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<div id="content">
<h1 class="title">Multiple Service Sanity with ClojureScript
<br />
<span class="subtitle">Part 1 - An Introduction to Protocols</span>
</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#org03e6c3e">1. Introduction</a></li>
<li><a href="#org449f779">2. Protocols and Classes</a>
<ul>
<li><a href="#orgee537a2">2.1. Classes</a></li>
<li><a href="#orgece7fe5">2.2. Protocols</a></li>
</ul>
</li>
<li><a href="#org0df85db">3. Protocol Declaration</a></li>
<li><a href="#orge6bba73">4. Protocol Implementation</a>
<ul>
<li><a href="#org09a4eb0">4.1. Google Maps</a></li>
<li><a href="#org8fa3651">4.2. Mapbox</a></li>
</ul>
</li>
<li><a href="#orgd8cdba5">5. Final Remarks</a></li>
</ul>
</div>
</div>
<div id="outline-container-org03e6c3e" class="outline-2">
<h2 id="org03e6c3e"><span class="section-number-2">1</span> Introduction</h2>
<div class="outline-text-2" id="text-1">
<p>
In early 2018, google announced a change in the pricing of their maps services,
finally implementing it half a year later. The new prices would have been
prohibitively expensive for many tech companies (think thousands of U.S. dollars a
month) and many scrambled to find alternatives. Along with everyone else who was
affected, we had to:
</p>
<ol class="org-ol">
<li>Find alternative service providers for each google maps service we used</li>
<li>Compare pricing, quality, and licensing (Many providers, while good, forbade
the combination of their service with others. Even more maddening, few
actually support the Philippines.)</li>
<li>Decide, and then implement across all affected platforms.</li>
</ol>
<p>
The problem is, of course, that step 3 is a lot of engineering effort, and steps
1 and 2 take time. When customer satisfaction and operation costs are on the
line, one does not simply say:
</p>
<blockquote>
<p>
"Nah, fam. I already implemented the thing. We're stuck with this suboptimal
service because it would take too much effort to switch."
</p>
</blockquote>
<p>
We can't wait 5 months for a final decision either.
</p>
<p>
The solution then, is to start early and build out infrastructure to reasonably
support every alternative.
</p>
<div class="figure">
<p><img src="https://i.imgflip.com/2peutr.jpg" alt="2peutr.jpg" />
</p>
<p><span class="figure-number">Figure 1: </span>Sometimes, overengineering IS the answer.</p>
</div>
<p>
We use two major features: the map and the geocoding service. We'll be focusing
on the map because it's "read only". Information only flows from the app state
into the map, and not the other way around. As for forward and reverse
geocoding, that's for the next post in the series.
</p>
<p>
We'll be making a very simple toy that switches between two mapping services
during runtime:
</p>
<iframe style="width: 512px; height: 548px"= src="https://by-implication.github.io/multimap-sanity/";></iframe>
</div>
</div>
<div id="outline-container-org449f779" class="outline-2">
<h2 id="org449f779"><span class="section-number-2">2</span> Protocols and Classes</h2>
<div class="outline-text-2" id="text-2">
<p>
Modern mapping libraries have roughly the same features. Beyond displaying the
map itself, one can expect to place markers or pins, draw lines and shapes, and
control what part of the map is shown. While these features are relatively
consistent, their APIs vary greatly across alternatives.
</p>
<p>
Javascript's prototypes and classes may help bring sanity to this, but Clojure
(and therefore Clojure cript) offers a slightly different approach with
protocols. Instead of dealing with a hierarchy, you deal with characteristics.
I'll give a quick example to illustrate the differences.
</p>
</div>
<div id="outline-container-orgee537a2" class="outline-3">
<h3 id="orgee537a2"><span class="section-number-3">2.1</span> Classes</h3>
<div class="outline-text-3" id="text-2-1">
<p>
The following is a sample class hierarchy involving pigeons, penguins, bats, and whales.
