The OpenACS Template System (ATS) is designed to allow developers to cleanly separate application logic from display logic. The intent is to have all of the logic related to manipulating the database and other application state data in one place, and all the logic related to displaying the state of the application in another place. This gives developer's quicker customization and easier upgrades, and also allows developers and graphic designers to work more independently.
In ATS, you write two files for every user-visible page in the
system. One is a plain .tcl
file and the other is a special .adp file. The .tcl file runs a script that sets up a set
of name/value bindings that we call data
sources. These data
sources are generally the results of Tcl and/or database
queries or some combination thereof. The template system
automatically makes them available to the .adp file, or the display part of the
template, which is written in a combination of HTML, special
template related tags, and data source substitutions.
In the overall context of our example OpenACS Notes application, this document will show you how to set up a simple templated page that displays a form to the user for entering new notes into the system. In later sections of the DG, we'll discuss how to develop the pages that actually add notes to the database, how to provide a separate instance of the Notes application to every user and how to design appropriate access control policies for the system.
In order for the Notes application to be useful, we have to allow users to enter data into the database. Typically, this takes two pages: one that displays a form for data entry, and another page that runs the code to update the database and tells the user whether the operation failed. In this document, we will use the template system to build the first of these pages. This isn't a very interesting use of the system since we won't be displaying much data, but we'll cover more on that end later.
The .tcl file for the form
entry template is pretty simple. Here, the only thing we need from
the database is a new ID for the note object to be inserted. Open
up a file called note-add.tcl
in the ROOT/packages/notes/www
directory, and put the following code in it:
ad_page_contract {
Form to add a note in OpenACS Notes.
\@author Jane Coder
\@creation-date 11 Oct 2000
} -properties {
note_id:onevalue
submit_label:onevalue
target:onevalue
page_title:onevalue
} -query {
}
set user_id [ad_conn user_id]
db_1row user_name {
select first_names || ' ' || last_name as user_name
from users
where user_id = :user_id
}
set page_title "Add a note for $user_name"
set submit_label "Add"
set target "note-add-2"
set note_id [db_nextval acs_object_id_seq]
ad_return_template "note-add"
Some things to note about this code:
The procedure ad_page_contract is always the first thing a
.tcl file calls, if it's
under the www/ directory (i.e. not a Tcl library file). It does
validation of input values from the HTTP request (i.e. form
variables) and in this case, the -properties clause is used to set up the
data sources that we will ship over to the .adp part of the page. In this case, we
only use the simplest possible kind of data source, called a
onevalue, which hold just a
single string value. Later on, we'll see how to use more
powerful kinds of data sources for representing multiple rows from
an SQL query. You also include overall documentation for the page
in the contract, and OpenACS has automatic tools that extract this
documentation and make it browsable.
After being declared in the ad_page_contract, each property is just a
simple Tcl variable. The template system passes the final value of
the variable to the .adp
template when the .tcl file is
processed.
The call ad_return_template
tells the template system what .adp template page to fetch to display the
properties that have been processed. By default, the template
system will look for a file by the same name as the .tcl file that just ran, but with an
.adp extension.
Next we write the corresponding .adp page. This page outputs HTML for the
form, and also contains placeholders whose values are substituted
in from the properties set up by the .tcl file. Create a file called
note-add.adp in your editor,
and insert this text:
<master src="master"> <property name="title">\@page_title;literal\@</property> <property name="context_bar">\@context_bar;literal\@</property> <form action="\@target\@"> <p>Title: <input type="text" name="title" value=""> </p> <p>Body: <input type="text" name="title" value=""> </p> <p> <center> <input type="submit" value="\@submit_label\@"> </center> </p> </form>
The main point to note here is: when you want to substitute a value into a page, you put the name of the data source between two "\@" characters. Another point to note is the use of a master template: Master templates allow you do centralize display code that is used throughout an application in a single file. In this case, we intend to have a master template that does the standard page headers and footers for us
After putting all these files into ROOT/packages/notes/www, you should be able
to go to /notes/ URL for your
server and see the input form.
Templates separate application logic from display logic by
requiring the developer to write pages in two stages, one file for
database queries and application logic, and another for display. In
OpenACS, the logic part of the page is just a .tcl that sets up data sources that are used by the
display part of the page. The display part of the page is an
.adp file with some special
tags and notations for dealing with display logic and inserting
properties into the text of the page. Later on we'll get into
templates more deeply, and show how to use database queries as data
sources.
Templating system documentation