Mac/Demo/example1.html

   1 <HTML><HEAD><TITLE>Using python to create Macintosh applications, part one</TITLE></HEAD>
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   3
   4 <H1>Using python to create Macintosh applications, part one</H1>
   5 <HR>
   6
   7 This document will show you how to create a simple mac-style
   8 application using Python. We will glance at how to use dialogs and
   9 resources. <p>
  10
  11 The example application we look at will be a simple program with a
  12 dialog that allows you to perform domain name lookups on IP addresses
  13 and hostnames.
  14 The <A HREF="example1/dnslookup-1.py">source code</A> and
  15 <A HREF="example1/dnslookup-1.rsrc">resource file</A>
  16 for this application are available in the <A
  17 HREF="example1">example1</A> folder (which you will have to download
  18 if you are reading this document over the net and if you want to look
  19 at the resources). <p>
  20
  21 We will use the builtin module "socket" that allows a
  22 Python program to perform all sorts of networking functions, and we
  23 will create the user interface around that. You should be able to run
  24 the sample code with the standard Python distribution.<p>
  25
  26 <CITE>
  27 If you are interested in building your own extensions to python you
  28 should check out the companion document <A
  29 HREF="plugins.html">Creating Macintosh Python C extensions</A>,
  30 which tells you how to build your own C extension.
  31 <p>
  32 </CITE>
  33
  34 <H2><A NAME="dialog-resources">Creating dialog resources</A></H2>
  35
  36 Let us start with the creative bit: building the dialogs and creating
  37 an icon for our program. For this you need ResEdit, and a reasonable
  38 working knowledge of how to use it. "Inside Mac" or various books on
  39 macintosh programming will help here. <p>
  40
  41 There is one fine point that deserves to be mentioned here: <A
  42 NAME="resource-numbering">resource numbering</A>.  Because often your
  43 resources will be combined with those that the Python interpreter and
  44 various standard modules need you should give your DLOG and DITL
  45 resources numbers above 512. 128 and below are reserved for Apple,
  46 128-228 are for extensions like Tk,
  47 228-255 for the Python interpreter and 256-511 for standard
  48 modules. If you are writing a module that you will be distributing for
  49 inclusion in other people's programs you may want to register a number
  50 in the 256-511 range, contact Guido or myself or whoever you think is
  51 "in charge" of Python for the Macintosh at the moment. Even though the
  52 application we are writing at the moment will keep its resources in a
  53 separate resource file it is still a good idea to make sure that no
  54 conflicts arise: once you have opened your resource file any attempt
  55 by the interpreter to open a dialog will also search your resource
  56 file. <p>
  57
  58 Okay, let's have a look at dnslookup-1.rsrc, our resource file.
  59 The DLOG and accompanying DITL resource both have number 512. Since
  60 ResEdit creates both with default ID=128 you should take care to
  61 change the number on both. The dialog itself is pretty basic: two
  62 buttons (Lookup and Quit), two labels and
  63 two text entry areas, one of which is used for output only.  Here's what
  64 the dialog will look like at run time<p>
  65 <div align=center>
  66 <img width=324 height=189 src="example1/dnslookup-1.gif" alt="dialog image">
  67 </div>
  68 <p>
  69
  70 <H2><A NAME="modal-dialog">An application with a modal dialog</A></H2>
  71
  72 Next, we will have to write the actual application. For this example,
  73 we will use a modal dialog. This means that we will put up the dialog
  74 and go into a loop asking the dialog manager for events (buttons
  75 pushed). We handle the actions requested by the user until the Quit
  76 button is pressed, upon which we exit our loop (and the program). This
  77 way of structuring your program is actually rather antisocial, since
  78 you force the user to do whatever you, the application writer, happen
  79 to want. A modal dialog leaves no way of escape whatsoever (except
  80 command-option-escape), and is usually not a good way to structure
  81 anything but the most simple questions.  Even then: how often have you
  82 been confronted with a dialog asking a question that you could not
  83 answer because the data you needed was obscured by the dialog itself?
  84 In the next example we will look at an application that does pretty
  85 much the same as this one but in a more user-friendly way. <p>
  86
  87 The code itself is contained in the file <A
  88 HREF="example1/dnslookup-1.py"> dnslookup-1.py</A>. Have
  89 a copy handy before you read on.  The file starts off with a
  90 textstring giving a short description. Not many tools do anything with
  91 this as yet, but at some point in the future we <EM>will</EM> have all
  92 sorts of nifty class browser that will display this string, so just
  93 include it. Just put a short description at the start of each module,
  94 class, method and function.  After the initial description and some
  95 comments, we import the modules we need. <p>
  96
  97 <A NAME="easydialogs"><CODE>EasyDialogs</CODE></A> is a handy standard
  98 module that provides you with routines that put up common text-only
  99 modal dialogs:
 100 <UL>
 101 <LI> <CODE>Message(str)</CODE>
 102 displays the message "str" and an OK button,
 103 <LI> <CODE>AskString(prompt, default)</CODE>
 104 asks for a string, displays OK and Cancel buttons,
 105 <LI> <CODE>AskYesNoCancel(question, default)</CODE>
 106 displays a question and Yes, No and Cancel buttons.
