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("

Hello, IVLE!

\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("

") try: req.sendfile(path) except IOError, msg: req.write("Error: %s" % msg) req.write("

") `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.