title: Leaflet detect when marker goes into and out of a circle
body: <p>I am trying out geofencing with leaflet and I have a marker that can be moved and a circle on a map.</p>
  
  <p>Here is the full code:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html lang="en" &gt;
  &lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;title&gt;&lt;/title&gt;
  
  &lt;/head&gt;
  &lt;body&gt;
  
  &lt;head&gt;
      &lt;link href='https://api.mapbox.com/mapbox.js/plugins/leaflet-locatecontrol/v0.43.0/css/font-awesome.min.css' rel='stylesheet' /&gt;
      &lt;link rel="stylesheet" href="https://unpkg.com/leaflet@1.5.1/dist/leaflet.css" /&gt;
  
      &lt;script src="https://unpkg.com/leaflet@1.5.1/dist/leaflet.js"&gt;&lt;/script&gt;
    &lt;/head&gt;
  &lt;body&gt;
  &lt;div id="mapid" style="height: 600px"&gt;&lt;/div&gt;
  &lt;script&gt;
  
    var mymap = L.map('mapid', {
      center: [50.897819, -1.150189],
      zoom: 16
    });
  
      L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1Ijoic3RldmVuc2F0Y2giLCJhIjoiY2p5eDR6MWgzMHRvbjNocnJkN2d2MjRwaSJ9.wd0OtBUQQfUtNxdduQA3lg', {
          maxZoom: 18,
          attribution: 'Map data &amp;copy; &lt;a href="https://www.openstreetmap.org/"&gt;OpenStreetMap&lt;/a&gt; contributors, ' +
              '&lt;a href="https://creativecommons.org/licenses/by-sa/2.0/"&gt;CC-BY-SA&lt;/a&gt;, ' +
              'Imagery © &lt;a href="https://www.mapbox.com/"&gt;Mapbox&lt;/a&gt;',
          id: 'mapbox.streets'
      }).addTo(mymap);
  
  var marker = new L.marker([50.898422, -1.148444],{
      draggable: true,
      autoPan: true
  }).addTo(mymap);
  
  
   var circle = L.circle([50.895763, -1.150556], {
      color: 'red',
      fillColor: '#f03',
      fillOpacity: 0.5,
      radius: 200
  }).addTo(mymap);
  
  
  &lt;/script&gt;
  &lt;/body&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>I need to detect when the marker going into and out of the circle.</p>
  
  <p>How can I do this?</p>