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Reactor core

A reactor is a process that receives input on form:

  {reactor_signal,Src,SrcField,Field,Value}

Where Src is an other reactor or undefined if sent from any process.
SrcField is the originating field when sent from a connected channel.

This event, trigger event handlers to evaluate and set output 
fields which in turn send reactor signals on connected outputs.

A reactor process consists of fields and event handlers.
A field may be assigned any value either by assign it in the event handler
using assign(Field, Value) or set(Field, Value) or set(self(),Field,Value)
Any field in one reactor may be connected to one or several fields
in other reactors. This makes the value propagtion automatic.

Example:

    Pid = reactor:create(
      name,
      [
        {x,1},  %% x::integer()
        {y,2}   %% y::integer()
      ],

      [
        {id1, 10, x, [x,y], fun(X,Y) -> X+Y end},
	{id2, 20, @exit, [y],   fun(0) -> normal; (_) -> false }
      ]).

This reactor has two fields x and y wih the default values 1 and 2 respectivily.
The create of the reactor can the use reactor:set(Pid,x,2) to make
the reactor trigger the first script (because x is in the input list) and
evaluate a new x value to X+Y = 2+2 = 4.
An other example is to call reactor:set(Pid,y,0) this will trigger the
reactor script number 20 that will set the special field @exit to normal
and will effectivly exit the reactor with reason normal.

Standard field in a reactor core:

@id :: atom()
    Name of reactor "class"

@init :: void()
    Set only when reactor is create and can / should be used
    as constructor. A @init constructor must be given as:
       {constructor, 1, [a,b,c], ['@init'],
       		     fun(_) -> initialize() end}
    The important thing here is that the input fields are ['@init'] to
    trigger on '@init'.
    
@terminate :: void()
    A reactor destructor, trigger before reactor terminates. For example
    after @exit has been set.

@exit :: atom() 
    Stop object when set.

@auto_exit :: boolean()
    Allow reactor to be terminated  - when available.

@dump :: boolean()
    Debug print all fields.

@add_field :: atom()|{atom(),term()}
    Add a dynamic field to the reactor.

@del_field  :: atom() 
    Delete a field from a reactor (even @del_field can be deleted)

@add_handler :: handler()
    Add a dynamic handler to the reactor.

@del_handler :: identifier() 
    Delete a handler from a reactor.

@connect    :: {atom(),pid(),atom()}
    Connect a field to an other reactor field.
	
@disconnect :: {atom(),pid(),atom()}
    Disconnect a field.

@child :: void()
    used to connect @parent in children.
    The {channel,'@child'} is used to get the list of children

@parent :: void()
    store 'parent'.
    
@event :: term()
    Set when an event a message that is not on form:

    {reactor_signal,Src,SrcField,Field,Value}

    is received by the reactor.

@error_handler :: term()
    Set when an error is detect in a destination reactor, when for example
    a reactor tries to send a value to a non existing field.