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scalamacrosmvp

Scala model-view-presenter, traits


I am a fan of Martin Fowler's (deprecated) model-view-presenter pattern. I am writing a Scala view class containing several button classes. I would like to include methods to set the action properties of the buttons, to be called by the presenter. A typical code fragment looks like this:

private val aButton = new JButton
def setAButtonAction(action: Action): Unit = { aButton.setAction(action) }

This code is repeated for each button. If Java/Scala had the C preprocessor, I would create a macro to generate this code, given the button name (no lectures on the evils of the C preprocessor, please). This code is obviously very verbose and repetitive. Is there any better way way to do this in Scala, perhaps using traits?

Please hold the lectures about scala.swing. I looking for a general pattern here.


Solution

  • Scala is less about explicit setter and getter methods than java. Instead use abstract fields to define the exposed interface. How about something like this:

    trait ActionUser {
      def setAction(action:Action):Unit
    }
    
    trait Container {
      val aButton:ActionUser
    }
    
    trait ContainerImpl {
      val aButton = new JButton with ActionUser
    }
    

    Classes operating against Container will only be able to access setAction while internal methods see it as a JButton.

    One more note: C uses macros to cut down on code duplication. Scala uses multi-inheritance of traits to accomplish the same thing.