如何在Scala swing中设置背景

我正在Scala中的步骤中查看示例6.9，他的背景设置方法不充分，因为当调整帧大小时，背景不会改变。示例代码（针对版本2.10的更改而更新）为：

import scala.swing._
import scala.swing.event._

object circle extends SimpleSwingApplication {
  def top = new MainFrame {
    var mouseX = 0
    var mouseY = 0
    var mouseclicked = false
    title = "Draw Circle"
    val rect = new java.awt.Rectangle(350,250)
    preferredSize = new Dimension(350,250)
    val canvas = new Panel {
      border = Swing.EmptyBorder(15, 15, 15, 15)
      opaque = false
      override def paintComponent(g: java.awt.Graphics2D) {
     // g.setBackground(java.awt.Color.magenta)
        g.setColor(java.awt.Color.magenta)
        g.fill(rect)
        g.setColor(java.awt.Color.blue)
          if ( mouseclicked ) {
            g.fillOval(mouseX-10, mouseY-10, 20, 20)
            mouseclicked = false
          }
      }
      listenTo(mouse.clicks)
      reactions += {
        case MouseClicked(_, p, _, 1, _) => {  
          mouseX = p.x
          mouseY = p.y
          mouseclicked = true 
          repaint
        }
      }
    }
    contents = canvas
  }
}

我尝试用g.setBackground（…）替换g.setColor（…）和g.fill（rect），如上面所述，它可以编译，但不起作用

有人能建议正确的方法吗

---

提前感谢。

简单的方法是让Swing进行背景绘制

val canvas = new Panel {
  opaque = true
  background = Color.magenta
  override def paintComponent(g: java.awt.Graphics2D) {
    super.paintComponent(g);
    g.setColor(Color.blue)
    if (mouseclicked) {
      g.fillOval(mouseX - 10, mouseY - 10, 20, 20)
      mouseclicked = false
    }
  }

---

简单的方法是让Swing进行背景绘制

val canvas = new Panel {
  opaque = true
  background = Color.magenta
  override def paintComponent(g: java.awt.Graphics2D) {
    super.paintComponent(g);
    g.setColor(Color.blue)
    if (mouseclicked) {
      g.fillOval(mouseX - 10, mouseY - 10, 20, 20)
      mouseclicked = false
    }
  }

---

简单的方法是让Swing进行背景绘制

val canvas = new Panel {
  opaque = true
  background = Color.magenta
  override def paintComponent(g: java.awt.Graphics2D) {
    super.paintComponent(g);
    g.setColor(Color.blue)
    if (mouseclicked) {
      g.fillOval(mouseX - 10, mouseY - 10, 20, 20)
      mouseclicked = false
    }
  }

---

简单的方法是让Swing进行背景绘制

val canvas = new Panel {
  opaque = true
  background = Color.magenta
  override def paintComponent(g: java.awt.Graphics2D) {
    super.paintComponent(g);
    g.setColor(Color.blue)
    if (mouseclicked) {
      g.fillOval(mouseX - 10, mouseY - 10, 20, 20)
      mouseclicked = false
    }
  }