+
+/* A note about Cocoa's event loops vs. run loops:
+
+ It's important to understand that Cocoa has a two-level event loop. The
+ outer level is run by NSApplication and can only ever happen on the main
+ thread. The nextEventMatchingMask:untilDate:inMode:dequeue: method returns
+ the next event which is then given to sendEvent: to send it. These
+ methods are defined in NSApplication and are thus part of AppKit.
+
+ Events (NSEvent) are only sent due to actual user actions like clicking
+ the mouse or moving the mouse or pressing a key and so on. There are no
+ paint events; there are no timer events; there are no socket events; there
+ are no idle events.
+
+ All of those types of "events" have nothing to do with the GUI at all.
+ That is why Cocoa's AppKit doesn't implement them. Instead, they are
+ implemented in Foundation's NSRunLoop which on OS X uses CFRunLoop
+ to do the actual work.
+
+ How NSApplication uses NSRunLoop is rather interesting. Basically, it
+ interacts with NSRunLoop only from within the nextEventMatchingMask
+ method. It passes its inMode: argument almost directly to NSRunLoop
+ and thus CFRunLoop. The run loop then runs (e.g. loops) until it
+ is told to exit. The run loop calls the callout functions directly.
+ From within those callout functions the run loop is considered to
+ be running. Presumably, the AppKit installs a run loop source to
+ receive messages from the window server over the mach port (like a
+ socket). For some messages (e.g. need to paint) the AppKit will
+ call application code like drawRect: without exiting the run loop.
+ For other messages (ones that can be encapsulated in an NSEvent)
+ the AppKit tells the run loop to exit which returns control to
+ the nextEventMatchingMask method which then returns the NSEvent
+ object. It's important to note that once the runloop has exited
+ it is no longer considered running and thus if you ask it which
+ mode it is running in it will return nil.
+
+ When manually pumping the event loop care should be taken to
+ tell it to run in the correct mode. For instance, if you are
+ using it to run a modal dialog then you want to run it in
+ the modal panel run loop mode. AppKit presumably has sources
+ or timers or observers that specifically don't listen on this
+ mode. Another interesting mode is the connection reply mode.
+ This allows Cocoa to wait for a response from a distributed
+ objects message without firing off user code that may result
+ in a DO call being made thus recursing. So basically, the
+ mode is a way for Cocoa to attempt to avoid run loop recursion
+ but to allow it under certain circumstances.
+ */
+