</p>
<ul class="org-ul">
<li>Animals
<ul class="org-ul">
<li>Birds
<ul class="org-ul">
<li>Flying Birds
<ul class="org-ul">
<li>Pigeons</li>
</ul></li>
<li>Aquatic Birds
<ul class="org-ul">
<li>Penguins</li>
</ul></li>
</ul></li>
<li>Mammals
<ul class="org-ul">
<li>Flying Mammals
<ul class="org-ul">
<li>Bats</li>
</ul></li>
<li>Aquatic Mammals
<ul class="org-ul">
<li>Whales</li>
</ul></li>
</ul></li>
</ul></li>
</ul>
<p>
Because a class hierarchy is a tree, the sharing of attributes and methods
outside of the parent-child relationship is, depending on the language, anywhere
between complicated and impossible.
</p>
</div>
</div>
<div id="outline-container-orgece7fe5" class="outline-3">
<h3 id="orgece7fe5"><span class="section-number-3">2.2</span> Protocols</h3>
<div class="outline-text-3" id="text-2-2">
<p>
In contrast, the same set of animals can be described in this way:
</p>
<ul class="org-ul">
<li>Pigeons (Flying, Avian)</li>
<li>Penguins (Aquatic, Avian)</li>
<li>Bats (Flying, Mammalian)</li>
<li>Whales (Aquatic, Mammalian)</li>
</ul>
<p>
While the Flying protocol shared by pigeons and bats may declare a set of
methods like <code>fly</code>, <code>land</code>, and <code>maneuver</code>, implementation will be specific to
pigeons and bats. (of course! they work differently.)
</p>
<p>
Protocols lend themselves very well to composition, but sometimes it's harder to
see structure. As the saying goes, use the right tool for the right job.
</p>
<div class="figure">
<p><img src="https://imgs.xkcd.com/comics/lisp_cycles.png" alt="lisp_cycles.png" />
</p>
<p><span class="figure-number">Figure 2: </span>Except for lisp. Lisp is always the right tool.</p>
</div>
<p>
Kidding aside, the great thing about Clojure and ClojureScript being hosted
languages is that you kind of get the best of both worlds. If the situation
calls for it, you can always dig down into interop and use the respective class
systems of Java and JavaScript.
</p>
</div>
</div>
</div>
<div id="outline-container-org0df85db" class="outline-2">
<h2 id="org0df85db"><span class="section-number-2">3</span> Protocol Declaration</h2>
<div class="outline-text-2" id="text-3">
<p>
The first step is in declaring the protocols. There are many ways of organizing
protocols and namespaces, but I like separating protocol declaration from
implementation. This is how I've structured the project below.
</p>
<p>
Please read the inline documentation and comments–they're part of this article!
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span><span style="color: #a020f0;">ns</span> <span style="color: #228b22;">sanity.protocols</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defprotocol</span> <span style="color: #228b22;">MapLike</span>
<span style="color: #8b2252;">"Things-that-are-like-maps."</span>
<span style="color: #7388d6;">(</span>get-dom-node <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #8b2252;">"Returns the dom-node that this map is attached to."</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>add-marker! <span style="color: #909183;">[</span>this marker-config<span style="color: #909183;">]</span>
<span style="color: #8b2252;">"For now, marker-config only has one relevant key: `</span><span style="color: #008b8b;">:position</span><span style="color: #8b2252;">`"</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>add-polyline! <span style="color: #909183;">[</span>this polyline-config<span style="color: #909183;">]</span>
<span style="color: #8b2252;">"Three relevant keys: `</span><span style="color: #008b8b;">:path</span><span style="color: #8b2252;">`, `</span><span style="color: #008b8b;">:stroke-color</span><span style="color: #8b2252;">`, `</span><span style="color: #008b8b;">:stroke-weight</span><span style="color: #8b2252;">`"</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defprotocol</span> <span style="color: #228b22;">MapEntity</span>
<span style="color: #8b2252;">"For anything that will appear on a map."</span>
<span style="color: #7388d6;">(</span>destroy! <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #8b2252;">"Everything can be destroyed!"</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>set-opacity! <span style="color: #909183;">[</span>this opacity<span style="color: #909183;">]</span>
<span style="color: #8b2252;">"Most map providers allow you to control the opacity of map entities.</span>
<span style="color: #8b2252;">This method can be moved to more specific protocols, but it can stay here."</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defprotocol</span> <span style="color: #228b22;">MapSingleEntity</span>
<span style="color: #8b2252;">"Entities that exist as single points on a map, as opposed to lines and polygons."</span>
<span style="color: #7388d6;">(</span>set-position! <span style="color: #909183;">[</span>this position<span style="color: #909183;">]</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
</div>
</div>
<div id="outline-container-orge6bba73" class="outline-2">
<h2 id="orge6bba73"><span class="section-number-2">4</span> Protocol Implementation</h2>
<div class="outline-text-2" id="text-4">
<p>
And now we can implement these protocols as appropriate. Note that in the
following section, I will be using <code>extend-type</code>, which modifies an existing
type (like <code>google.maps.Marker</code>) with additional methods that conform to the
attached protocols. <code>extend-type</code> is not the only way to use protocols: there's
<code>defrecord</code> and <code>reify</code>, and while they're safer to use than <code>extend-type</code>,
<code>extend-type</code> makes a lot of things more convenient for us. Instead of having to
store the native javascript object as a field in a ClojureScript data structure,
we can just use the object itself.
</p>
</div>
<div id="outline-container-org09a4eb0" class="outline-3">
<h3 id="org09a4eb0"><span class="section-number-3">4.1</span> Google Maps</h3>
<div class="outline-text-3" id="text-4-1">
<p>
Let's declare a separate namespace
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span><span style="color: #a020f0;">ns</span> <span style="color: #228b22;">sanity.google</span>
<span style="color: #7388d6;">(</span><span style="color: #008b8b;">:require</span> <span style="color: #909183;">[</span>sanity.protocols <span style="color: #008b8b;">:as</span> sp<span style="color: #909183;">]</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
And then extend the marker type. A marker is both a generic map entity, but it's
also a single map entity representing a single point on the map.
</p>
<p>
We are returning nil in <code>destroy!</code> because it should no longer exist.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span>extend-type <span style="color: #228b22;">js</span>/google.maps.Marker
<span style="color: #228b22;">sp</span>/MapEntity
<span style="color: #7388d6;">(</span>destroy! <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.setMap this <span style="color: #008b8b;">nil</span><span style="color: #909183;">)</span>
<span style="color: #008b8b;">nil</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>set-opacity! <span style="color: #909183;">[</span>this opacity<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.setOpacity this opacity<span style="color: #909183;">)</span>
this<span style="color: #7388d6;">)</span>
<span style="color: #228b22;">sp</span>/MapSingleEntity
<span style="color: #7388d6;">(</span>set-position! <span style="color: #909183;">[</span>this position<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.setPosition this position<span style="color: #909183;">)</span>
this<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
In contrast, the polyline does <i>not</i> represent a single point, so
<code>set-position!</code> makes no sense. So, we only want to extend polyline with just
the <code>MapEntry</code> protocol.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span>extend-type <span style="color: #228b22;">js</span>/google.maps.Polyline
<span style="color: #228b22;">sp</span>/MapEntity
<span style="color: #7388d6;">(</span>destroy! <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.setMap this <span style="color: #008b8b;">nil</span><span style="color: #909183;">)</span>
<span style="color: #008b8b;">nil</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>set-opacity! <span style="color: #909183;">[</span>this opacity<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.setOptions this #js <span style="color: #707183;">{</span><span style="color: #008b8b;">:strokeOpacity</span> opacity<span style="color: #707183;">}</span><span style="color: #909183;">)</span>
this<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
Probably the most straightforward implementation here is <code>get-dom-node</code>,
effectively an alias for <code>getDiv</code>. We go through this trouble because we want a
consistent API for all maps we could possibly want to use.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span>extend-type <span style="color: #228b22;">js</span>/google.maps.Map
<span style="color: #228b22;">sp</span>/MapLike
<span style="color: #7388d6;">(</span>get-dom-node <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.getDiv this<span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>add-marker! <span style="color: #909183;">[</span>this marker-config<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #228b22;">js</span>/google.maps.Marker. <span style="color: #707183;">(</span>clj->js <span style="color: #7388d6;">(</span>assoc marker-config
<span style="color: #008b8b;">:map</span> this<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>add-polyline! <span style="color: #909183;">[</span>this polyline-config<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #228b22;">js</span>/google.maps.Polyline. <span style="color: #707183;">(</span>clj->js <span style="color: #7388d6;">(</span>assoc polyline-config
<span style="color: #008b8b;">:map</span> this<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
Finally, we explicitly define a <code>new-google-map</code> function to instantiate a…
new google map. We could have named this as simply <code>new-map</code> to allow for
consistency of this "constructor" function across the other services, but in
