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_verify_and_get_user_id]

db_1row user_name {
    select first_names || ' ' || last_name as user_name 
    from users
    where forum_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:

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@</property>
<property name="context_bar">@context_bar@</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 - create the master.adp file, which looks like this:

<%= [ad_header $title] %> 
<h2>@title@</h2> 
<%= [eval ad_context_bar $context_bar] %> 
<hr> 
<slave> 
<br clear="all"> 
<%= [ad_footer] %>

The main subtlety in this code is the inline Tcl code for running procs to build the header, footer, context bar, etc. Also, note the property substitutions that happen here, the values of which are set up in the <property> tags in the slave page.

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.