 107 </UL>
 108
 109 <A NAME="res"><CODE>Res</CODE></A> is a pretty complete interface to
 110 the MacOS Resource Manager, described fully in Inside Mac. There is
 111 currently no documentation of it, but the Apple documentation (or
 112 Think Ref) will help you on your way if you remember two points:
 113 <UL>
 114 <LI> Resources are implemented as Python objects, and each routine
 115 with a resource first argument is implemented as a python method.
 116 <LI> When in doubt about the arguments examine the routines docstring,
 117 as in <CODE>print Res.OpenResFile.__doc__</CODE>
 118 </UL>
 119
 120 Similarly, <A NAME="dlg"><CODE>Dlg</CODE></A> is an interface to the
 121 Dialog manager (with Dialogs being implemented as python objects and
 122 routines with Dialog arguments being methods). The sys module you
 123 know, I hope.  The string module is an often used module that enables
 124 you to perform many string related operations.  In this case however, we
 125 are only using the "digits" constant from the string module.  We could
 126 have simply defined "digits" as "0123456789".  The socket module enables
 127 us to perform the domain name lookups. We
 128 use two calls from it:
 129 <UL>
 130 <LI> <CODE>gethostbyaddr()</CODE>
 131 returns the hostname associated with an IP address
 132 <LI> <CODE>gethostbyname()</CODE>
 133 returns the IP address associated with a hostname
 134 </UL>
 135
 136 Next in the source file we get definitions for our dialog resource
 137 number and for the item numbers in our dialog. These should match the
 138 situation in our resource file dnslookup-1.rsrc,
 139 obviously.<p>
 140
 141 On to the main program. We start off with opening our resource file,
 142 which should live in the same folder as the python source. If we
 143 cannot open it we use <CODE>EasyDialogs</CODE> to print a message and
 144 exit. You can try it: just move the resource file somewhere else for a
 145 moment. Then we call do_dialog() to do the real work. <p>
 146
 147 <CODE>Do_dialog()</CODE> uses <CODE>Dlg.GetNewDialog()</CODE> to open
 148 a dialog window initialized from 'DLOG' resource ID_MAIN and putting
 149 it on screen in the frontmost position.  Next, we go into a loop,
 150 calling <CODE>Dlg.ModalDialog()</CODE> to wait for the next user
 151 action. <CODE>ModalDialog()</CODE> will return us the item number that
 152 the user has clicked on (or otherwise activated). It will handle a few
 153 slightly more complicated things also, like the user typing into
 154 simple textfields, but it will <EM>not</EM> do things like updating
 155 the physical appearance of radio buttons, etc. See Inside Mac or
 156 another programming guide for how to handle this
 157 yourself. Fortunately, our simple application doesn't have to bother with this,
 158 since buttons and textfields are the only active elements we have. So, we do a
 159 simple switch on item number and call the appropriate routine to implement the
 160 action requested. Upon the user pressing "Quit" we simply leave the loop and,
 161 hence, <CODE>do_dialog()</CODE>. This will cause the python dialog object
 162 <CODE>my_dlg</CODE> to be deleted and the on-screen dialog to disappear. <p>
 163
 164 <A NAME="dialog-warning">Time for a warning</A>: be very careful what
 165 you do as long as a dialog is on-screen.  Printing something, for
 166 instance, may suddenly cause the standard output window to appear over
 167 the dialog, and since we took no measures to redraw the dialog it will
 168 become very difficult to get out of the dialog. Also, command-period
 169 may or may not work in this situation. I have also seen crashes in
 170 such a situation, probably due to the multiple event loops involved or
 171 some oversight in the interpreter. You have been warned. <p>
 172
 173 The implementation of the "Lookup" command can use a bit more
 174 explaining: we get the necessary information with <CODE>dnslookup()</CODE>
 175 but now we have to update the on-screen dialog to present this
 176 information to the user. The <CODE>GetDialogItem()</CODE> method of
 177 the dialog returns three bits of information about the given item: its
 178 type, its data handle and its rect (the on-screen <CODE>x,y,w,h</CODE>
 179 coordinates). We are only interested in the data handle here, on which
 180 we call <CODE>SetDialogItemText()</CODE> to set our new text.  Note
 181 here that python programmers need not bother with the C-string versus
 182 pascal-string controversy: the python glue module knows what is needed
 183 and converts the python string to the correct type. <p>
 184
 185 And that concludes our first example of the use of resources and
 186 dialogs. Next, you could have a look at the source of EasyDialogs for
 187 some examples of using input fields and filterprocs. Or, go on with
 188 reading the <A HREF="example2.html">second part</A> of this document
 189 to see how to implement a better version of this application.<p>
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