Scala Swing Script Not Working

Question

I just started learning Scala, and as part of the process, I've been trying to write some simple scripts that use Swing.

Here is an extremely stripped down example which exhibits the problem that I am seeing.

SimpleSwingApp:

import scala.swing._
object SimpleSwingApp extends SimpleSwingApplication {
  def top = new MainFrame {
    println ("Starting")
    title = "First Swing App"
    contents = new Button { text = "Click me" }
  }
}

When I run the script through the Eclipse Scala IDE as a Scala Application, everything works as expected. I get the expected "Starting" text printed to the Eclipse console and the little GUI pops up and waits for a click.

When I try to run it as a script from the command line, I get nothing.

> scala SimpleSwingApp.scala

No error message, no "Starting", no GUI.

So what is going on, and how can I achieve the result that I want (ie, starting a Scala GUI script from the command line)?

QFH · Accepted Answer · 2012-08-24 21:11:53Z

4

The problem is that the Scala script runner is looking for a main method in your script file, but it isn't finding one.

The reason it isn't finding one is because you are inheriting main from SimpleSwingApplication, so there isn't a main method in your script file.

You can work around the problem by adding a main method to your script that immediately invokes super.main, as follows:

import scala.swing._
object SimpleSwingApp extends SimpleSwingApplication {

  override def main(args: Array[String]) = super.main(args)

  def top = new MainFrame {
    println("Starting")
    title = "First Swing App"
    contents = new Button { text = "Click me" }
  }
}

Now, the script will not raise an error in the Scala IDE, and it can be executed by calling

scala SimpleSwingApp.scala

rather than the less aesthetic

scala -i SimpleSwingApp.scala -e "SimpleSwingApp.main(args)"

edited Aug 24, 2012 at 21:11

answered Aug 24, 2012 at 20:53

QFH

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Comments

stew · Accepted Answer · 2012-08-23 18:56:13Z

2

If you want to run it from the command line, add an explicit call to main at the bottom of the file:

SimpleSwingApp.main(Array())

If you compile this with scalac first, then run, it will work without explicitly calling main.

answered Aug 23, 2012 at 18:56

stew

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1 Comment

HarryM Over a year ago

Thanks for you answer. That does indeed work. Unfortunately, it raises an error in the Scala IDE for Eclipse. Finding the solution to that lead me to this answer, which solves the IDE error. Essentially, if I call my script with scala -i SimpleSwingApp.scala -e "SimpleSwingApp.main(args)" it works.

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