liufeigit/dnscript

Hook into Mac OS X distributed notifications without using Objective-C

dnscript

dnscript is a Mac OS X user agent that allows for simple integration into Mac OS X's distributed notification system - for example, running shell scripts when they are dispatched.

The specific use case this tool was developed for was refreshing GeekTool "geeklets"; I have a set of geeklets displaying the currently playing artist/track/album in iTunes. As wonderful of a program as GeekTool is, it only allows geeklets to be updated on a timer, which forces a compromise between having song info update less immediately and having constant small drain on the CPU. This program allows the distributed notifications sent by iTunes (and other music players) to be set to run shell scripts that update the geeklet display immediately.

Configuration

dnscript reads its configuration from a .dnscriptrc file in the current user's home directory. As an example, my .dnscriptrc file is as follows:

# refresh geektool on music change
;com.apple.iTunes.playerInfo
;com.catpigstudios.Radium.stateChange
;VLCPlayerStateDidChange
osascript ~/scripts/refreshall.scpt

Lines preceeded with #'s are, as you could've guessed, comments. Lines beginning with ; contain names of notifications to listen for; all other lines are read as single-line shell commands. Notifications names are associated with each other by alternating groups; so, ignoring comments and empty lines, a group of consecutive notification names will be associated with the following consecutive group of shell commands, and the next notification line found will start a new group. To make this more concrete, if you wanted to rewrite my .dnscriptrc to run a separate script for each of the music player notifications instead of of one for all three, you could rewrite it as:

;com.apple.iTunes.playerInfo
osascript ~/scripts/refreshitunes.scpt

;com.catpigstudios.Radium.stateChange
osascript ~/scripts/refreshradium.scpt

;VLCPlayerStateDidChange
osascript ~/scripts/refreshvlc.scpt

I'll be the first to admit that this format is somewhat bizaare - the semicolon was chosen to denote notification names because I could see no reason for it to need to be at the start of a shell command. The format is also inherently broken, in that it eschews support for certain pathological examples of notification names - for example, names containing newlines, though I have yet to see anything of the sort.

As a bonus, because I was interested in how it would be implemented, dnscript will listen for any changes to .dnscriptrc and process them automatically - all configuration changes will be refelcted immediately.

Installation

To install dnscript from the Xcode project, you need to run xcodebuild install from the project directory, which will generate a dnscript.pkg installer that you can run to install. Or, you can just download the installer from GitHub. dnscript only needs two files: the dnscript binary itself, which is installed to /usr/local/bin, and a configuration file com.github.stephanwilliams.dnscript, installed to /Library/LaunchAgents, which will tell launchd how to manage the dnscript process. After installation you should restart your computer.

Other Files

Since the use case I've presented here for dnscript is using it in conjunction with GeekTool, it seems appropriate to include the relevant GeekTool scripts - these are located in the scripts directory. refreshall.scpt is a dead-simple script that simply tells GeekTool to refresh all geeklets. musicinfo.scpt is more complicated, being a one-stop-shop to get the name, artist, or album of the currently playing song, from iTunes, Radium, or VLC (though, I couldn't figure out how to get an album name from VLC). The latter script can be used in a shell geeklet with the command osascript path/to/musicinfo.scpt name, for name, artist, or album.

Future Improvement

It's unlikely I'll ever get around to doing this, but dnscript would definitely benefit from being able to pass the userInfo dictionary in the NSNotification object to the scripts being executed. iTunes, for example, passes a large amount track info as part of its notifications - if this could be passed in as some sort of JSON or something, perhaps it could be parsed by some Perl/Python/Ruby/etc. script to do some interesting stuff. Though, most of the same information should also be obtainable through AppleScript, at least in iTunes' case. Also useful would be a mode where dnscript would simply print out names of all distributed notifications, which is about the only way to discover what notifications are being dispatched by applications.