How to "force" user to enter a valid value in a TextBox

Question

How can one create a TextBox in which the user is not able to leave (commit the value, change focus, or perform the action of another control) unless a valid value (the value to be committed) is displayed?
The following is the best I've come up with so far.
Good: It doesn't permit a value to be committed unless it is valid.
Good: It doesn't permit the focus to be changed when TAB is pressed unless it is valid.
Bad: It permits the user to perform the action associated with a button at any time.
In the following code pressing on the button prints "I've been pushed", no matter the value of the TextBoxes. Can I stop this (in general, not for specific buttons)?
class ValidTextBox extends TextBox {
  public-init var validValue:String = "";
  function isValid():Boolean {
    return (rawText == validValue);
  var isFocused = bind focused on replace {
    if (not focused and not isValid()) {
      requestFocus();
  override function commit():Void {
    if (isValid()) { super.commit(); }
    else {};
var string1:String = "" on replace { println ("String1 = '{string1}'") };
var string2:String = "" on replace { println ("String2 = '{string2}'") };
def textbox1 = ValidTextBox { text: bind string1 with inverse; promptText: "String1: enter abc"; columns: 20; validValue: "abc"; }
def textbox2 = ValidTextBox { text: bind string2 with inverse; promptText: "String2: enter def"; columns: 20; validValue: "def"; }
def aButton = Button {
  text: "Push me";
  action: function():Void { println("I've been pushed!"); }
Stage {
  scene: Scene {
    width: 250
    height: 300
    content: [ VBox { spacing: 10; content: [ textbox1, textbox2, aButton ] } ]
}

Answer 1

It sounds like you are looking for a concept of validation that groups items together. In days long gone Oracle used to have a product called Oracle Forms - that had field validation, record validation and block validation. So in your example if string1 and string2 were in the same persistable data object then the equivalent concept would be 'record validation'. The code in your button would say "if the record is valid print". A record is only valid if all its fields are valid.
JavaFX really provides a 'GUI toolkit'. I think you are looking for a fairly advanced binding framework - a framework that builds on the concept of field level validation. You can approximate such a thing by creating a 'validation group' class. This class would be able to have nodes added to it and have an 'isValid()' function which only returns true if all the node items are valid.