Elkjs Tree

Like our dagre example, this example shows how you can integrate elkjs with Svelte Flow for more advanced tree layouts. The code for this example builds a similar tree to the dagre example, but you can look at the reference here to see what you can configure (hint: it’s a lot).

<script lang="ts">
  import { writable } from 'svelte/store';
  import ELK from 'elkjs/lib/elk.bundled.js';
  import {
    SvelteFlow,
    Background,
    Position,
    ConnectionLineType,
    Panel,
    useSvelteFlow,
    type Node,
    type Edge,
  } from '@xyflow/svelte';
 
  import '@xyflow/svelte/dist/style.css';
 
  import { initialNodes, initialEdges } from './nodes-and-edges';
  import { onMount } from 'svelte';
 
  const nodes = writable<Node[]>([]);
  const edges = writable<Edge[]>([]);
 
  const { fitView } = useSvelteFlow();
 
  const elk = new ELK();
 
  // Elk has a *huge* amount of options to configure. To see everything you can
  // tweak check out:
  //
  // - https://www.eclipse.org/elk/reference/algorithms.html
  // - https://www.eclipse.org/elk/reference/options.html
  const elkOptions = {
    'elk.algorithm': 'layered',
    'elk.layered.spacing.nodeNodeBetweenLayers': '100',
    'elk.spacing.nodeNode': '80',
  };
 
  function getLayoutedElements(nodes: Node[], edges: Edge[], options = {}) {
    const isHorizontal = options?.['elk.direction'] === 'RIGHT';
    const graph = {
      id: 'root',
      layoutOptions: options,
      children: nodes.map((node) => ({
        ...node,
        // Adjust the target and source handle positions based on the layout
        // direction.
        targetPosition: isHorizontal ? Position.Left : Position.Top,
        sourcePosition: isHorizontal ? Position.Right : Position.Bottom,
 
        // Hardcode a width and height for elk to use when layouting.
        width: 150,
        height: 50,
      })),
      edges: edges,
    };
 
    return elk
      .layout(graph)
      .then((layoutedGraph) => ({
        nodes: layoutedGraph.children.map((node) => ({
          ...node,
          // React Flow expects a position property on the node instead of `x`
          // and `y` fields.
          position: { x: node.x, y: node.y },
        })),
 
        edges: layoutedGraph.edges,
      }))
      .catch(console.error);
  }
 
  function onLayout(direction: string, useInitialNodes = false) {
    const opts = { 'elk.direction': direction, ...elkOptions };
    const ns = useInitialNodes ? initialNodes : $nodes;
    const es = useInitialNodes ? initialEdges : $edges;
 
    getLayoutedElements(ns, es, opts).then(
      ({ nodes: layoutedNodes, edges: layoutedEdges }) => {
        $nodes = layoutedNodes;
        $edges = layoutedEdges;
 
        fitView();
      },
    );
  }
 
  onMount(() => {
    onLayout('DOWN', true);
  });
</script>
 
<div style="height:100vh;">
  <SvelteFlow
    {nodes}
    {edges}
    fitView
    connectionLineType={ConnectionLineType.SmoothStep}
    defaultEdgeOptions={{ type: 'smoothstep', animated: true }}
  >
    <Panel position="top-right">
      <button on:click={() => onLayout('DOWN')}>vertical layout</button>
      <button on:click={() => onLayout('RIGHT')}>horizontal layout</button>
    </Panel>
    <Background />
  </SvelteFlow>
</div>