[英]In Scala, how can I use implicit conversion to “add” methods to subclasses of a common parent?
假设我在“哑”模型中有一些数据。 在这个例子中,我将使用Circle
和Triangle
,它们扩展了一个trait Shape
。
我正在寻找一种方法来隔离可能使用这些形状的行为,但我不确定构建它的最佳方法。 如果我试图将这些形状绘制到文档上,我希望能够编写如下代码:
shapes.foreach(doc.add)
这里的诀窍是shapes
是Seq[Shape]
,而add
方法是我想要隐式添加的东西,因为我无法修改形状本身(我也不想将这些特定的功能融入其中)。
我遇到困难的地方是,我不知道如何将隐式转换与子类混合在一起。 有关详细信息,请参阅代码中的QUESTION:
// Let's assume I'm working with some shape models that are defined in some
// external library that's out of my control.
sealed trait Shape
case class Circle() extends Shape
case class Triangle() extends Shape
// Now I'm building an add that adds stuff to a Document
// and I want to locally implement methods that work on these general shapes.
case class Document()
// Using implicit conversion to add methods to a case class that's just holding data
implicit class DocumentExtensions(doc: Document) {
// I don't want this to be called
def add(shape: Shape): Unit = println("Add a shape")
// I want to use shape-specific methods
def add(shape: Circle): Unit = println("Add a circle")
def add(shape: Triangle): Unit = println("Add a triangle")
}
val doc = Document()
val shapes = Seq(Circle(), Triangle())
// This just prints "Add a shape" for the Circle and Triangle.
// I want to it to print "Add a circle" and "Add a triangle".
shapes.foreach { shape =>
// QUESTION:
// Is there a way or pattern to have this call the add for the
// subclass instead of for Shape? I want this to be fully dynamic
// so that I don't have to list out each subclass. Even more ideally,
// the compiler could warn me if there was a subclass that there wasn't
// an implicit add for.
doc.add(shape)
}
// This would work, but I'm wondering if there's a way to do this more
// dynamically without listing everything out.
shapes.foreach {
case c: Circle => doc.add(c)
case t: Triangle => doc.add(t)
}
我确定我正在寻找什么名称,但我不知道它是什么或搜索什么。
问题:编译器无法选择和使用特定于处理子类的隐式值。 当你只知道它是一个Shape
时,基本上不可能决定调用什么方法(对于Triangle
或Circle
)。 这实际上是一个经典问题,它有标准的解决方案。
解决方案1
DocumentExtension.add
模式匹配
优点:
trait Shape
被定义为sealed
,如果您错过了某个祖先的案例,编译器会给您。 缺点:
解决方案2
古典访客模式
sealed trait Shape {
def addToDoc(doc: Document, visitor: ShapeDrawer)
}
final class Triangle extends Shape {
def addToDoc(doc: Document, visitor: ShapeDrawer) = visitor.draw(doc, this)
}
final class Circle extends Shape {
def addToDoc(doc: Document, visitor: ShapeDrawer) = visitor.draw(doc, this)
}
trait ShapeDrawer {
def draw(doc: Document, t: Circle)
def draw(doc: Document, t: Triangle)
}
val drawer: ShapeDrawer = ???
val doc: Document = ???
val shapes = Seq.empty[Shape]
shapes.foreach(_.addToDoc(doc, drawer))
此解决方案还符合在编译时确保已处理Shape的每个子类的要求,但需要向特征本身添加奇怪的方法。
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