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## Using Templates in OpenACS

By Pete Su

OpenACS docs are written by the named authors, and may be edited by OpenACS documentation staff.

### Overview

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.

### Entering Notes

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:

- 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@</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.

### Summary

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.

### Documentation

Templating system documentation
($Id$)

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