CSE116的学习笔记-Lec5-3:Actors

Actors

actors again!

Stock Trader Example

• Simulate stock changing prices and a trader purchasing stocks

• Stock
  • Receives a Tick message and changes its price
    • Price changes are random for this simulation
    • Tick messages are sent to all stocks at regular intervals
  • Receives a GetPrice message and responds with its current price

• Trader
  • Knows each stock’s ticker symbol and actor reference
  • Receives a CheckStocks message and checks the price of all known stocks
  • Receives Price messages from stocks and decides [randomly] whether or not buy some shares

To the Code

Traffic Example

• Intersections
  • North / South road intersects with an East / West road
  • Initially Green light East / West
  • Alternate green light at a fixed interval

• Cars
  • Have a List of directions to follow
  • Each direction has:
    • A destination Intersection
    • The time it takes to each that Intersection
    • Whether the intersection will be approached from the East / West or North / South

• When a car approaches an intersection:
  • Inform the Intersection
  • Intersection gives the GreenLight for the car to go
    • If the light is red, the Intersection will wait until it’s green before sending the GreenLight message

• When a car receives a GreenLight message
  • Proceed to next Intersection in the direction list

Delayed Messages

• There are two cases where we’ll have message that need to be sent with a delay
  • Intersections changing lights at fixed intervals
  • Cars waiting the appropriate amount of time before checking an intersection for a green light
• Similar syntax as repeatedly sending a message (Supervisor example)
• Use scheduleOnce

  import context.dispatcher
  // ...
  context.system.scheduler.scheduleOnce(
      timeInterval.milliseconds, 
      self,
      ChangeLight
  )

• Must import the dispatcher from inside the Actor’s class
• Speccify:
  • The time delay
  • The recipient
    • self contains an ActorRef to this actor
  • The message to be sent

Actors Creating Actors

• this.context can be used to add new actors to the system
• Use the same “actorOf” syntax as used when adding actors using the system directly

  this.context.actorOf(Props(classOf[TimerActor]))

To the Code

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Lecture Question

Task:
Create and test an actor class to set and sound an alarm

• In a package named actors, create a class named AlarmActor that extends Actor
• Create the following case class in the actors package that will be used as messages
  • A case class named SetAlarm that takes a Double and a String in its constructor
  • A case class named Alarm that takes a String in its constructor
• The AlarmActor class must respond to messages of type SetAlarm by:
  • Interpreting the Double as the number of seconds to wait before sounding the alarm
  • When the alarm “sounds” send an Alarm message back to the actor that sent the SetAlarm message containing the same String that was in the SetAlarm message

Testing:
Write a test suite named tests.TestAlarmActor to test this functionality

No test after this LQ!!!!