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Added safeguard against failure in __del__.
author Guido van Rossum <guido@python.org>
date Wed, 27 Nov 1996 19:39:58 +0000
parents a71d2fd90fa8
children 149a54798896
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1 <HTML><HEAD><TITLE>Using python to create Macintosh applications, part one</TITLE></HEAD>
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2 <BODY>
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3 <H1>Using python to create Macintosh applications, part one</H1>
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4 <HR>
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5
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6 This document will show you how to create a simple mac-style
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7 application using Python. We will glance at how to use dialogs and
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8 resources. <p>
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9
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10 The example application we look at will be a simple program with a
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11 dialog that allows you to control and monitor InterSLIP, a device
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12 driver that connects your mac to the Internet via a modem connection.
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13 <A HREF="example1/InterslipControl-1.py">Source</A> and resource file
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14 (in binary and <A
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15 HREF="example1/InterslipControl-1.rsrc.hqx">BinHex</A> form for
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16 downloading) for this application are available in the <A
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17 HREF="example1">example1</A> folder (which you will have to download
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18 if you are reading this document over the net and if you want to look
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19 at the resources). <p>
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20
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21 We will use a C extension module module "interslip" that allows a
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22 Python program to control and monitor the behaviour of the low-level
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23 driver, and we will create the user interface around that. If you want
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24 to actually run the code, you will obvously need InterSLIP and the
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25 interslip module. The latter is available as a dynamically loadable
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26 extension for PowerPC macs, and may be compiled in your Python
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27 interpreter for 68K macs. As of this writing there is still a slight
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28 problem with the Python interslip module causing it to say "file not
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29 found" if the driver is not loaded yet. The workaround is to load the
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30 driver by starting InterSLIP Control and quitting it. <p>
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31
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32 <CITE>
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33 If you are interested in building your own extensions to python you
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34 should check out the companion document <A
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35 HREF="plugins.html">Creating Macintosh Python C extensions</A>,
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36 which tells you how to build your own C extension. Not completely
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37 coincidental this document uses the interslip module that we will use
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38 here as an example. <p>
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39 </CITE>
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40
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41 <H2><A NAME="dialog-resources">Creating dialog resources</A></H2>
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42
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43 Let us start with the creative bit: building the dialogs and creating
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44 an icon for our program. For this you need ResEdit, and a reasonable
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45 working knowledge of how to use it. "Inside Mac" or various books on
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46 macintosh programming will help here. <p>
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47
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48 There is one fine point that deserves to be mentioned here: <A
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49 NAME="resource-numbering">resource numbering</A>. Because often your
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50 resources will be combined with those that the Python interpreter and
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51 various standard modules need you should give your DLOG and DITL
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52 resources numbers above 512. 128 and below are reserved for Apple,
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53 128-255 for the Python interpreter and 256-511 for standard
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54 modules. If you are writing a module that you will be distributing for
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55 inclusion in other people's programs you may want to register a number
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56 in the 256-511 range, contact Guido or myself or whoever you think is
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57 "in charge" of Python for the Macintosh at the moment. Even though the
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58 application we are writing at the moment will keep its resources in a
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59 separate resource file it is still a good idea to make sure that no
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60 conflicts arise: once you have opened your resource file any attempt
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61 by the interpreter to open a dialog will also search your resource
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62 file. <p>
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63
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64 Okay, let's have a look at InterslipControl-1.rsrc, our resource file.
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65 The DLOG and accompanying DITL resource both have number 512. Since
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66 ResEdit creates both with default ID=128 you should take care to
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67 change the number on both. The dialog itself is pretty basic: four
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68 buttons (connect, disconnect, update status and quit), two labels and
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69 two status fields. <p>
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70
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71 <H2><A NAME="modal-dialog">An application with a modal dialog</A></H2>
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72
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73 Next, we will have to write the actual application. For this example,
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74 we will use a modal dialog. This means that we will put up the dialog
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75 and go into a loop asking the dialog manager for events (buttons
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76 pushed). We handle the actions requested by the user until the quit
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77 button is pressed, upon which we exit our loop (and the program). This
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78 way of structuring your program is actually rather antisocial, since
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79 you force the user to do whatever you, the application writer, happen
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80 to want. A modal dialog leaves no way of escape whatsoever (except
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81 command-option-escape), and is usually not a good way to structure
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82 anything but the most simple questions. Even then: how often have you
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83 been confronted with a dialog asking a question that you could not
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84 answer because the data you needed was obscured by the dialog itself?
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85 In the next example we will look at an application that does pretty
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86 much the same as this one but in a more user-friendly way. <p>
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87
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88 On to the code itself, in file <A
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89 HREF="example1/InterslipControl-1.py"> InterslipControl-1.py</A>. Have
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90 a copy handy before you read on. The file starts off with a
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91 textstring giving a short description. Not many tools do anything with
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92 this as yet, but at some point in the future we <EM>will</EM> have all
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93 sorts of nifty class browser that will display this string, so just
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94 include it. Just put a short description at the start of each module,
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95 class, method and function. After the initial description and some
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96 comments, we import the modules we need. <p>
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97
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98 <A NAME="easydialogs"><CODE>EasyDialogs</CODE></A> is a handy standard
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99 module that provides you with routines that put up common text-only
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100 modal dialogs:
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101 <UL>
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102 <LI> <CODE>Message(str)</CODE>
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103 displays the message "str" and an OK button,
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104 <LI> <CODE>AskString(prompt, default)</CODE>
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105 asks for a string, displays OK and Cancel buttons,
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106 <LI> <CODE>AskYesNoCancel(question, default)</CODE>
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107 displays a question and Yes, No and Cancel buttons.
