class Pigeon::Task
  # == Constants ============================================================
  
  # == Properties ===========================================================
  
  attr_reader :state
  attr_accessor :context
  attr_reader :engine
  attr_reader :exception
  attr_reader :created_at
  attr_reader :started_at

  # == Class Methods ========================================================

  # Defines the initial state of this type of task. Default is :initialized
  # but this can be customized in a subclass.
  def self.initial_state
    :initialized
  end
  
  # Returns an array of the terminal states for this task. Default is
  # :failed, :finished but this can be customized in a subclass.
  def self.terminal_states
    @terminal_states ||= [ :failed, :finished ].freeze
  end

  # == Instance Methods =====================================================
  
  # Creates a new instance of a Task with a series of context. An optional
  # engine parameter indicates which engine this task should be associated
  # with. An arbitrary context object can be specified which will persist in
  # the context property.
  def initialize(context = nil, engine = nil)
    @context = context
    @engine = engine || Pigeon::Engine.default_engine
    @created_at = Time.now
    
    after_initialized
  end
  
  # Kicks off the task. An optional callback is executed just before each
  # state is excuted and is passed the state name as a symbol.
  def run!(initial_state = nil, &callback)
    @callback = callback if (block_given?)
    
    @state = initial_state || self.class.initial_state
    @started_at = Time.now

    run_state!(@state)
  end

  # Returns true if the task is in the finished state, false otherwise.
  def finished?
    @state == :finished
  end

  # Returns true if the task is in the failed state, false otherwise.
  def failed?
    @state == :failed
  end
  
  # Returns true if an exception was thrown, false otherwise.
  def exception?
    !!@exception
  end
  
  # Returns true if the task is in any terminal state.
  def terminal_state?
    self.class.terminal_states.include?(@state)
  end
  
  # Dispatches a block to be run as soon as possible.
  def dispatch(&block)
    @engine.dispatch(&block)
  end
  
  # Returns a numerical priority order. If redefined in a subclass,
  # should return a comparable value.
  def priority
    @created_at
  end
  
  def inspect
    "<#{self.class}\##{self.object_id}>"
  end
  
  def <=>(task)
    self.priority <=> task.priority
  end
  
protected
  def run_state!(state)
    # Grab the current state and save it here, as it may switch at any time
    @state = state
    terminate = self.class.terminal_states.include?(state)

    before_state(state)

    send_callback(state) if (@callback)
    
    unless (terminate)
      state_method = :"state_#{state}!"

      # Only perform this state action if it is defined, otherwise ignore
      # as some states may be deliberately NOOP in order to wait for some
      # action to be completed asynchronously.
      if (respond_to?(state_method))
        send(state_method)
      end
    end
    
  rescue Object => e
    @exception = e

    handle_exception(e) rescue nil
    
    transition_to_state(:failed) unless (self.failed?)
    
    self.after_failed
    self.after_terminated
  ensure
    after_state(state)

    if (terminate)
      self.after_finished
      
      # Send a final notification callback
      if (@callback and @callback.arity == 0)
        @callback.call
      end
      
      self.after_terminated
    end
  end

  # Schedules the next state to be executed. This method should only be
  # called once per state or it may result in duplicated state actions.
  def transition_to_state(state)
    @engine.dispatch do
      run_state!(state)
    end
    
    state
  end

  def send_callback(state)
    # State-notificaton callbacks are not made to blocks that do not take
    # arguments, but instead a singe final callback is made.
    case (@callback.arity)
    when 2
      @callback.call(self, state)
    when 1
      @callback.call(state)
    end
  end
  
  # Called just after the task is initialized.
  def after_initialized
  end
  
  # Called before a particular state is executed.
  def before_state(state)
  end

  # Called after a particular state is executed.
  def after_state(state)
  end

  # Called just after the task is finished.
  def after_finished
  end

  # Called just after the task fails.
  def after_failed
  end
  
  # Called after the task finishes or terminates.
  def after_terminated
  end

  # Called when an exception is thrown during processing with the exception
  # passed as the first argument. Default behavior is to do nothing but
  # this can be customized in a subclass. Any exceptions thrown by this
  # method are ignored.
  def handle_exception(exception)
  end
  
  # This defines the behaivor of the intialized state. By default this
  # simply transitions to the finished state.
  def state_initialized!
    transition_to_state(:finished)
  end
end