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<article>
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    <h1>A microdata enhanced HTML Webcomponent for Leaflet</h1>
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  <p>The people in my RSS reader have been talking some more about Web Components in the 
past couple of months. Jim Nielsen had <a href="https://blog.jim-nielsen.com/2023/html-web-components/">a post</a>
referring to Jeremy Keith coining the term <a href="https://adactio.com/journal/20618">HTML Web Components</a>
for a special kind of Web Components. The idea essentially is that you have regular HTML markup that is wrapped
by a custom Web Component which enhances the user experience.</p>

<p>I thought that this was quite an interesting idea and thought about situations in which
it could be applied. Working with open data quite a bit, we often find ourselves in 
situations where we build maps. Additionally, the German open data scene has been lobbying for
more linked open data in the past couple of months. I think I came up with an example which
combines these three things - HTML Web Components, maps and Linked Data (or a flavor 
thereof) quite nicely.</p>

<p>As an example, assume that we want to render schools on a map. The data in
our example is taken from our <a href="https://jedeschule.codefor.de/docs">jedeschule.de API</a> 
which has the goal of making all primary and secondary schools in Germany
searchable and queryable.</p>

<p>With the approach I’m proposing, we can author markup that looks like this:</p>

<div class="language-html highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;leaflet-map&gt;</span>
  <span class="nt">&lt;div</span> <span class="na">id=</span><span class="s">"map"</span><span class="nt">&gt;&lt;/div&gt;</span>
  <span class="nt">&lt;ul</span> <span class="na">class=</span><span class="s">"locations"</span><span class="nt">&gt;</span>
      <span class="nt">&lt;li</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/School"</span> <span class="na">data-fresh-key=</span><span class="s">"NW-153710"</span><span class="nt">&gt;</span>
          <span class="nt">&lt;h2</span> <span class="na">itemprop=</span><span class="s">"name"</span><span class="nt">&gt;</span>Dahlingschule, Städt. Förderschule im integr. Verbund, FSP Lernen u. Emot. und soziale Entwicklung,-Primarstufe u. SekI<span class="nt">&lt;/h2&gt;</span>
          <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"address"</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/PostalAddress"</span><span class="nt">&gt;</span>
              <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"streetAddress"</span><span class="nt">&gt;</span>Dahlingstr. 40<span class="nt">&lt;/div&gt;</span>
              <span class="nt">&lt;span</span> <span class="na">itemprop=</span><span class="s">"postalCode"</span><span class="nt">&gt;</span>47229<span class="nt">&lt;/span&gt;</span>
              <span class="nt">&lt;span</span> <span class="na">itemprop=</span><span class="s">"addressLocality"</span><span class="nt">&gt;</span>Duisburg<span class="nt">&lt;/span&gt;</span>
          <span class="nt">&lt;/div&gt;</span>
          <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"geo"</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/GeoCoordinates"</span><span class="nt">&gt;</span>
              <span class="nt">&lt;meta</span> <span class="na">itemprop=</span><span class="s">"latitude"</span> <span class="na">content=</span><span class="s">"51.38331874331818"</span><span class="nt">/&gt;</span>
              <span class="nt">&lt;meta</span> <span class="na">itemprop=</span><span class="s">"longitude"</span> <span class="na">content=</span><span class="s">"6.700606101666003"</span><span class="nt">/&gt;</span>
          <span class="nt">&lt;/div&gt;</span>
      <span class="nt">&lt;/li&gt;</span>
      <span class="nt">&lt;li</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/School"</span> <span class="na">data-fresh-key=</span><span class="s">"NW-166480"</span><span class="nt">&gt;</span>
          <span class="nt">&lt;h2</span> <span class="na">itemprop=</span><span class="s">"name"</span><span class="nt">&gt;</span>Montessori-Gymnasium Städt. Gymnasium für Jungen und Mädchen<span class="nt">&lt;/h2&gt;</span>
          <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"address"</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/PostalAddress"</span><span class="nt">&gt;</span>
              <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"streetAddress"</span><span class="nt">&gt;</span>Rochusstr. 145<span class="nt">&lt;/div&gt;</span>
              <span class="nt">&lt;span</span> <span class="na">itemprop=</span><span class="s">"postalCode"</span><span class="nt">&gt;</span>50827<span class="nt">&lt;/span&gt;</span>
              <span class="nt">&lt;span</span> <span class="na">itemprop=</span><span class="s">"addressLocality"</span><span class="nt">&gt;</span>Köln<span class="nt">&lt;/span&gt;</span>
