Adding to contents of scala.swing.Panel

[Alan Mimms]

OK, I have had coffee and am more awake now :)

I am wrestling with "assignment to val" in the following

package trythings

import scala.swing._

class TryPanel extends Panel {

  var subPanel = new GridBagPanel()

  _contents += subPanel          // No error here

  contents += subPanel           // "Assignment to val" error here

}

To me this looks incorrect. Panel is defined as

abstract class Panel extends Component with Container.Wrapper

and doesn't directly override contents. Nor does Component.

In scala.swing.Container.Wrapper, we have 

object Container {

...

  trait Wrapper extends Container with Publisher {

    override def peer: javax.swing.JComponent

    

    protected val _contents = new Content

    def contents: Seq[Component] = _contents

  

    protected class Content extends BufferWrapper[Component] {

      override def clear() { peer.removeAll() }

      override def remove(n: Int): Component = { 

        val c = peer.getComponent(n)

        peer.remove(n)

        UIElement.cachedWrapper[Component](c)

      }

      protected def insertAt(n: Int, c: Component) { peer.add(c.peer, n) }

      def +=(c: Component): this.type = { peer.add(c.peer) ; this }

      def length = peer.getComponentCount

      def apply(n: Int) = UIElement.cachedWrapper[Component](peer.getComponent(n))

    }

...

}

This appears to define contents as Seq[Component] and sets its value to _contents, which is a Container.Wrapper.Content. Container.Wrapper.Content defines a += method to forward to peer.add().

And yet I get "assignment to val" error here.

Via another path, if I hack and make my class TryPanel extend BoxPanel(Orientation.Horizontal) the error goes away. BoxPanel extends SequentialContainer.Wrapper instead of Container.Wrapper which directly defines contents as a Buffer[Component].

What is going on here? Am I (a newbie with Scala) missing something obvious to you wise old sages of Scala-dom? I'm guessing that contents' the use of Seq[Component] in Container.Wrapper is where the problem comes in, but I am not confident enough in my Scala skills to be sure. Is this a bug?

Thanks!

[pagoda_5b]

I believe the point here is why you're trying to create a subclass of Panel.

If you only need a specific panel (let's say BoxPanel) to add your components, you should just:

 val myPane = new BoxPanel {

   contents += new GridBagPanel()

   ...

 }

where you could use an anonimous class to layout your application gui.

If you're trying to create your custom panel, then I guess you should override the panel contents method, which returns a generic Seq[Component], which in turn doesn't sports any += method

if you look at the signatures of the Content inner class you can figure it out:

    protected val _contents = new Content

    def contents: Seq[Component] = _contents

As you already noted, the specific subclasses like BoxPanel override the public method to return a specific type (like Buffer[Component]) that have a += method

I think you could override the content method with a val to just return the already available protected Content value:

    override val contents: Content = _contents

Does it makes sense to you?

Ivano

[Alan Mimms]

Yes, I think that makes sense. I do not understand the reason for the original implementation in Panel, though.