Question

当我使用Java（或类似语言）进行编程时，我经常使用简单版本的策略模式，使用接口和实现类，在我的代码中提供特定概念的运行时可选实现。

作为一个非常人为的例子，我可能希望拥有一个可以在我的Java代码中产生噪音的动物的一般概念，并希望能够在运行时选择动物的类型。所以我会按照这些方式编写代码：

interface Animal {
    void makeNoise();
}

class Cat extends Animal {
    void makeNoise() { System.out.println("Meow"); }
}

class Dog extends Animal {
    void makeNoise() { System.out.println("Woof"); }
}

class AnimalContainer {
    Animal myAnimal;

    AnimalContainer(String whichOne) {
        if (whichOne.equals("Cat"))
            myAnimal = new Cat();
        else
            myAnimal = new Dog();
    }

    void doAnimalStuff() {
        ...
        // Time for the animal to make a noise
        myAnimal.makeNoise();
        ...
    }

足够简单。不过，最近，我一直在Scala开展一个项目，我想做同样的事情。使用特征这样做似乎很容易，例如：

trait Animal {
    def makeNoise:Unit
}

class Cat extends Animal {
    override def makeNoise:Unit = println("Meow")
}

class AnimalContainer {
    val myAnimal:Animal = new Cat
    ...
}

然而，这看起来非常类似Java而且不是很实用 - 更不用说特征和接口不是真的同样的东西。所以我想知道在我的Scala代码中是否有更惯用的方法来实现策略模式 - 或类似的东西 - 以便我可以在运行时选择抽象概念的具体实现。或者使用特征是实现这一目标的最佳方式？

Answer 1

它可能就像“Design pattern in scala”中的那个例子：

就像任何函数是一流对象或闭包可用的语言一样，策略模式是显而易见的例如。考虑“征税”的例子：

trait TaxPayer
case class Employee(sal: Long) extends TaxPayer
case class NonProfitOrg(funds: BigInt) extends TaxPayer

//Consider a generic tax calculation function. (It can be in TaxPayer also).
def calculateTax[T <: TaxPayer](victim: T, taxingStrategy: (T => long)) = {
  taxingStrategy(victim)
}

val employee = new Employee(1000)
//A strategy to calculate tax for employees
def empStrategy(e: Employee) = Math.ceil(e.sal * .3) toLong
calculateTax(employee, empStrategy)

val npo = new NonProfitOrg(100000000)
//The tax calculation strategy for npo is trivial, so we can inline it
calculateTax(nonProfit, ((t: TaxPayer) => 0)

这样我就可以在运行时选择抽象概念的具体实现。

您正在使用 upper bound ，以便将子类中T的特化限制为TaxPayer的子类型。

Answer 2

您可以对蛋糕模式进行修改。

trait Animal {
    def makenoise: Unit
}

trait Cat extends Animal {
    override def makeNoise { println("Meow") }
}

trait Dog extends Animal {
    override def makeNoise { println("Woof") }
}

class AnimalContaineer {
    self: Animal =>

    def doAnimalStuff {
         // ...
         makeNoise
         // ...
     }
}

object StrategyExample extends Application {
    val ex1 = new AnimalContainer with Dog
    val ex2 = new AnimalContainer with Cat

    ex1.doAnimalStuff
    ex2.doAnimalStuff
}

就策略模式而言，策略中的自我类型表明它必须与某种算法的特定实现混合。

Answer 3

来自Java，我仍然喜欢OO样式语法。我也只是观看了Deriving Scalaz的第一部分（免责声明），并将其作为一个小练习向我自己展示了Pimp My Library和Implicits的概念。我想我也可以分享我的发现。一般来说，以这种方式进行设置会产生更多的编程开销，但我个人认为用法更清晰。

第一个片段演示了如何添加Pimp My Library模式。

trait TaxPayer

/**
 * This is part of the Pimp My Library pattern which converts any subclass of
 * TaxPayer to type TaxPayerPimp
 */
object TaxPayer {
  implicit def toTaxPayerPimp[T <: TaxPayer](t: T) : TaxPayerPimp[T] =
    new TaxPayerPimp[T] {
      val taxPayer = t
    }
}

