How to use contents = to add to a Scala Panel? - swing

Question

Sorry this must be a very silly question.. but everywhere I've been seeing Scala code examples where you just do
contents+= on a BoxPanel or some layout Panel. I figured because they have contents as mutable.buffer so you can just add and remove components.
But how do you add a component to Scala Panel? It accepts a seq so do you have to give it a list or something? I know you can just call peer.add but I want to see how Scala code does it. :)
For example contents = new Button {} isn't working.
Sorry for this simple question I'm very new to Scala..
EDIT:
Thanks for the replies. My question now though becomes.. can you ever just have a class extending Panel? Would you be able to set contents for it at all? Or is it never done and everyone always just uses the Panels associated with a layout manager?

Answer 1

The Panel class itself is abstract, meaning it can't be instantiated directly, and is intended as a "base" for concrete implementations of panels.
It doesn't seem to have a "common" method for adding components probably because each subclass implements its own, sometimes mutually incompatible custom one:
BoxPanel, as you've noted, has a settable Buffer,
FlowPanel seems to mandate adding components as constructor arguments,
GridBagLayout and some others implement addition via the layout Map,
etc.
As you might see from the above examples, it would be hard to specify what a general "add" method would mean in all of those cases.
EDIT in response: of course you can, there's nothing stopping you from subclassing a Panel yourself and override the contents method, e.g.:
val myPanel = new Panel() {
private val myContents = (new Content += new Button())
override def contents = myContents
}
You can also use Panel as a type parameter for your methods that process panels in a general way, etc. It's just that you can't have an instance that's just a Panel, because, again, the class is abstract, so you can't instantiate it.
Note that this is not unique to Scala, if JPanel was abstract in Java (like Component is) the outcome would be the same.

Answer 2

I want to see how Scala code does it.
https://github.com/scala/scala-swing/blob/v1.0.0-RC2/src/main/scala/scala/swing/Container.scala#L35
I, too, practiced on some Swing code when I first learned some Scala.
Here is a Panel component that renders itself as a simple game grid:
https://github.com/som-snytt/House-of-Mirrors-Fork/blob/act/src/main/scala/hom/LightBox.scala#L286
To see how the Scala and Swing pieces fit together, see SuperMixin:
https://github.com/scala/scala-swing/blob/v1.0.0-RC2/src/main/scala/scala/swing/Component.scala#L51
Assembly:
https://github.com/som-snytt/House-of-Mirrors-Fork/blob/act/src/main/scala/hom/HouseOfMirrors.scala#L18
This is what you asked about directly:
https://github.com/som-snytt/House-of-Mirrors-Fork/blob/act/src/main/scala/hom/HouseOfMirrors.scala#L45

Answer 3

If you have a button:
val button=new Button{
text="Click me!"
}
or
val label=new Label{
text="Look, i'm a label!"
}
or
object myField extends TextField{ columns=2 }
then you just use:
contents=new BoxPanel(Orientation.Vertical){
contents+=button
border=Swing.EmptyBorder(10,20,10,20)
}
or in a more simpler form:
contents=new FlowPanel(){
contents+=new Label("This is my button:")
contents+=new Button("Click me!")
border=Swing.EmptyBorder(10,20,10,20)
}