Nested interfaces example

class PipelineNodesGraph {
    // nested groovy trait, same as nested interface in java
    static trait NodeVisitor {
        void enter(GraphNode node) {}
        void visit(GraphNode node) {}
        void exit(GraphNode node) {}
    }
    static class CycleDetector implements NodeVisitor {
        // ... graph cycles detecting implementation ...
    }

    private List<Node> findCycle() {
        def cycleDetector = new CycleDetector()
        def cycle = []
        try {
            traverseDepthFirst([cycleDetector])
        } catch (CyclicConnectionException e) {
            cycle = e.cycle
        }
        return cycle
    }
}

class NodeBuildingVisitor implements NodeVisitor {
    void visit(GraphNode graphNode) {
        if (!graphNode.node.assembled) {
            graphNode.node.assemble()
        }
    }
}

class Pipeline {
    void build() {
         // build all nodes
        def graph = new PipelineNodesGraph(nodes, links)
        def buildVisitor = new NodeBuildingVisitor(this)
        graph.traverseDepthFirst([buildVisitor])
        assert nodes.every { it.assembled }
    }
}