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  28 <h1><a name="hooks_scheme_extension_howto" id="hooks_scheme_extension_howto">Hooks/Scheme Extension HOWTO</a></h1>
  29 <div class="level1">
  30 <pre class="code">gEDA - GPL Electronic Design Automation
  31
  32 HOOKS AND SCHEME EXTENSION IN GSCHEM
  33 ====================================
  34
  35 Copyright (C) 2000 Stefan Petersen
  36
  37 This program is free software; you can redistribute it and/or modify
  38 it under the terms of the GNU General Public License as published by
  39 the Free Software Foundation; either version 2 of the License, or
  40 (at your option) any later version.
  41
  42 This program is distributed in the hope that it will be useful,
  43 but WITHOUT ANY WARRANTY; without even the implied warranty of
  44 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  45 GNU General Public License for more details.
  46
  47 You should have received a copy of the GNU General Public License
  48 along with this program; if not, write to the Free Software
  49 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  50
  51
  52 Introduction
  53 ------------
  54 gschem has a scheme interpreter (called Guile) built in. Though not
  55 complete, there are extensions to this interpreter to get access to
  56 different parts of the schematic.
  57
  58 There are a couple of other scheme extensions available that will not be
  59 described here. They belong mainly to rc-files (resource files in
  60 gEDA programs are really scheme scripts) and to the keymapping system
  61 (described in separate keymapping documentation).
  62
  63 The rest I will try to describe here.
  64
  65
  66 Scheme functions
  67 ----------------
  68 There are two function available for handling attributes in the schematic.
  69
  70 * get-attribute-name-value
  71 Inparameter  : an attribute
  72 Outparameter : a pair with the name of the attribute as string in the
  73                car element and the value of the attribute in the cdr
  74                element.
  75 Description  : Simply an accessor to the information hidden in the type
  76                attribute. The functionality of this is placed in libgeda
  77                since the C-type ATTRIBUTE is defined there.
  78
  79 * set-attribute-value!
  80 Inparameter  : an attribute and a string.
  81 Outparameter : undefined.
  82 Description  : Sets a new value to an attribute. The attribute must
  83                be defined, the function can&#039;t create a new attribute.
  84                Defined both in gschem and libgeda, mainly because
  85                where different variables and information are available.
  86
  87
  88 Hooks
  89 -----
  90 Hooks are a way to define functions that will be called during different
  91 part of a programs execution. In gschem there are (currently) three
  92 different hooks available:
  93 * add-component-hook
  94 * copy-component-hook
  95 * move-component-hook
  96
  97 As their name indicate, they are called at different occasions. When
  98 you add a component add-component-hook is called, etc.
  99
 100 To add a function to be called you simply use the Guile funtion add-hook!.
 101 An example; to run the function auto-uref when you add a component you
 102 simply add the following line, preferrably in ${HOME}/.gEDA/gschemrc:
 103 (add-hook! add-component-hook auto-uref)
 104
 105 The function to be called from a hook (for example auto-uref above) has
 106 to accept one parameter, a list of attributes.
 107
 108 A small example that prints all attributes on a component to be placed:
 109
 110 (define (print-all-attributes attribute-list)
 111   (foreach (lambda (attribute) (display attribute)) attribute-list))
 112
 113
 114 How to use this
 115 ---------------
 116 The most complete example utilizing all of the above functions are in fact
 117 the auto-uref scheme script that currently is part of the gschem distribution.
 118 You can find it &lt;where gschem is installed&gt;/share/gEDA/scheme/auto-uref.scm.
 119 Uninstalled it&#039;s available at gschem/scheme/auto-uref.scm
 120
 121 All components have a reference designator that must be unique so
 122 gnetlist can handle it properly. By automatically assigning a number
 123 to each instance of a component when you place and copy it, you can
 124 simplify the naming operation.
 125
 126 All components has, per default, an uref attribute, for example uref=R?.
 127 The letter varies with component type. The auto-uref script enumerates
 128 uref based on what prefix the component has and assigns a number.
 129
 130 For example, the first component you place has per default uref=U? gets
 131 the attribute uref=U1. Next component with uref=U? gets uref=U2 and so on.
 132
 133 To be able to use the auto-uref script you simply add two lines in
 134 ${HOME}/.gEDA/gschemrc. They are:
 135 (load &quot;&lt;where gschem is installed&gt;/share/gEDA/scheme/auto-uref.scm&quot;)
 136 (add-hook! add-component-hook auto-uref)
 137
 138 If you want auto enumeration to work when you copy the component too, you
 139 simply add the following line:
 140 (add-hook! copy-component-hook auto-uref)
 141
 142 Good luck!
 143
 144 </pre>
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