title: how to invoke a method from an Entry Widget in Ruby Tk
body: <p>I'm looking for an option or a way to bind an event, which invokes a method from an Entry-Widget.</p>
  
  <p>Like the <code>command</code> option for button-widgets: <code>command { method }</code> or </p>
  
  <p>like binding an event for combobox-widgets: 
  <code>$combobox.bind("&lt;ComboboxSelected&gt;") { method }</code></p>
  
  <p>Now I'm looking for something similar for Entry-Widgets. I want to invoke a method every time an entry (value) has been edited. That's my initial situation:</p>
  
  <pre><code>$foo = TkVariable.new
  $entry = Tk::Tile::Entry.new(parent) {
    validate 'key'; validatecommand method; textvariable $foo
    }
  </code></pre>
  
  <p><code>validatecommand</code> works only at the first time.</p>