What is the trick behind "launch application holding [modifier key]"?

iTunes and Reeder (and I'm sure lots of other) applications has an ability to modify startup behavior whenever they are launched while holding ⌥ (option) key. I tried looking at NSApplicationDelegate methods, but none seem to add any sort of hint to what I'm looki开发者_StackOverflow中文版ng for. How is this functionality achieved?

In your application delegate's applicationDidFinishLaunching: method

NSUInteger flags = ([NSEvent modifierFlags] & NSDeviceIndependentModifierFlagsMask);

BOOL isOptionPressed = (flags == NSAlternateKeyMask);

(Edited to match awesome answer Declaring and checking/comparing (bitmask-)enums in Objective-C Previously this was BOOL isOptionPressed = (0 != (flags & NSAlternateKeyMask)); But the zero checking is not necessary to check if a bit mask for equality, unless the bit mask itself represents all zeros in binary. The provided link gives greater detail. )

Francis McGrew's answer needs improvement before it can work, but I still see no solution via that route. The answer as stated does not compile. An actual NSEvent pointer is needed for modifierFlags, which is not a class method as the answer would suggest.

One would hope that the needed event might be obtained by the following.

NSEvent *event = [NSApp currentEvent];

However when called from applicationDidFinishLaunching the resulting event is nil. At least this is true in my testing on Snow Leopard.

applicationDidFinishLaunching has an NSNotification argument but I don't know how to make any use of it.

I found a similar question on cocobuilder.com from 2007 and the answer there is basically to call the carbon function GetCurrentKeyModifiers and convert the carbon flags to cocoa form.

See http://www.cocoabuilder.com/archive/cocoa/176882-detecting-modifier-keys-at-launch.html