Swtbot.syncExec() problem when used in custom class

Question

I'm learning to use SWTBot and I met this problem.

Here's my code in groovy.

class A{

  SWTBot bot = new SWTBot()

  def foo(){
      bot.syncExec {
         ...
      }
  }
}

The function foo() works fine when its not a class method.

But when I put it in class, I got the error message:

No signature of method: org.eclipse.swtbot.swt.finder.SWTBot.syncExec() is applicable for argument types: (A$_foo_closure1)

Can someone help me with this? Thanks a lot!