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Watching NSEvents

In the midst of investigating how events are passed around within Cocoa and I found some interesting information. Based on the Apple Tech Note - Mac OS X Debugging Magic, it is possible to watch all the events as they are sent.

I was skeptical, but pleasantly surprised to discover that this still works if you have a PyObjC app, including one that is sym-linked! (ie. python py2app -A)

The magical incantation looks something like:

% defaults write com.mysite.MyApp NSTraceEvents YES

This will launch the app (MyApp) from the command line, with the events being sent back to the command line. Opening the application as per normal will have the events sent to the Console.

The following will turn of the feature:

% defaults write com.mysite.MyApp NSTraceEvents NO

The results look something like (line breaks added for clarity):

2005-07-27 18:35:57.599 MyApp [2790]     In Application: NSEvent: type=KeyUp loc=(-395,428) time=152170.8 flags=0x180128 win=0 winNum=15994 ctxt=0xf45f chars='ƒ' unmodchars='f' repeat=0 keyCode=3

Note: the com.mysite.MyApp bit is actually the value of the CFBundleIdentifier in the Info.plist file. See here for details on setting this up.