This layer gives direct wrapped access to the fltk API.

It maintains a map from ViewIds to window pointers, which represents the on screen state. All functions here take ViewIds, and will throw an exception if the view_id isn't in the pointer map. C++ exceptions coming from FLTK should be converted and thrown as haskell exceptions.

TODO exceptions are not implemented yet

There are three methods for sharing data with C++:

• Note tracks pass a callback, that will return events in the given ScoreTime range. This means the complete map of events exists only on the Haskell side, and changing events means passing a new callback to C++. This is because events change a lot, and it seems wasteful to copy over a whole track of events just because one was added.
• Rulers have marklists, which are like events but much denser. They're entirely marshalled to a C array so C++ has direct access to the marks. Because many tracks will share the same ruler, marklists use a reference counting scheme so Haskell can allocate the array once, and then C++ and Haskell can cooperate on the memory management.
• Control signals are just copied, and ownership is given to C++. This is because control signals are large but are already dense arrays so they can be copied very quickly with memcpy. They could use a refcounting scheme like ruler marklists, but they're rarely shared so it's probably not worth it.