This is the first step towards more type-safety in Actor interactions,
comprising:
* generic ActorRef[T] that only accepts T messages
* generic ActorSystem[T] extends ActorRef[T] (sending to the guardian,
whose Props[T] are provided for ActorSystem construction)
* removed the Actor trait: everything in there has been made into
messages and signals
* new Behavior[T] abstraction that consumes messages (of type T) or
Signals (lifecycle hooks, Terminated, ReceiveTimeout, Failed),
producing the next Behavior[T] as the result each time
* the ask pattern is provided and yields properly typed Futures
* variants of ActorContext are provided for synchronous testing of
Behaviors
All of this is implemented without touching code outside akka-typed
(apart from making guardianProps configurable), creating wrapper objects
around ActorRef, ActorContext, ActorSystem, Props and providing an Actor
implementation that just runs a Behavior.
* Dispatchers need Scheduler to be able to shutdown themselves. Stop Scheduler after dispatchers.
* Changed CallingThreadDispatcher global object to Extension, since it holds map of references to mailboxes. Will be GC:ed when system is GC:ed.
* Made testActor lazy, since it is not used in all tests, and it creates CallingThreadDispatcher.
* Activated some java tests that were not running
* Many tests were not stopping created ActorSystems. VERY IMPORTANT TO STOP ActorSystem in tests. Use AkkaSpec as much as possible.
* Used profiler to verify (and find) dangling ActorSystemImpl and threads from dispatchers.
* FutureSpec creates ForkJoinPool threads that are not cleared, but number of threads don't grow so it's not a problem.
