尝试在ScalaFx中绑定鼠标事件侦听器时发生编译错误

Question

I'm coding a little test application that uses the Canvas class in Scala with scalafx . The problem is with:

gui.canvas.handleEvent(MouseEvent.MouseClicked){
     a: MouseEvent => {
          println("Mouse pos:" + a.sceneX+ "  " + a.sceneY)
     }      
}

Throwing the error:

[error] E:\dev\sg\src\main\scala\Test.scala:27: type mismatch;
[error]  found   : scalafx.scene.input.MouseEvent => Unit
[error]  required: _2.HandlerMagnet[javafx.scene.input.MouseEvent,?] 
where val _2: scalafx.scene.canvas.Canvas
[error]       a: MouseEvent => {
[error]                     ^

I don't get why this exact same snippet compiles when it appears in the method setupGraphics but NOT in when it's in the Main object's code.

BTW, having browsed quite a bit of ScalaFX code, I'm getting a bit dizzy because I've seen lots different ways to bind to an event, there's the one I'm using , setting the onMouseClicked variable (with and without a first class function), using filterEvent . Also, for each you can use an anonymous function or create an event handler object.

Of all those, this is the only one that worked in this example; trying to create an EventHandler[MouseEvent] and override handle() also fails in this case.

I'm using Scala 2.10.5

Full Source:

object Test {
  import scalafx.Includes._
  import scalafx.application.JFXApp
  import scalafx.application.JFXApp.PrimaryStage
  import scalafx.stage.Stage

  import scalafx.scene.Scene
  import scalafx.scene.canvas.Canvas
  import scalafx.scene.canvas.GraphicsContext

  import scalafx.scene.input.MouseEvent
  import scalafx.event.EventHandler



  class GUI(val canvas:Canvas
    ,val stage: Stage
    ,val gc: GraphicsContext){}

  object Main extends JFXApp {

    var gui=setupGraphics()

    gui.canvas.handleEvent(MouseEvent.MouseClicked){
      a: MouseEvent => {
        println("Mouse pos:" + a.sceneX+ "  " + a.sceneY)
      }
    }

    def setupGraphics():GUI={
      val canvas = new Canvas(400, 400)
      canvas.translateX=0
      canvas.translateY=0
      val gc = canvas.graphicsContext2D

      canvas.handleEvent(MouseEvent.MouseClicked){
        a: MouseEvent => {
          println("Mouse pos:" + a.sceneX+ "  " + a.sceneY)
        }
      }

      stage = new PrimaryStage {
        title = "asd"
        height = 600
        width = 800
        scene=new Scene{content=canvas}
      }

      return new GUI(canvas,stage,gc)
    }    
  }    
}

Answer 1

This looks to me like a bug in Scala compiler, especially that you can correct this by using an intermediate variable for canvas without modifying anything else. Instead of calling gui.canvas.handleEvent , first assign canvas to a variable then call handleEvent on that:

var gui=setupGraphics()
val canvas = gui.canvas
canvas.handleEvent(MouseEvent.MouseClicked){
  a: MouseEvent => {
    println("Mouse pos:" + a.sceneX+ "  " + a.sceneY)
  }
}