this case it's important to be very clear about what it is you're constructing.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span><span style="color: #a020f0;">defn</span> <span style="color: #0000ff;">new-google-map</span> <span style="color: #7388d6;">[</span>map-config<span style="color: #7388d6;">]</span>
<span style="color: #7388d6;">(</span><span style="color: #a020f0;">let</span> <span style="color: #909183;">[</span><span style="color: #707183;">{</span><span style="color: #008b8b;">:keys</span> <span style="color: #7388d6;">[</span>dom-node center zoom<span style="color: #7388d6;">]</span><span style="color: #707183;">}</span> map-config<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #228b22;">js</span>/google.maps.Map. dom-node
#js <span style="color: #707183;">{</span><span style="color: #008b8b;">:center</span> <span style="color: #7388d6;">(</span>clj->js center<span style="color: #7388d6;">)</span>
<span style="color: #008b8b;">:zoom</span> zoom<span style="color: #707183;">}</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
</div>
</div>
<div id="outline-container-org8fa3651" class="outline-3">
<h3 id="org8fa3651"><span class="section-number-3">4.2</span> Mapbox</h3>
<div class="outline-text-3" id="text-4-2">
<p>
Now let's implement the same protocol for mapbox. Notice that in our namespace
declaration, <code>mapbox-gl</code> is in the require statement. Unlike google maps, mapbox
exists as an npm library. Unfortunately, it's huge–it takes up about 23% the
size of final sakay webapp artifact, (in contrast, ClojureScript takes up 17%,
and the sakay-specific code is another 17%) but there's not much we can do about that.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span><span style="color: #a020f0;">ns</span> <span style="color: #228b22;">sanity.mapbox</span>
<span style="color: #7388d6;">(</span><span style="color: #008b8b;">:require</span> <span style="color: #909183;">[</span>sanity.protocols <span style="color: #008b8b;">:as</span> sp<span style="color: #909183;">]</span>
<span style="color: #909183;">[</span><span style="color: #8b2252;">"mapbox-gl"</span> <span style="color: #008b8b;">:as</span> mapbox<span style="color: #909183;">]</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
Mapbox markers are interesting. They're actually dom elements that are
positioned relative to the map. So, to set the marker's opacity, we can just
apply css styling. I'll leave the implementation of this method as an exercise
for the reader.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span>extend-type <span style="color: #228b22;">mapbox</span>/Marker
<span style="color: #228b22;">sp</span>/MapEntity
<span style="color: #7388d6;">(</span>destroy! <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.remove this<span style="color: #909183;">)</span>
<span style="color: #008b8b;">nil</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>set-opacity! <span style="color: #909183;">[</span>this opacity<span style="color: #909183;">]</span>
this<span style="color: #7388d6;">)</span>
<span style="color: #228b22;">sp</span>/MapSingleEntity
<span style="color: #7388d6;">(</span>set-position! <span style="color: #909183;">[</span>this <span style="color: #707183;">{</span><span style="color: #008b8b;">:keys</span> <span style="color: #7388d6;">[</span>lat lng<span style="color: #7388d6;">]</span><span style="color: #707183;">}</span><span style="color: #909183;">]</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">Unfortunately, mapbox and google maps do not agree</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">on the representation of map positions.</span>
<span style="color: #909183;">(</span>.setLngLat this #js <span style="color: #707183;">{</span><span style="color: #008b8b;">:lon</span> lng <span style="color: #008b8b;">:lat</span> lat<span style="color: #707183;">}</span><span style="color: #909183;">)</span>
this<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
Mapbox doesn't have polylines as a first-class entity, so we have to make a
record that implements the appropriate protocols. You can think of records as
clojure maps that have methods associated with them.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span><span style="color: #a020f0;">defrecord</span> <span style="color: #228b22;">MapboxPolyline</span> <span style="color: #7388d6;">[</span>street-map id<span style="color: #7388d6;">]</span>
<span style="color: #228b22;">sp</span>/MapEntity
<span style="color: #7388d6;">(</span>destroy! <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #a020f0;">when</span> <span style="color: #707183;">(</span>.getLayer street-map id<span style="color: #707183;">)</span>
<span style="color: #707183;">(</span>.removeLayer street-map id<span style="color: #707183;">)</span><span style="color: #909183;">)</span>
