title: Remove connection between checked states of parent and children in ControlsFX CheckTreeView
body: <p>I'm using <a href="https://controlsfx.bitbucket.io/org/controlsfx/control/CheckTreeView.html" rel="nofollow noreferrer">CheckTreeView</a> to represent hierachical structure. The current checking behaviour is if I check/uncheck a parent, all of its children will automatically get check/unchecked. If I check/uncheck all the children the parent also gets automatically checked/unchecked. Is there any way to remove this behaviour? I want to be able to check all the children without changing the checked state of the parent to true. I also want to be able to check the parent and some or no children but not all of them. In short I want the check boxes to be totally independent from the others. I have attached an edited screenshot to demonstrate what I describe</p>
  
  <p><a href="https://i.stack.imgur.com/PpJap.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PpJap.png" alt="This modified screenshot is what I want to achieve"></a></p>
  
  <p>Example code to build the tree:</p>
  
  <pre><code>@FXML
  private void initialize() {
      CheckBoxTreeItem&lt;String&gt; parent1 = new CheckBoxTreeItem&lt;&gt;("Parent 1");
      addChildren(parent1);
      CheckBoxTreeItem&lt;String&gt; parent2 = new CheckBoxTreeItem&lt;&gt;("Parent 2");
      addChildren(parent2);
      CheckBoxTreeItem&lt;String&gt; root = new CheckBoxTreeItem&lt;&gt;("Root");
      root.getChildren().addAll(parent1, parent2);
      ctv.setRoot(root);
  }
  
  private void addChildren(TreeItem&lt;String&gt; parent) {
      parent.getChildren().add( new CheckBoxTreeItem&lt;&gt;("Child 1"));
      parent.getChildren().add( new CheckBoxTreeItem&lt;&gt;("Child 2"));
  }
  </code></pre>