/**
 * This is an extra trait defining tax calculation which will be overloaded by
 * individual TaxCalculator strategies.
 */
trait TaxCalculator[T <: TaxPayer] {
  def calculate(t: T) : Long
}

/**
 * This is the other part of the Pimp My Library pattern and is analogus to
 * Scalaz's Identity trait.
 */
trait TaxPayerPimp[T <: TaxPayer] {
  val taxPayer: T
  def calculateTax(tc: TaxCalculator[T]) : Long = tc.calculate(taxPayer)
}


case class Employee(sal: Long) extends TaxPayer

/**
 *  This is the employee companion object which defines the TaxCalculator
 *  strategies.
 */
object Employee {
  object DefaultTaxCalculator extends TaxCalculator[Employee] {
    def calculate(e: Employee) = Math.ceil(e.sal * .3) toLong
  }

  object BelgianTaxCalculator extends TaxCalculator[Employee] {
    def calculate(e: Employee) = Math.ceil(e.sal * .5) toLong
  }
}

case class NonProfitOrg(funds: BigInt) extends TaxPayer

/**
 * This is the NonProfitOrg companion which defines it's own TaxCalculator
 * strategies.
 */
object NonProfitOrg {
  object DefaultTaxCalculator extends TaxCalculator[NonProfitOrg] {
    def calculate(n: NonProfitOrg) = 0
  }
}



object TaxPayerMain extends Application {

  //The result is a more OO style version of VonC's example
  val employee = new Employee(1000)
  employee.calculateTax(Employee.DefaultTaxCalculator)
  employee.calculateTax(Employee.BelgianTaxCalculator)

  val npo = new NonProfitOrg(100000000)
  npo.calculateTax(NonProfitOrg.DefaultTaxCalculator)

  //Note the type saftey, this will not compile
  npo.calculateTax(Employee.DefaultTaxCalculator)

}

我们可以使用implicits进一步了解这一点。

trait TaxPayer
object TaxPayer {
  implicit def toTaxPayerPimp[T <: TaxPayer](t: T) : TaxPayerPimp[T] =
      new TaxPayerPimp[T] {
        val taxPayer = t
      }
}

trait TaxCalculator[T <: TaxPayer] {
  def calculate(t: T) : Long
}

/**
 * Here we've added an implicit to the calculateTax function which tells the
 * compiler to look for an implicit TaxCalculator in scope.
 */
trait TaxPayerPimp[T <: TaxPayer] {
  val taxPayer: T
  def calculateTax(implicit tc: TaxCalculator[T]) : Long = tc.calculate(taxPayer)
}

case class Employee(sal: Long) extends TaxPayer

/**
 * Here we've added implicit to the DefaultTaxCalculator.  If in scope
 * and the right type, it will be implicitely used as the parameter in the
 * TaxPayerPimp.calculateTax function.
 *
 *
 */
object Employee {
  implicit object DefaultTaxCalculator extends TaxCalculator[Employee] {
    def calculate(e: Employee) = Math.ceil(e.sal * .3) toLong
  }

  object BelgianTaxCalculator extends TaxCalculator[Employee] {
    def calculate(e: Employee) = Math.ceil(e.sal * .5) toLong
  }
}

/**
 * Added implicit to the DefaultTaxCalculator...
 */
case class NonProfitOrg(funds: BigInt) extends TaxPayer
object NonProfitOrg {
  implicit object DefaultTaxCalculator extends TaxCalculator[NonProfitOrg] {
    def calculate(n: NonProfitOrg) = 0
  }
}

object TaxPayer2 extends Application {

    println("TaxPayer2")

    val taxPayer = new Employee(1000)

    //Now the call to calculateTax will
    //implicitely use Employee.DefaultTaxCalculator
    taxPayer.calculateTax
    //But if we want, we can still explicitely pass in the BelgianTaxCalculator
    taxPayer.calculateTax(Employee.BelgianTaxCalculator)

    val npo = new NonProfitOrg(100000000)

    //implicitely uses NonProfitOrg.defaultCalculator
    npo.calculateTax


}

Scala中战略模式的更好替代方案？

3 个答案: