ontoref-outreach/site/htmx-assets/ext/preload.js

// This adds the "preload" extension to htmx.  By default, this will
// preload the targets of any tags with `href` or `hx-get` attributes
// if they also have a `preload` attribute as well.  See documentation
// for more details
htmx.defineExtension('preload', {

  onEvent: function(name, event) {
    // Only take actions on "htmx:afterProcessNode"
    if (name !== 'htmx:afterProcessNode') {
      return
    }

    // SOME HELPER FUNCTIONS WE'LL NEED ALONG THE WAY

    // attr gets the closest non-empty value from the attribute.
    var attr = function(node, property) {
      if (node == undefined) { return undefined }
      return node.getAttribute(property) || node.getAttribute('data-' + property) || attr(node.parentElement, property)
    }

    // load handles the actual HTTP fetch, and uses htmx.ajax in cases where we're
    // preloading an htmx resource (this sends the same HTTP headers as a regular htmx request)
    var load = function(node) {
      // Called after a successful AJAX request, to mark the
      // content as loaded (and prevent additional AJAX calls.)
      var done = function(html) {
        if (!node.preloadAlways) {
          node.preloadState = 'DONE'
        }

        if (attr(node, 'preload-images') == 'true') {
          document.createElement('div').innerHTML = html // create and populate a node to load linked resources, too.
        }
      }

      return function() {
        // If this value has already been loaded, then do not try again.
        if (node.preloadState !== 'READY') {
          return
        }

        // Special handling for HX-GET - use built-in htmx.ajax function
        // so that headers match other htmx requests, then set
        // node.preloadState = TRUE so that requests are not duplicated
        // in the future
        var hxGet = node.getAttribute('hx-get') || node.getAttribute('data-hx-get')
        if (hxGet) {
          htmx.ajax('GET', hxGet, {
            source: node,
            handler: function(elt, info) {
              done(info.xhr.responseText)
            }
          })
          return
        }

        // Otherwise, perform a standard xhr request, then set
        // node.preloadState = TRUE so that requests are not duplicated
        // in the future.
        if (node.getAttribute('href')) {
          var r = new XMLHttpRequest()
          r.open('GET', node.getAttribute('href'))
          r.onload = function() { done(r.responseText) }
          r.send()
        }
      }
    }

    // This function processes a specific node and sets up event handlers.
    // We'll search for nodes and use it below.
    var init = function(node) {
      // If this node DOES NOT include a "GET" transaction, then there's nothing to do here.
      if (node.getAttribute('href') + node.getAttribute('hx-get') + node.getAttribute('data-hx-get') == '') {
        return
      }

      // Guarantee that we only initialize each node once.
      if (node.preloadState !== undefined) {
        return
      }

      // Get event name from config.
      var on = attr(node, 'preload') || 'mousedown'
      const always = on.indexOf('always') !== -1
      if (always) {
        on = on.replace('always', '').trim()
      }

      // FALL THROUGH to here means we need to add an EventListener

      // Apply the listener to the node
      node.addEventListener(on, function(evt) {
        if (node.preloadState === 'PAUSE') { // Only add one event listener
          node.preloadState = 'READY' // Required for the `load` function to trigger

          // Special handling for "mouseover" events.  Wait 100ms before triggering load.
          if (on === 'mouseover') {
            window.setTimeout(load(node), 100)
          } else {
            load(node)() // all other events trigger immediately.
          }
        }
      })

      // Special handling for certain built-in event handlers
      switch (on) {
        case 'mouseover':
          // Mirror `touchstart` events (fires immediately)
          node.addEventListener('touchstart', load(node))

          // WHhen the mouse leaves, immediately disable the preload
          node.addEventListener('mouseout', function(evt) {
            if ((evt.target === node) && (node.preloadState === 'READY')) {
              node.preloadState = 'PAUSE'
            }
          })
          break

        case 'mousedown':
          // Mirror `touchstart` events (fires immediately)
          node.addEventListener('touchstart', load(node))
          break
      }

      // Mark the node as ready to run.
      node.preloadState = 'PAUSE'
      node.preloadAlways = always
      htmx.trigger(node, 'preload:init') // This event can be used to load content immediately.
    }

    // Search for all child nodes that have a "preload" attribute
    event.target.querySelectorAll('[preload]').forEach(function(node) {
      // Initialize the node with the "preload" attribute
      init(node)

      // Initialize all child elements that are anchors or have `hx-get` (use with care)
      node.querySelectorAll('a,[hx-get],[data-hx-get]').forEach(init)
    })
  }
})