Question

I’m developing an XTEXT 2.0 plugin. I’d like to group some nodes inside my outline in a “virtual” node. Which is the right way to achieve this result?

Currently if i want to group nodes of type “A”, in my OutlineTreeProvider I define the following method

protected void _createNode(IOutlineNode parentNode, A node) {
 if(this.myContainerNode == null){
  A container = S3DFactoryImpl.eINSTANCE.createA();
  super._createNode(parentNode, container);
  List<IOutlineNode> children = parentNode.getChildren();
  this.myContainerNode = children.get(children.size()-1);
 }
 super._createNode(this.myContainerNode, node);
}

Reading the Xtext 2.0 documentation i saw also that there is a EStructuralFeatureNode. I didn’t understand exactly what this type of node is and how to use it. Can you explain what EStructuralFeatureNode is used for?

Many thanks

Answer 1

There are a couple of problems with your code:

this.myContainerNode: There is no guarantee that your provider is a prototype; someone could configure the instance as singleton. Therefore, avoid instance fields.

There are two solutions to this problem:

1. Search the parent node for your container node whenever you need it (slow but simple)
2. Add a cache to your instance (see How do I attach some cached information to an Eclipse editor or resource?)

super._createNode(): Don’t call the methods with _, always call the plain version (super.createNode()). That method will figure out which overloaded _create* method to call for you. But in your case, you can’t call any of these methods because you’d get a loop. Call createEObjectNode() instead.

Lastely, you don’t need to create an instance of A (S3DFactoryImpl.eINSTANCE.createA()). Nodes can be backed by model elements but that’s optional.

For grouping, I use this class:

public class VirtualOutlineNode extends AbstractOutlineNode {
    protected VirtualOutlineNode( IOutlineNode parent, Image image, Object text, boolean isLeaf ) {
        super( parent, image, text, isLeaf );
    }
}

In your case, the code would look like so:

protected void _createNode(IOutlineNode parentNode, A node) {
    VirtualOutlineNode group = findExistingNode();
    if( null == group ) {
        group = new VirtualOutlineNode( parentNode, null, "Group A", false );
    }
    // calling super._createNode() or super.createNode() would create a loop
    createEObjectNode( group, node );
}