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Objective-C multithreaded actors & futures
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	ActorKit is a framework for multithreaded actors with transparent futures in Objective-C.

	Sending an "asActor" message to any object returns an actor proxy for the object.
	Sending messages to the actor will queue them to be processed in first-in-first-out order 
	by the actor's thread and immediately returns a "future" object.
	If its message queue exceeds a given limit (set with setActorQueueLimit:), 
	the calling threads that exceeded the limit will be paused until the more
        of the queue is processed.


	A future is a transparent proxy for the result which, when accessed before the 
	result is ready, pauses any threads attempting to access it until it is ready. 	
	Futures auto detect and raise an exception in deadlock situations.

	// the fetch return a future immediately

	NSData *aFuture = [(NSURL *)[[@"" asURL] asActor] fetch];

	// ... do stuff ...
	// now when we try to access the values, 
        // the thread waits if the values aren't ready

	NSLog(@"request returned %i bytes", (int)[aFuture length]); 


	Calling asThreadSafe on any object returns a proxy that ensures only one thread
	can access it at a time. Example:

	NSMutableDictionary *dict = [[NSMutableDictionary dictionary] asThreadSafe];


	Calling asBatch on an NSArray returns a BatchProxy which can be used to do
	a parallel "map" using GCD (BSD workerqueues). Example:

	NSArray *results = [[urls asBatch] fetch];
	Sends concurrent fetch messages to each element of the urls array and returns
	an array containing the results.


	Exceptions that occur while an actor processes a message will be
	passed to the future and raised in all the threads that attempt 
        to access the future.
	It's ok for multiple threads to look at the same future. 
	ActorKit does no busy waits.
	Objects store their proxies as an associated objects so the same 
	proxy is returned for a given instance.

To Do

	- handle BatchProxy exceptions
	- make sure all locks deal with exceptions
	- future notifications of some kind

	- more tests
	- auto deadlock detection for actor queue limit and batches
	- add a total queue and/or total actor limits
	- better respondsToSelector implementation
	- (maybe) chainable persistent batch groups with in, out, and error queues

	- (maybe) integrate with distibuted objects to allow bundles and data to
	  be distributed...

	- (maybe) explore synchronization via ownership 


	Thanks to Mark Papadakis for tips on mutex conditions.
	IIRC I saw the BatchProxy pattern first used in Io by quag.
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