<CODE>Do_dialog()</CODE> uses <CODE>Dlg.GetNewDialog()</CODE> to open
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a dialog window initialized from 'DLOG' resource ID_MAIN and putting
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it on screen in the frontmost position. Next, we go into a loop,
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calling <CODE>Dlg.ModalDialog()</CODE> to wait for the next user
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action. <CODE>ModalDialog()</CODE> will return us the item number that
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the user has clicked on (or otherwise activated). It will handle a few
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slightly more complicated things also, like the user typing into
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simple textfields, but it will <EM>not</EM> do things like updating
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the physical appearance of radio buttons, etc. See Inside Mac or
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another programming guide for how to handle this
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yourself. Fortunately, our simple application doesn't have to bother
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with this, since buttons are the only active elements we have. So, we
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do a simple switch on item number and call the appropriate routine to
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implement the action requested. Upon the user pressing "quit" we
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simply leave the loop and, hence, <CODE>do_dialog()</CODE>. This will
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cause the python dialog object <CODE>my_dlg</CODE> to be deleted and
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the on-screen dialog to disappear. <p>
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<A NAME="dialog-warning">Time for a warning</A>: be very careful what
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you do as long as a dialog is on-screen. Printing something, for
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instance, may suddenly cause the standard output window to appear over
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the dialog, and since we took no measures to redraw the dialog it will
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become very difficult to get out of the dialog. Also, command-period
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may or may not work in this situation. I have also seen crashes in
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such a situation, probably due to the multiple event loops involved or
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some oversight in the interpreter. You have been warned. <p>
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The implementation of the "update status" command can use a bit more
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explaining: we get the new information with <CODE>do_status()</CODE>
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but now we have to update the on-screen dialog to present this
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information to the user. The <CODE>GetDialogItem()</CODE> method of
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the dialog returns three bits of information about the given item: its
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type, its data handle and its rect (the on-screen <CODE>x,y,w,h</CODE>
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coordinates). We are only interested in the data handle here, on which
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we call <CODE>SetDialogItemText()</CODE> to set our new text. Note
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here that python programmers need not bother with the C-string versus
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pascal-string controversy: the python glue module knows what is needed
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and converts the python string to the correct type. <p>
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Finally, the three implementation routines <CODE>do_connect()</CODE>,
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<CODE>do_disconnect()</CODE> and <CODE>do_status()</CODE> are simply
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boring wrappers around the corresponding interslip methods that will
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put up a dialog in case of an error. <p>
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And that concludes our first example of the use of resources and
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dialogs. Next, you could have a look at the source of EasyDialogs for
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some examples of using input fields and filterprocs. Or, go on with
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reading the <A HREF="example2.html">second part</A> of this document
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to see how to implement a better version of this application. Not only
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will it allow the user to go back to the finder (or other apps) when
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your application is running, it will also free her of the RSI-inducing
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chore of pressing "update status" continuously... <p>
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