          <span class="nt">&lt;/div&gt;</span>
          <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"geo"</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/GeoCoordinates"</span><span class="nt">&gt;</span>
              <span class="nt">&lt;meta</span> <span class="na">itemprop=</span><span class="s">"latitude"</span> <span class="na">content=</span><span class="s">"50.963204818343755"</span><span class="nt">/&gt;</span>
              <span class="nt">&lt;meta</span> <span class="na">itemprop=</span><span class="s">"longitude"</span> <span class="na">content=</span><span class="s">"6.904840537750957"</span><span class="nt">/&gt;</span>
          <span class="nt">&lt;/div&gt;</span>
      <span class="nt">&lt;/li&gt;</span>
      <span class="nt">&lt;li</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/School"</span> <span class="na">data-fresh-key=</span><span class="s">"NW-167782"</span><span class="nt">&gt;</span>
          <span class="nt">&lt;h2</span> <span class="na">itemprop=</span><span class="s">"name"</span><span class="nt">&gt;</span>Städt. Grillo-Gymnasium<span class="nt">&lt;/h2&gt;</span>
          <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"address"</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/PostalAddress"</span><span class="nt">&gt;</span>
              <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"streetAddress"</span><span class="nt">&gt;</span>Hauptstr. 60<span class="nt">&lt;/div&gt;</span>
              <span class="nt">&lt;span</span> <span class="na">itemprop=</span><span class="s">"postalCode"</span><span class="nt">&gt;</span>45879<span class="nt">&lt;/span&gt;</span>
              <span class="nt">&lt;span</span> <span class="na">itemprop=</span><span class="s">"addressLocality"</span><span class="nt">&gt;</span>Gelsenkirchen<span class="nt">&lt;/span&gt;</span>
          <span class="nt">&lt;/div&gt;</span>
          <span class="nt">&lt;div</span> <span class="na">itemprop=</span><span class="s">"geo"</span> <span class="na">itemscope</span> <span class="na">itemtype=</span><span class="s">"https://schema.org/GeoCoordinates"</span><span class="nt">&gt;</span>
              <span class="nt">&lt;meta</span> <span class="na">itemprop=</span><span class="s">"latitude"</span> <span class="na">content=</span><span class="s">"51.5130837882258"</span><span class="nt">/&gt;</span>
              <span class="nt">&lt;meta</span> <span class="na">itemprop=</span><span class="s">"longitude"</span> <span class="na">content=</span><span class="s">"7.099798427442939"</span><span class="nt">/&gt;</span>
          <span class="nt">&lt;/div&gt;</span>
      <span class="nt">&lt;/li&gt;</span>
  <span class="nt">&lt;/ul&gt;</span>
<span class="nt">&lt;/leaflet-map&gt;</span>
<span class="nt">&lt;style&gt;</span>
<span class="nf">#map</span> <span class="p">{</span>
    <span class="nl">width</span><span class="p">:</span> <span class="m">800px</span><span class="p">;</span>
    <span class="nl">height</span><span class="p">:</span> <span class="nb">auto</span><span class="p">;</span>
    <span class="py">aspect-ratio</span><span class="p">:</span> <span class="m">16</span><span class="p">/</span><span class="m">9</span><span class="p">;</span>
    <span class="nl">background-image</span><span class="p">:</span> <span class="sx">url("/map.png")</span><span class="p">;</span>
    <span class="nl">background-size</span><span class="p">:</span> <span class="n">contain</span><span class="p">;</span>
<span class="p">}</span>
<span class="nt">&lt;/style&gt;</span>
</code></pre></div></div>

<p>In this form, this isn’t interactive yet but it can already be easily consumed by both humans
and computers. Humans will see a static map (I simply took a screenshot and set is as a
background image for the <code class="highlighter-rouge">#map</code> node) and a list of schools with their addresses. 
Computers will be able to also parse the schema.org annotations to extract structured data out of the list.</p>

<p>Since the data is semi-structured and machine readable, we can now also consume it from
a Web Component to add interactivity with <a href="https://leafletjs.com">Leaflet</a>. The code to do so
looks like this:</p>
<div class="language-js highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">class</span> <span class="nx">LeafletMap</span> <span class="kd">extends</span> <span class="nx">HTMLElement</span> <span class="p">{</span>
  <span class="nx">connectedCallback</span><span class="p">()</span> <span class="p">{</span>
    <span class="kd">const</span> <span class="nx">mapElement</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="s2">"#map"</span><span class="p">);</span>
    <span class="nx">mapElement</span><span class="p">.</span><span class="nx">style</span><span class="p">.</span><span class="nx">backgroundImage</span> <span class="o">=</span> <span class="s2">"none"</span><span class="p">;</span>

