====== Show Footprints ======

This page describes how to track Resource updates to render [[glossary#Footprint|Footprints]] on the Reference Map.

===== Listening ==== 

You as a developer should listen for changes to the Reference Map extent.  When it changes, footprints should be updated.

To listen to resource changes, use these signals : 
  * onResourceOpened : fired when a resource is opened.
  * onResourceClosed : fired when a resource is closed.

When any of these events occur, footprints should be refreshed.

===== Loading =====

Use the ''getStreamingFootprints2D'' to retrieve footprints for a 2D map.  

<note>
Depending on your host environment, it may be the case that "map extent" events are fired faster than you can handle them.  In such a case, loading footprints should be rate-limited by limiting the calls to the ''getStreamingFootprints2D'' function.

For javascript, see [[https://www.npmjs.com/package/debounce]]
</note>

<code javascript>
/**
 * Update footprint data.
 * Assumes reference map scale,crs,minX,minY,maxX and maxY properties are available in this context.
 */
function updateFootprints() {
  // Get map extent
  var bounds = Bounds.fromXY(crs,minX,minY,maxX,maxY);
  // Get map objects
  var mapObjects = viewer.getStreamingFootprints2D(null/*resourceIds*/,bounds,scale).then(renderFootprints);
  // Render map objects
  renderMapObjects(mapObjects);
}
// Listen to signals
viewer.onResourceOpened.add(updateFootprints);
viewer.onResourceClosed.add(updateFootprints);
</code>

===== Rendering =====

See [[mapobjects]].