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
---------------------

The application directory must have two special files:

* `__init__.py` is the application entrypoint, discussed below.
* `help.html` is the application's help file. Only required if `hashelp` is
    set to True in the application database.

The directory may contain any other files you wish, including other Python
modules to import. Note that no files places in the application directory or
its subdirectories will be directly visible from the web.

If you wish to make static files such as images, JavaScript and CSS content
available, they must be placed in `media/apps/yourapp`. Import common.util and
use `util.make_path` to generate URLs which point to the media directory.

`__init__.py` must contain a function `handle(req)` which takes 1 argument.
The argument, "req", will be passed an IVLE Request object. This is very
similar to the mod_python/Apache Request object but has an
independently-defined interface. The Request object provides input data in its
fields. It also provides a `write` method through which the application sends
its output.

The `handle` function has no return value.

The application should operate by reading input from req, setting its fields
accordingly, then writing the output data. The HTTP response status is set by
one of the fields of the Request. It also provides error and redirection
functions which halt execution by throwing an exception.

See the documentation on `dispatch.request` for the details of this object.

### Help file ###

Applications with `hashelp` set to True in the database are required to have
an additional file, "help.html". This file's contents are displayed by the
Help app.

This is not an ordinary HTML file. It should be a valid XHTML file except
should not contain html or body tags (its contents should just be the inside
of a body tag). This is equivalent to the output of an app which has
`write_html_head_foot` set to True. Note that this means it is not a valid XML
file, but it will be valid once rendered by the Help app.

The help file will be styled by IVLE's default style sheet. Please use h2 for
headings (h1 will be used for the main page heading). Use other HTML elements
in a natural way and they will be styled accordingly.

Important notes
---------------

* The settings of the Request object can only be set before any writing takes
  place. This is necessary to ensure the correct HTTP and HTML headers can be
  written before the first actual piece of data is written. Any settings which
  are set after the first write will be ignored.
* Similarly, throwing errors or redirects after the first write will not have
  the intended effects, as the HTTP headers will not be written.
* Never generate absolute URLs directly (either site-absolute or
  world-absolute). Applications should not guess where IVLE is located in the
  site's URL hierarchy. Instead use `common.util.make_path`, and supply it
  with a path relative to the IVLE site root.
* All HTML pages generated by the app should set `req.write_html_head_foot` to
  True (which will decorate the page in the IVLE theme and interface). All
  non-HTML pages should set it to False or the output will be corrupted by
  HTML headers. An exception to this rule is an app such as "serve" which
  serves user applications which should not be decorated by the IVLE
  interface.
* Applications which wish to access the student's file system or subversion
  dynamically (using Ajax) can do so through the `fileservice` app. This will
  be described in detail in another document. See the `files` app for an
  example.

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("<div id="ivle_padding">\n")
        req.write("  <p>Hello, IVLE!</p>\n")
        req.write("</div>\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.

Note that by default, there is no padding around the application's HTML
output. This is because more complex apps (most of ours) don't work properly
with padding. Simpler apps (which simply write HTML paragraph content) look
better with padding.  Apps can wrap all of their output in a
'<div id="ivle_padding">' element (as shown above) to get padding.

### 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.