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IVLE - App Authoring Guide
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==========================
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Intended audience: IVLE developers who wish to write a new application for
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IVLE's plugin interface.
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IVLE's modular architecture allows new applications ("apps") to be easily
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written and added to the site. An app is just a Python program which conforms
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to a small API, plus a few additional configurations.
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Application Specification
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-------------------------
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An application consists of two parts:
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* A Python package in the `apps` directory of IVLE. (That is, a directory with
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the name of the application containing a file `__init__.py`).
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* An entry in the applications database, stored in the file `conf/apps.py`.
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The entry in the apps database allows IVLE to locate and run the application.
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The package contains the application's code.
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Applications may be known by three distinct names:
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* The "directory name" ("`dir`") is the most common identifier used for an
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app. This is the name of the app's package directory.
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* The "URL name" is the URL path segment used to publically identify the
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application. Is is usually the same as the dir name but may be distinct.
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* The "friendly name" ("`name`") is the name shown to users, eg, in the title
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Applications Database Entry
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---------------------------
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The file `conf/apps.py` is the applications database. (No, it isn't a real
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database, just a Python file with a dictionary in it). Each application is
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defined in a variable. This is merely a convenience so they don't all have
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to be defined inside the dictionary.
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Each application should be created by calling the App constructor, with the
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* `dir` : string - The "directory name" of the app.
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* `name` : string - The "friendly name" of the app.
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* `requireauth` : bool - If True, will automatically require authentication
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(but not authorization).
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* `hashelp` : bool - If True, this app will be given a help entry.
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Each application should be given an entry in the `app_url` dict, mapping its
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"url name" to the App variable.
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Applications which require a tab in the IVLE interface should have their "url
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names" added to the `apps_in_tabs` list as well.
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The application directory must have two special files:
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* `__init__.py` is the application entrypoint, discussed below.
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* `help.html` is the application's help file. Only required if `hashelp` is
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set to True in the application database.
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The directory may contain any other files you wish, including other Python
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modules to import. Note that no files places in the application directory or
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its subdirectories will be directly visible from the web.
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If you wish to make static files such as images, JavaScript and CSS content
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available, they must be placed in `media/apps/yourapp`. Import common.util and
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use `util.make_path` to generate URLs which point to the media directory.
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`__init__.py` must contain a function `handle(req)` which takes 1 argument.
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The argument, "req", will be passed an IVLE Request object. This is very
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similar to the mod_python/Apache Request object but has an
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independently-defined interface. The Request object provides input data in its
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fields. It also provides a `write` method through which the application sends
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The `handle` function has no return value.
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The application should operate by reading input from req, setting its fields
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accordingly, then writing the output data. The HTTP response status is set by
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one of the fields of the Request. It also provides error and redirection
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functions which halt execution by throwing an exception.
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See the documentation on `dispatch.request` for the details of this object.
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Applications with `hashelp` set to True in the database are required to have
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an additional file, "help.html". This file's contents are displayed by the
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This is not an ordinary HTML file. It should be a valid XHTML file except
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should not contain html or body tags (its contents should just be the inside
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of a body tag). This is equivalent to the output of an app which has
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`write_html_head_foot` set to True. Note that this means it is not a valid XML
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file, but it will be valid once rendered by the Help app.
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The help file will be styled by IVLE's default style sheet. Please use h2 for
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headings (h1 will be used for the main page heading). Use other HTML elements
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in a natural way and they will be styled accordingly.
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* The settings of the Request object can only be set before any writing takes
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place. This is necessary to ensure the correct HTTP and HTML headers can be
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written before the first actual piece of data is written. Any settings which
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are set after the first write will be ignored.
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* Similarly, throwing errors or redirects after the first write will not have
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the intended effects, as the HTTP headers will not be written.
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* Never generate absolute URLs directly (either site-absolute or
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world-absolute). Applications should not guess where IVLE is located in the
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site's URL hierarchy. Instead use `common.util.make_path`, and supply it
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with a path relative to the IVLE site root.
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* All HTML pages generated by the app should set `req.write_html_head_foot` to
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True (which will decorate the page in the IVLE theme and interface). All
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non-HTML pages should set it to False or the output will be corrupted by
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HTML headers. An exception to this rule is an app such as "serve" which
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serves user applications which should not be decorated by the IVLE
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* Applications which wish to access the student's file system or subversion
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dynamically (using Ajax) can do so through the `fileservice` app. This will
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be described in detail in another document. See the `files` app for an
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This section shows the creation of a "Hello World" application which simply
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prints some text, inside the IVLE interface. The app's name will be "hello",
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"hello" and "Hello World" respectively.
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Firstly, create a directory, `apps/hello`. Create a file
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`apps/hello/__init__.py` with the following contents:
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req.content_type = "text/html"
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req.write_html_head_foot = True
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req.write("<p>Hello, IVLE!</p>\n")
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Now, edit the file `conf/apps.py`, and add the following lines:
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app_hello.dir = "hello"
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app_hello.name = "Hello World"
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app_hello.requireauth = False
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app_hello.hashelp = False
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Add `"hello" : app\_hello,` to the app\_url dictionary. Add `"hello"` to the
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Now restart the web server, and the "Hello World" tab should appear. Click
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the tab to call your new app.
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Note that the page output includes the standard IVLE interface and style
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theme (by virtue of setting `req.write_html_head_foot` to True). The data that
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the Hello World app itself outputs should be written assuming it is inside an
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XHTML body element. The final output will be valid XHTML 1.0 Strict if the
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application's output is.
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### Making a file dump ###
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You can modify the application to dump files from the students directories
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easily. You may wish to set `requireauth` to True, which will require that a
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user is logged in. (Note that it doesn't say anything about which user must be
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logged in - any student will still be able to read any other student's files).
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You will need to import the `studpath` module from `common` - this provides
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utilities for accessing student files.
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from common import studpath
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req.content_type = "text/html"
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req.write_html_head_foot = True
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(user, path) = studpath.url_to_local(req.path)
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req.write("Error: %s" % msg)
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`studpath.url_to_local` gives you a path on the local file system which the
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file corresponds to. (It also gives you the name of the user or group who owns
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the file, though we don't use that here).
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**Important**: This simple example does not escape characters for HTML, so it
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will not display some files correctly, and could be vulnerable to JavaScript
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injections. In a real app, characters (at the very least, '<', '>' and '&')
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should be escaped correctly.
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doc/app_howto.txt