    <span class="kd">const</span> <span class="nx">schools</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">querySelectorAll</span><span class="p">(</span>
      <span class="s1">'[itemType="https://schema.org/School"]'</span><span class="p">,</span>
    <span class="p">);</span>

    <span class="kd">var</span> <span class="nx">map</span> <span class="o">=</span> <span class="nx">L</span><span class="p">.</span><span class="nx">map</span><span class="p">(</span><span class="nx">mapElement</span><span class="p">).</span><span class="nx">setView</span><span class="p">([</span><span class="mf">51.505</span><span class="p">,</span> <span class="o">-</span><span class="mf">0.09</span><span class="p">],</span> <span class="mi">13</span><span class="p">);</span>
    <span class="nx">L</span><span class="p">.</span><span class="nx">tileLayer</span><span class="p">(</span><span class="s2">"https://tile.openstreetmap.org/{z}/{x}/{y}.png"</span><span class="p">,</span> <span class="p">{</span>
      <span class="na">attribution</span><span class="p">:</span>
        <span class="s1">'&amp;copy; &lt;a href="https://www.openstreetmap.org/copyright"&gt;OpenStreetMap&lt;/a&gt; contributors'</span><span class="p">,</span>
    <span class="p">}).</span><span class="nx">addTo</span><span class="p">(</span><span class="nx">map</span><span class="p">);</span>

    <span class="kd">const</span> <span class="nx">markers</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">L</span><span class="p">.</span><span class="nx">featureGroup</span><span class="p">();</span>
    <span class="nx">schools</span><span class="p">.</span><span class="nx">forEach</span><span class="p">((</span><span class="nx">school</span><span class="p">)</span> <span class="o">=&gt;</span> <span class="p">{</span>
      <span class="kd">const</span> <span class="nx">latitude</span> <span class="o">=</span> <span class="nx">school</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="s1">'[itemProp="latitude"]'</span><span class="p">).</span><span class="nx">content</span><span class="p">;</span>
      <span class="kd">const</span> <span class="nx">longitude</span> <span class="o">=</span> <span class="nx">school</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="s1">'[itemProp="longitude"]'</span><span class="p">).</span><span class="nx">content</span><span class="p">;</span>
      <span class="kd">const</span> <span class="nx">name</span> <span class="o">=</span> <span class="nx">school</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="s1">'[itemProp="name"]'</span><span class="p">).</span><span class="nx">textContent</span><span class="p">;</span>
      <span class="kd">const</span> <span class="nx">address</span> <span class="o">=</span> <span class="nx">school</span><span class="p">.</span><span class="nx">querySelector</span><span class="p">(</span><span class="s1">'[itemProp="address"]'</span><span class="p">);</span>

      <span class="kd">const</span> <span class="nx">h2</span> <span class="o">=</span> <span class="nb">document</span><span class="p">.</span><span class="nx">createElement</span><span class="p">(</span><span class="s2">"h2"</span><span class="p">);</span>
      <span class="nx">h2</span><span class="p">.</span><span class="nx">innerText</span> <span class="o">=</span> <span class="nx">name</span><span class="p">;</span>
      <span class="kd">const</span> <span class="nx">popup</span> <span class="o">=</span> <span class="nb">document</span><span class="p">.</span><span class="nx">createElement</span><span class="p">(</span><span class="s2">"div"</span><span class="p">);</span>
      <span class="nx">popup</span><span class="p">.</span><span class="nx">classList</span><span class="p">.</span><span class="nx">add</span><span class="p">(</span><span class="s2">"popup"</span><span class="p">);</span>
      <span class="nx">popup</span><span class="p">.</span><span class="nx">appendChild</span><span class="p">(</span><span class="nx">h2</span><span class="p">);</span>
      <span class="nx">popup</span><span class="p">.</span><span class="nx">appendChild</span><span class="p">(</span><span class="nx">address</span><span class="p">.</span><span class="nx">cloneNode</span><span class="p">(</span><span class="kc">true</span><span class="p">));</span>

      <span class="kd">const</span> <span class="nx">marker</span> <span class="o">=</span> <span class="nx">L</span><span class="p">.</span><span class="nx">marker</span><span class="p">([</span><span class="nx">latitude</span><span class="p">,</span> <span class="nx">longitude</span><span class="p">]).</span><span class="nx">bindPopup</span><span class="p">(</span><span class="nx">popup</span><span class="p">);</span>
      <span class="nx">marker</span><span class="p">.</span><span class="nx">addTo</span><span class="p">(</span><span class="nx">markers</span><span class="p">);</span>
    <span class="p">});</span>
    <span class="nx">markers</span><span class="p">.</span><span class="nx">addTo</span><span class="p">(</span><span class="nx">map</span><span class="p">);</span>