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108 </UL>
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109
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110 <A NAME="res"><CODE>Res</CODE></A> is a pretty complete interface to
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111 the MacOS Resource Manager, described fully in Inside Mac. There is
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112 currently no documentation of it, but the Apple documentation (or
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113 Think Ref) will help you on your way if you remember two points:
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114 <UL>
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115 <LI> Resources are implemented as Python objects, and each routine
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116 with a resource first argument is implemented as a python method.
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117 <LI> When in doubt about the arguments examine the routines docstring,
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118 as in <CODE>print Res.OpenResFile.__doc__</CODE>
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119 </UL>
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120
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121 Similarly, <A NAME="dlg"><CODE>Dlg</CODE></A> is an interface to the
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122 Dialog manager (with Dialogs being implemented as python objects and
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123 routines with Dialog arguments being methods). The sys module you
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124 know, I hope. <A NAME="interslip"><CODE>Interslip</CODE></A>,
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125 finally, is the module with the interface to the InterSLIP driver. We
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126 use four calls from it:
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127 <UL>
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128 <LI> <CODE>open()</CODE>
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129 opens the driver
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130 <LI> <CODE>connect()</CODE>
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131 asks it to initiate a connection procedure (without waiting)
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132 <LI> <CODE>disconnect()</CODE>
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133 asks it to initiate a disconnection procedure (without waiting)
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134 <LI> <CODE>status()</CODE>
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135 returns the current connection status in the form of an integer state,
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136 an integer "message sequence number" and a message string.
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137 </UL>
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138
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139 Next in the source file we get definitions for our dialog resource
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140 number and for the item numbers in our dialog. These should match the
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141 situation in our resource file InterslipControl-1.rsrc,
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142 obviously. Then we get an array converting numeric state codes
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143 returned by <CODE>interslip.status()</CODE> to textual messages. <p>
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144
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145 On to the main program. We start off with opening our resource file,
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146 which should live in the same folder as the python source. If we
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147 cannot open it we use <CODE>EasyDialogs</CODE> to print a message and
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148 exit. You can try it: just move the resource file somewhere else for a
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149 moment. Then, we try to open the interslip driver, again catching an
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150 error. All modules that raise <A NAME="macos-errors">MacOS error
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151 exceptions</A> will pass a 2-tuple to the exception handler with the
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152 first item being the numeric <CODE>OSErr</CODE> code and the second
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153 one being an informative message. If no informative message is
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154 available it will be the rather uninformative <CODE>"MacOS Error
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155 -12345"</CODE>, but at least the second item will always be a
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156 printable string. Finally we call do_dialog() to do the real work. <p>
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157
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158 <CODE>Do_dialog()</CODE> uses <CODE>Dlg.GetNewDialog()</CODE> to open
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159 a dialog window initialized from 'DLOG' resource ID_MAIN and putting
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160 it on screen in the frontmost position. Next, we go into a loop,
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161 calling <CODE>Dlg.ModalDialog()</CODE> to wait for the next user
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162 action. <CODE>ModalDialog()</CODE> will return us the item number that
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163 the user has clicked on (or otherwise activated). It will handle a few
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164 slightly more complicated things also, like the user typing into
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165 simple textfields, but it will <EM>not</EM> do things like updating
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166 the physical appearance of radio buttons, etc. See Inside Mac or
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167 another programming guide for how to handle this
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168 yourself. Fortunately, our simple application doesn't have to bother
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169 with this, since buttons are the only active elements we have. So, we
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170 do a simple switch on item number and call the appropriate routine to
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171 implement the action requested. Upon the user pressing "quit" we
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172 simply leave the loop and, hence, <CODE>do_dialog()</CODE>. This will
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173 cause the python dialog object <CODE>my_dlg</CODE> to be deleted and
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174 the on-screen dialog to disappear. <p>
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175
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176 <A NAME="dialog-warning">Time for a warning</A>: be very careful what
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177 you do as long as a dialog is on-screen. Printing something, for
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178 instance, may suddenly cause the standard output window to appear over
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179 the dialog, and since we took no measures to redraw the dialog it will
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180 become very difficult to get out of the dialog. Also, command-period
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181 may or may not work in this situation. I have also seen crashes in
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182 such a situation, probably due to the multiple event loops involved or
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183 some oversight in the interpreter. You have been warned. <p>
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184
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185 The implementation of the "update status" command can use a bit more
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186 explaining: we get the new information with <CODE>do_status()</CODE>
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187 but now we have to update the on-screen dialog to present this
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188 information to the user. The <CODE>GetDialogItem()</CODE> method of
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189 the dialog returns three bits of information about the given item: its
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190 type, its data handle and its rect (the on-screen <CODE>x,y,w,h</CODE>
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191 coordinates). We are only interested in the data handle here, on which
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192 we call <CODE>SetDialogItemText()</CODE> to set our new text. Note
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193 here that python programmers need not bother with the C-string versus
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194 pascal-string controversy: the python glue module knows what is needed
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195 and converts the python string to the correct type. <p>
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196
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197 Finally, the three implementation routines <CODE>do_connect()</CODE>,
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198 <CODE>do_disconnect()</CODE> and <CODE>do_status()</CODE> are simply
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199 boring wrappers around the corresponding interslip methods that will
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200 put up a dialog in case of an error. <p>
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201
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202 And that concludes our first example of the use of resources and
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203 dialogs. Next, you could have a look at the source of EasyDialogs for
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204 some examples of using input fields and filterprocs. Or, go on with
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205 reading the <A HREF="example2.html">second part</A> of this document
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206 to see how to implement a better version of this application. Not only
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207 will it allow the user to go back to the finder (or other apps) when
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208 your application is running, it will also free her of the RSI-inducing
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209 chore of pressing "update status" continuously... <p>
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210
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211