<span style="color: #909183;">(</span><span style="color: #a020f0;">when</span> <span style="color: #707183;">(</span>.getSource street-map id<span style="color: #707183;">)</span>
<span style="color: #707183;">(</span>.removeSource street-map id<span style="color: #707183;">)</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>set-opacity! <span style="color: #909183;">[</span>this opacity<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.setPaintProperty street-map id <span style="color: #8b2252;">"line-opacity"</span> opacity<span style="color: #909183;">)</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
And since polylines aren't a first-class entity, "constructing" it is a little
different. Also notice the trivially simple implementation for <code>get-dom-node</code>.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span>extend-type <span style="color: #228b22;">mapbox</span>/Map
<span style="color: #228b22;">sp</span>/MapLike
<span style="color: #7388d6;">(</span>get-dom-node <span style="color: #909183;">[</span>this<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.getContainer this<span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>add-marker! <span style="color: #909183;">[</span>this <span style="color: #707183;">{</span><span style="color: #008b8b;">:keys</span> <span style="color: #7388d6;">[</span>position<span style="color: #7388d6;">]</span> <span style="color: #008b8b;">:as</span> marker-config<span style="color: #707183;">}</span><span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #a020f0;">-></span> <span style="color: #707183;">(</span><span style="color: #228b22;">mapbox</span>/Marker. #js <span style="color: #7388d6;">{}</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #228b22;">sp</span>/set-position! position<span style="color: #707183;">)</span>
<span style="color: #707183;">(</span>.addTo this<span style="color: #707183;">)</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>add-polyline! <span style="color: #909183;">[</span>this <span style="color: #707183;">{</span>path <span style="color: #008b8b;">:path</span>
stroke-color <span style="color: #008b8b;">:strokeColor</span>
stroke-weight <span style="color: #008b8b;">:strokeWeight</span>
<span style="color: #008b8b;">:as</span> polyline-config<span style="color: #707183;">}</span><span style="color: #909183;">]</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">This bit is long because mapbox treats polylines differently from google maps.</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">Remember what I said about vastly different APIs? :P</span>
<span style="color: #909183;">(</span><span style="color: #a020f0;">let</span> <span style="color: #707183;">[</span>polyline-id <span style="color: #7388d6;">(</span>random-uuid<span style="color: #7388d6;">)</span>
line-source <span style="color: #7388d6;">{</span><span style="color: #008b8b;">:type</span> <span style="color: #8b2252;">"geojson"</span>
<span style="color: #008b8b;">:data</span> <span style="color: #909183;">{</span><span style="color: #008b8b;">:type</span> <span style="color: #8b2252;">"Feature"</span>
<span style="color: #008b8b;">:geometry</span> <span style="color: #707183;">{</span><span style="color: #008b8b;">:type</span> <span style="color: #8b2252;">"LineString"</span>
<span style="color: #008b8b;">:properties</span> <span style="color: #7388d6;">{}</span>
<span style="color: #008b8b;">:coordinates</span> <span style="color: #7388d6;">(</span>map <span style="color: #909183;">(</span><span style="color: #a020f0;">fn</span> <span style="color: #707183;">[</span><span style="color: #7388d6;">{</span><span style="color: #008b8b;">:keys</span> <span style="color: #909183;">[</span>lat lng<span style="color: #909183;">]</span><span style="color: #7388d6;">}</span><span style="color: #707183;">]</span>
<span style="color: #707183;">[</span>lng lat<span style="color: #707183;">]</span><span style="color: #909183;">)</span>
path<span style="color: #7388d6;">)</span><span style="color: #707183;">}</span><span style="color: #909183;">}</span><span style="color: #7388d6;">}</span>
line-layer <span style="color: #7388d6;">(</span>clj->js
<span style="color: #909183;">{</span><span style="color: #008b8b;">:id</span> polyline-id
<span style="color: #008b8b;">:type</span> <span style="color: #8b2252;">"line"</span>
<span style="color: #008b8b;">:layout</span> <span style="color: #707183;">{</span><span style="color: #008b8b;">:line-join</span> <span style="color: #8b2252;">"round"</span>
