Plug-in Architectures
Presented by Truc Nguyen
What’s a plug-in?
“a type of program that tightly integrates with
a larger application to add a special capability
to it”
- CNET
“a set of software components that adds
specific abilities to a larger software
application”
- Wikipedia
What’s a plug-in?
• Component architectures
• Semantics
– Plug-ins
– Extensions
– Add-ons
Plug-ins
• Not hard-coded into host app
– Unknown at compile time
• Not a standalone application
• One step further: compose an entire
application out of plugins
Why do have plug-ins?
•
•
•
•
Extensible
Modular
Customizable
Keep It Small and Simple!
Traditional vs Pure
Traditional Plug-in
• Extend capabilities of base application
• Not compiled into the application
Pure Plug-in
• Everything is a plug-in
• Plug-ins extend other plug-ins
• Where does that leave the base
application?
– A run-time engine for plug-ins
– Plug-ins become hosts to other plug-ins by
providing extension points
• How do plug-ins play nicely with each
other?
– Abide by the framework’s or each other’s
Pure Plug-in Engine
• Keeping track of where the right plug-in
code is and running it
• Maintain a registry of plug-ins and
functions that go with each
• Manage extension model and
dependencies between plug-ins
Plug-in Design Patterns
• Best practice solutions to solve a design
problem
• How to support dynamic loading of
modules unknown during compilation
Plug-in Pattern
Eclipse
• Well known as a Java IDE
• Pure plug-in architecture
– Runtime engine itself is a bunch of core plugins except for tiny bootstrap code
• Universal plug-in architecture for creating
“anything, but nothing in particular”
• Each plug-in has a manifest
Eclipse
Gecko (NPAPI)
• Netscape Plugin Application Programming
Interface
• Plug-in is completely controlled by web
page that calls it
• On start, Gecko looks for plug-in modules
in a specified locations on the system
• On a page with certain embedded media,
checks for plug-in with matching MIME
type creates new instance and runs it
Photoshop
• Central Plug-in loader
– Scans PiPL (Plug-in Property List)
• Fixed types
– filter, import, export, file format, automation
– selection and parser (not implemented by 3rd
party)
• Non-Photoshop applications can support
Photoshop plug-ins
Chrome
• Scripting API vs Browser API
• Extensions can modify the DOM
• Can have access to the history, current
tabs, bookmarks, etc
• Composed of files, can be packaged as
.crx
– manifest.json
– HTML files
– JavaScript
– Other resources (e.g CSS, images)
Chrome
• Background pages
– Persistent or event
• Content scripts are JS that executes in the
context of the page just loaded
– Can change the DOM on the current page
– Can’t change the DOM on the background
page of parent extension
• Callbacks execute on a user action
Challenges
• Installing and updating
– Configuration plugin
• Security
• Concurrent plug-in version
– Eclipse convention: Only plug-ins that
contribute code libraries can coexist
• Scalability
Questions?