IVLE - App Authoring Guide
==========================

    Author: Matt Giuca
    Date: 17/12/2007

Intended audience: IVLE developers who wish to write a new application for
IVLE's plugin interface.

IVLE's modular architecture allows new applications ("apps") to be easily
written and added to the site. An app is just a Python program which conforms
to a small API, plus a few additional configurations.

Application Specification
-------------------------

An application consists of two parts:

* A Python package in the `apps` directory of IVLE. (That is, a directory with
  the name of the application containing a file `__init__.py`).
* An entry in the applications database, stored in the file `conf/apps.py`.

The entry in the apps database allows IVLE to locate and run the application.
The package contains the application's code.

### App name ###

Applications may be known by three distinct names:

* The "directory name" ("`dir`") is the most common identifier used for an
  app. This is the name of the app's package directory.
* The "URL name" is the URL path segment used to publically identify the
  application. Is is usually the same as the dir name but may be distinct.
* The "friendly name" ("`name`") is the name shown to users, eg, in the title
  bar and in the tabs.

Applications Database Entry
---------------------------

The file `conf/apps.py` is the applications database. (No, it isn't a real
database, just a Python file with a dictionary in it). Each application is
defined in a variable. This is merely a convenience so they don't all have
to be defined inside the dictionary.

Each application should be created by calling the App constructor, with the
following fields:

* `dir` : string - The "directory name" of the app.
* `name` : string - The "friendly name" of the app.
* `requireauth` : bool - If True, will automatically require authentication
      (but not authorization).
* `hashelp` : bool - If True, this app will be given a help entry.

Each application should be given an entry in the `app_url` dict, mapping its
"url name" to the App variable.

Applications which require a tab in the IVLE interface should have their "url
names" added to the `apps_in_tabs` list as well.

Application Interface
---------------------

Example Application
-------------------

This section shows the creation of a "Hello World" application which simply
prints some text, inside the IVLE interface. The app's name will be "hello",
"hello" and "Hello World" respectively.

Firstly, create a directory, `apps/hello`. Create a file
`apps/hello/__init__.py` with the following contents:

    def handle(req):
        req.content_type = "text/html"
        req.write_html_head_foot = True
        req.write("<p>Hello, IVLE!</p>\n")

Now, edit the file `conf/apps.py`, and add the following lines:

    app_hello = App()
    app_hello.dir = "hello"
    app_hello.name = "Hello World"
    app_hello.requireauth = False
    app_hello.hashelp = False

Add `"hello" : app_hello,` to the app_url dictionary. Add `"hello"` to the
apps_in_tabs list.

Now restart the web server, and the "Hello World" tab should appear. Click
the tab to call your new app.

Note that the page output includes the standard IVLE interface and style
theme (by virtue of setting `req.write_html_head_foot` to True). The data that
the Hello World app itself outputs should be written assuming it is inside an
XHTML body element. The final output will be valid XHTML 1.0 Strict if the
application's output is.

### Making a file dump ###

You can modify the application to dump files from the students directories
easily. You may wish to set `requireauth` to True, which will require that a
user is logged in. (Note that it doesn't say anything about which user must be
logged in - any student will still be able to read any other student's files).

You will need to import the `studpath` module from `common` - this provides
utilities for accessing student files.

    from common import studpath

    def handle(req):
        req.content_type = "text/html"
        req.write_html_head_foot = True

        (user, path) = studpath.url_to_local(req.path)
        req.write("<p>")
        try:
            req.sendfile(path)
        except IOError, msg:
            req.write("Error: %s" % msg)
        req.write("</p>")

`studpath.url_to_local` gives you a path on the local file system which the
file corresponds to. (It also gives you the name of the user or group who owns
the file, though we don't use that here).

 **Important**: This simple example does not escape characters for HTML, so it
will not display some files correctly, and could be vulnerable to JavaScript
injections. In a real app, characters (at the very least, '<', '>' and '&')
should be escaped correctly.