<span style="color: #008b8b;">:line-cap</span> <span style="color: #8b2252;">"round"</span><span style="color: #707183;">}</span>
<span style="color: #008b8b;">:paint</span> <span style="color: #707183;">{</span><span style="color: #008b8b;">:line-color</span> stroke-color
<span style="color: #008b8b;">:line-width</span> stroke-weight<span style="color: #707183;">}</span>
<span style="color: #008b8b;">:source</span> line-source<span style="color: #909183;">}</span><span style="color: #7388d6;">)</span><span style="color: #707183;">]</span>
<span style="color: #707183;">(</span>.addLayer this line-layer<span style="color: #707183;">)</span>
<span style="color: #707183;">(</span>map->MapboxPolyline <span style="color: #7388d6;">{</span><span style="color: #008b8b;">:street-map</span> this
<span style="color: #008b8b;">:id</span> polyline-id<span style="color: #7388d6;">}</span><span style="color: #707183;">)</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defn</span> <span style="color: #0000ff;">new-mapbox-map</span> <span style="color: #7388d6;">[</span><span style="color: #909183;">{</span><span style="color: #008b8b;">:keys</span> <span style="color: #707183;">[</span>dom-node center zoom style<span style="color: #707183;">]</span><span style="color: #909183;">}</span><span style="color: #7388d6;">]</span>
<span style="color: #7388d6;">(</span><span style="color: #a020f0;">let</span> <span style="color: #909183;">[</span><span style="color: #707183;">{</span><span style="color: #008b8b;">:keys</span> <span style="color: #7388d6;">[</span>lat lng<span style="color: #7388d6;">]</span><span style="color: #707183;">}</span> center<span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #228b22;">mapbox</span>/Map. #js <span style="color: #707183;">{</span><span style="color: #008b8b;">:container</span> dom-node
<span style="color: #008b8b;">:center</span> #js <span style="color: #7388d6;">[</span>lng lat<span style="color: #7388d6;">]</span>
<span style="color: #008b8b;">:zoom</span> zoom
<span style="color: #008b8b;">:style</span> <span style="color: #8b2252;">"https://tiles.stadiamaps.com/styles/alidade_smooth.json"</span><span style="color: #707183;">}</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
Now that our implementation is complete for both providers, we can use them.
</p>
<div class="org-src-container">
<pre class="src src-clojure"><span style="color: #707183;">(</span><span style="color: #a020f0;">ns</span> <span style="color: #228b22;">sanity.core</span>
<span style="color: #7388d6;">(</span><span style="color: #008b8b;">:require</span> <span style="color: #909183;">[</span>sanity.protocols <span style="color: #008b8b;">:as</span> sp<span style="color: #909183;">]</span>
<span style="color: #909183;">[</span>sanity.google<span style="color: #909183;">]</span>
<span style="color: #909183;">[</span>sanity.mapbox<span style="color: #909183;">]</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">def</span> <span style="color: #a0522d;">use-google</span> <span style="color: #8b2252;">"So we can switch between google and mapbox."</span> <span style="color: #7388d6;">(</span>atom <span style="color: #008b8b;">false</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">def</span> <span style="color: #a0522d;">app-map</span> <span style="color: #8b2252;">"Stores the map object. Can be either google or mapbox."</span> <span style="color: #7388d6;">(</span>atom <span style="color: #008b8b;">nil</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defn</span> <span style="color: #0000ff;">init-map</span> <span style="color: #7388d6;">[]</span>
<span style="color: #7388d6;">(</span><span style="color: #a020f0;">let</span> <span style="color: #909183;">[</span>map-config <span style="color: #707183;">{</span><span style="color: #008b8b;">:dom-node</span> <span style="color: #7388d6;">(</span><span style="color: #228b22;">js</span>/document.getElementById <span style="color: #8b2252;">"map"</span><span style="color: #7388d6;">)</span>
<span style="color: #008b8b;">:zoom</span> <span style="color: #008b8b;">12</span>
<span style="color: #008b8b;">:center</span> <span style="color: #7388d6;">{</span><span style="color: #008b8b;">:lat</span> <span style="color: #008b8b;">14.6091</span>
<span style="color: #008b8b;">:lng</span> <span style="color: #008b8b;">121.0223</span><span style="color: #7388d6;">}</span><span style="color: #707183;">}</span><span style="color: #909183;">]</span>
<span style="color: #909183;">(</span><span style="color: #a020f0;">if</span> @use-google
