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geda:hse_howto [2007/05/24 22:27] external edit
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 +====== Hooks/​Scheme Extension HOWTO ======
 +<​code>​gEDA - GPL Electronic Design Automation
 +==================================== ​
 +Copyright (C) 2000 Stefan Petersen
 +This program is free software; you can redistribute it and/or modify
 +it under the terms of the GNU General Public License as published by
 +the Free Software Foundation; either version 2 of the License, or
 +(at your option) any later version.
 +This program is distributed in the hope that it will be useful,
 +but WITHOUT ANY WARRANTY; without even the implied warranty of
 +GNU General Public License for more details.
 +You should have received a copy of the GNU General Public License
 +along with this program; if not, write to the Free Software
 +Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 +gschem has a scheme interpreter (called Guile) built in. Though not 
 +complete, there are extensions to this interpreter to get access to 
 +different parts of the schematic.
 +There are a couple of other scheme extensions available that will not be 
 +described here. They belong mainly to rc-files (resource files in 
 +gEDA programs are really scheme scripts) and to the keymapping system ​
 +(described in separate keymapping documentation).
 +The rest I will try to describe here. 
 +Scheme functions
 +There are two function available for handling attributes in the schematic.
 +* get-attribute-name-value
 +Inparameter ​ : an attribute
 +Outparameter : a pair with the name of the attribute as string in the 
 +        car element and the value of the attribute in the cdr 
 +        ​element.
 +Description ​ : Simply an accessor to the information hidden in the type
 +        ​attribute. The functionality of this is placed in libgeda
 +        since the C-type ATTRIBUTE is defined there.
 +* set-attribute-value!
 +Inparameter ​ : an attribute and a string.
 +Outparameter : undefined.
 +Description ​ : Sets a new value to an attribute. The attribute must 
 +               be defined, the function can't create a new attribute. ​
 +        ​Defined both in gschem and libgeda, mainly because ​
 +        where different variables and information are available.
 +Hooks are a way to define functions that will be called during different
 +part of a programs execution. In gschem there are (currently) three 
 +different hooks available:
 +* add-component-hook
 +* copy-component-hook
 +* move-component-hook
 +As their name indicate, they are called at different occasions. When 
 +you add a component add-component-hook is called, etc.
 +To add a function to be called you simply use the Guile funtion add-hook!.
 +An example; to run the function auto-uref when you add a component you
 +simply add the following line, preferrably in ${HOME}/​.gEDA/​gschemrc:​
 +(add-hook! add-component-hook auto-uref)
 +The function to be called from a hook (for example auto-uref above) has 
 +to accept one parameter, a list of attributes.
 +A small example that prints all attributes on a component to be placed:
 +(define (print-all-attributes attribute-list)
 +  (foreach (lambda (attribute) (display attribute)) attribute-list))
 +How to use this
 +The most complete example utilizing all of the above functions are in fact
 +the auto-uref scheme script that currently is part of the gschem distribution.
 +You can find it <where gschem is installed>/​share/​gEDA/​scheme/​auto-uref.scm.
 +Uninstalled it's available at gschem/​scheme/​auto-uref.scm
 +All components have a reference designator that must be unique so 
 +gnetlist can handle it properly. By automatically assigning a number
 +to each instance of a component when you place and copy it, you can 
 +simplify the naming operation.
 +All components has, per default, an uref attribute, for example uref=R?​. ​
 +The letter varies with component type. The auto-uref script enumerates ​
 +uref based on what prefix the component has and assigns a number. ​
 +For example, the first component you place has per default uref=U? gets 
 +the attribute uref=U1. Next component with uref=U? gets uref=U2 and so on.
 +To be able to use the auto-uref script you simply add two lines in
 +${HOME}/​.gEDA/​gschemrc. They are:
 +(load "<​where gschem is installed>/​share/​gEDA/​scheme/​auto-uref.scm"​)
 +(add-hook! add-component-hook auto-uref)
 +If you want auto enumeration to work when you copy the component too, you 
 +simply add the following line:
 +(add-hook! copy-component-hook auto-uref)
 +Good luck!
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