~azzar1/unity/add-show-desktop-key

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("<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.