<span style="color: #707183;">(</span><span style="color: #228b22;">sanity.google</span>/new-google-map map-config<span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #228b22;">sanity.mapbox</span>/new-mapbox-map map-config<span style="color: #707183;">)</span><span style="color: #909183;">)</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defn</span> <span style="color: #0000ff;">setup</span> <span style="color: #7388d6;">[]</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">Replace the `</span><span style="color: #008b8b;">app-map</span><span style="color: #b22222;">` atom with the value of a newly initialized map.</span>
<span style="color: #7388d6;">(</span>reset! app-map <span style="color: #909183;">(</span>init-map<span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">Notice that it doesn't care if the map is google or mapbox.</span>
<span style="color: #b22222;">;; </span><span style="color: #b22222;">The correct implementation will be used regardless.</span>
<span style="color: #7388d6;">(</span><span style="color: #228b22;">sp</span>/add-marker! @app-map <span style="color: #909183;">{</span><span style="color: #008b8b;">:position</span> <span style="color: #707183;">{</span><span style="color: #008b8b;">:lat</span> <span style="color: #008b8b;">14.6091</span>
<span style="color: #008b8b;">:lng</span> <span style="color: #008b8b;">121.0223</span><span style="color: #707183;">}</span><span style="color: #909183;">}</span><span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defn</span> ^<span style="color: #008b8b;">:export</span> <span style="color: #0000ff;">switch-provider</span> <span style="color: #7388d6;">[]</span>
<span style="color: #7388d6;">(</span>swap! use-google not<span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>setup<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
<span style="color: #707183;">(</span><span style="color: #a020f0;">defn</span> <span style="color: #0000ff;">init</span> <span style="color: #7388d6;">[]</span>
<span style="color: #7388d6;">(</span><span style="color: #a020f0;">let</span> <span style="color: #909183;">[</span>switch-button <span style="color: #707183;">(</span><span style="color: #228b22;">js</span>/document.getElementById <span style="color: #8b2252;">"switch-button"</span><span style="color: #707183;">)</span><span style="color: #909183;">]</span>
<span style="color: #909183;">(</span>.addEventListener switch-button <span style="color: #8b2252;">"click"</span> switch-provider<span style="color: #909183;">)</span><span style="color: #7388d6;">)</span>
<span style="color: #7388d6;">(</span>setup<span style="color: #7388d6;">)</span><span style="color: #707183;">)</span>
</pre>
</div>
<p>
At the time of writing, there is a <code>#wontfix</code> <a href="https://issuetracker.google.com/issues/35821412#comment32">memory leak</a> in google maps making
it difficult to properly destroy a map instance. Having said this, I need to
point out a few important things regarding this leak, and the exercise we just
did.
</p>
<ol class="org-ol">
<li>The google map we're instantiating here is very bare, and the memory leak is
unlikely to affect this demo app much. For an actually useful webapp though,
this leak will be non-trivial.</li>
<li>You probably shouldn't even be switching between different map providers
during runtime. Just comment things out, and refresh the browser. Our runtime
switch is for illustrative purposes only.</li>
<li>If things are really that bad that you need to support service switching
right up to deployment, you can come up with compiler flags via <code>goog-define</code>
that are basically variables that can be initialized based on build flags.</li>
</ol>
</div>
</div>
</div>
<div id="outline-container-orgd8cdba5" class="outline-2">
<h2 id="orgd8cdba5"><span class="section-number-2">5</span> Final Remarks</h2>
<div class="outline-text-2" id="text-5">
<p>
Protocols are a powerful tool for creating and managing abstractions or
interfaces to external services. They are not limited to our example above.
Protocols can be used for database connections, display rendering, and more.
</p>
<p>
In the next part of this series, I'll show you how we dealt with different
geocoding providers using a webapp framework called fulcro.
</p>
<p>
Finally, here is the working sample again, for your reference:
</p>
<iframe style="width: 512px; height: 548px"= src="https://by-implication.github.io/multimap-sanity/";></iframe>
</div>
</div>
</div>
<div id="postamble" class="status">
<p class="author">Author: Levi Tan Ong</p>
<p class="date">Created: 2019-01-10 Thu 16:23</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
</div>
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