D3 Examples - Vega-Lite listener

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://cdn.jsdelivr.net/npm/vega@5"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-lite@3"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-embed@4"></script>
</head>
​
<body>  
  <div id="vis"></div>
  
  <script>
    const spec = 'bar.vl.json';
    vegaEmbed('#vis', spec, {
      patch: (spec) => {
        spec.signals.push({
            "name": "barClick",
            "value": 0,
            "on": [{"events": "rect:mousedown", "update": "barClick + 1"}]
        })
        return spec;
      }
    }).then(result => {
        result.view.addSignalListener('barClick', console.log);
        })
      .catch(console.warn);
  </script>
</body>
    