Execute external application from within Cocoa application

Question

First-off, let me make clear that what I need it not simple (and obviously what I want is not just to execute another application).

What I need :

• Extend my Main application with the functionality of external applications (sort of "Plugins")
• These external plugins have to be separate from the Main application (in terms of code/implementation)
• External plugins have to be able to be seamlessly integrated into the Main application
• The external application should not be seen as something "external" by the user
• The implementation must adhere to App Store terms and SandBoxing guidelines.

Example (though the idea is not 100% clear in my head) :

• Host application launches
• External app has a window (and its own functionality)
• Once the external app is executed, its window now becomes a "sheet" of the Host application

I don't know if what I am describing is even doable. However, I'd still need to hear your input on this. I've done a lot of research on the matter and though I'm about to base the whole concept of making the host app extendable by external scripts (which get input and serve some output - via php,python,AppleScript,whatever...), this does not cover the case where the "plugin" has its own user interface.

So, any ideas? (An example or some specific reference to point me to would be ideal...)

Answer 1

There's a new API that addresses exactly what you want:

The XPC Services API, part of libSystem, provides a lightweight mechanism for basic interprocess communication integrated with Grand Central Dispatch (GCD) and launchd. The XPC Services API allows you to create lightweight helper tools, called XPC services, that perform work on behalf of your application.

Daemons and Services Programming Guide

There's a good talk on this in the WWDC 2012 videos.

So much of what you describe is indeed possible -- and not necessarily very difficult. However, your "Example" is not -- as you say yourself -- entirely coherent. You want the second application to be separate, but you don't want it to seem "external"; these forces are not fully reconcilable. What you are unlikely to be able to do, for obvious reasons, is to take over an application that's already running on its own. But you can start your own processes.

The usual architecture would be to have several daemons or services doing tasks for you, and one control process that also handles the user interface for everything. Mail, as noted above, works this way.