    <span class="nx">map</span><span class="p">.</span><span class="nx">fitBounds</span><span class="p">(</span><span class="nx">markers</span><span class="p">.</span><span class="nx">getBounds</span><span class="p">());</span>
  <span class="p">}</span>
<span class="p">}</span>

<span class="nb">window</span><span class="p">.</span><span class="nx">customElements</span><span class="p">.</span><span class="nx">define</span><span class="p">(</span><span class="s2">"leaflet-map"</span><span class="p">,</span> <span class="nx">LeafletMap</span><span class="p">);</span>
</code></pre></div></div>

<p>I quite like the way that this reads. The <code class="highlighter-rouge">itemProp</code>s make for very nice selectors and actually
allow developers to completely change the structure of the HTML. As long as the annotations 
are kept, the Web Component will be able to still extract the locations and to load them into
leaflet.</p>

<p>You can try this on CodePen:</p>

<p>This of course is nowhere from production ready but shall only serve as a proof of concept.
In a more advanced version, we wouldn’t limit our initial query selector to <code class="highlighter-rouge">[itemType="https://schema.org/School]</code>
but would probably want to find a way to query for everything that is a child of <code class="highlighter-rouge">https://schema.org/Place</code>.
We would also need to adapt the static image if we wanted to change the underlying data. If we have server
side rendering at our disposal, we could use something like the <a href="https://docs.mapbox.com/api/maps/static-images/">Mapbox Static Images
API</a> to dynamically create the fallback
images for clients that do not support JavaScript or Web Components.</p>

<p>All in all, this feels like a nice approach to me though. It provides content
that can be consumed by computers and humans with up to date or legacy (or privacy concious) 
devices easily. Using the <code class="highlighter-rouge">itemProp</code>s for query selectors also makes for a nice 
authoring experience with good separation of concerns.</p>

<p>What do you think? Let me know on <a href="https://berlin.social/@knut">mastodon</a>.</p>
</article>


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      <form>
          <fieldset>
              <legend><svg class="icon icon-brightness-contrast">
                <use xlink:href="/static/david/icons2/symbol-defs-2021-12.svg#icon-brightness-contrast"></use>
              </svg> Thème</legend>
              <label>
                  <input type="radio" value="auto" name="chosen-color-scheme" checked> Auto
              </label>
              <label>
                  <input type="radio" value="dark" name="chosen-color-scheme"> Foncé
              </label>
              <label>
                  <input type="radio" value="light" name="chosen-color-scheme"> Clair
              </label>
          </fieldset>
      </form>
  </template>
</footer>
<script src="/static/david/js/instantpage-5.1.0.min.js" type="module"></script>
<script>
    function loadThemeForm(templateName) {
        const themeSelectorTemplate = document.querySelector(templateName)
        const form = themeSelectorTemplate.content.firstElementChild
        themeSelectorTemplate.replaceWith(form)

        form.addEventListener('change', (e) => {
            const chosenColorScheme = e.target.value
            localStorage.setItem('theme', chosenColorScheme)
            toggleTheme(chosenColorScheme)
        })

        const selectedTheme = localStorage.getItem('theme')
        if (selectedTheme && selectedTheme !== 'undefined') {
            form.querySelector(`[value="${selectedTheme}"]`).checked = true
        }
    }

    const prefersColorSchemeDark = '(prefers-color-scheme: dark)'
    window.addEventListener('load', () => {
        let hasDarkRules = false
        for (const styleSheet of Array.from(document.styleSheets)) {
            let mediaRules = []
            for (const cssRule of styleSheet.cssRules) {
                if (cssRule.type !== CSSRule.MEDIA_RULE) {
                    continue
                }
                // WARNING: Safari does not have/supports `conditionText`.
                if (cssRule.conditionText) {
                    if (cssRule.conditionText !== prefersColorSchemeDark) {
                        continue
                    }
                } else {
                    if (cssRule.cssText.startsWith(prefersColorSchemeDark)) {
                        continue
                    }
                }
                mediaRules = mediaRules.concat(Array.from(cssRule.cssRules))
            }

            // WARNING: do not try to insert a Rule to a styleSheet you are
            // currently iterating on, otherwise the browser will be stuck
            // in a infinite loop…
            for (const mediaRule of mediaRules) {
                styleSheet.insertRule(mediaRule.cssText)
                hasDarkRules = true
            }
        }
        if (hasDarkRules) {
            loadThemeForm('#theme-selector')
        }
    })
</script>
</body>
</html>