NSResponder – NSResponder inherits from NSObject. Responders have the ability to handle NSEvent
See: http://developer.apple.com/documentation/Cocoa/Reference/ApplicationKit/Classes/NSResponder_Class/Reference/Reference.html
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One of the methods declared by NSResponder is _cancelKey:. interpretKeyEvents: converts ESC to a _cancelKey: action in the same way it converts the up-arrow to the moveUp: action.
Panther introduces cancelOperation:, a public responder method that does the same thing.
If you want to customize the handling of selectors, perhaps to pass them on to a delegate, it is not enough to override tryToPerform:with: and doCommand:. You must also override respondsToSelector: and performSelector:withObject:. NSApplication’s sendAction:to:from: uses the latter two to follow the responder chain, rather than the former two. –DustinVoss