Create a hover effect on Mapbox

I am sharing my experience in trying to create a hover effect on Mapbox. The first thing to do is to read their example and understand what is going on. Let’s unpack a little bit:

<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title>Create a hover effect</title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.49.0/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.49.0/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>

It’s the <head> of the HTML that has scripts from Mapbox. Just follow them and you’ll be fine. Change the text in <title> to have your own page title.

<body>


—- Your real code goes here —–

</body>

Your real work will go between and . The <div> is a container that refers to the map you are working on. The next part is Mapbox token

mapboxgl.accessToken = '<your access token here>';

To get a token, just create a free account on Mapbox. A free account is allowed up to 50,000 requests a month if I am not mistaken. It should be enough for a student or an enthusiast wishing to try it out. Once you have a token, just put it in between ” in the above line.

Let’s have a base map

var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v9',
    center: [-100.486052, 37.830348],
    zoom: 2
});

The “center” feature’s coordinates refer to where you want to focus on. Get your chosen destination’s coordinates and just put them there. Alternate the two figures in coordinates if you don’t get it right in the first try. “Zoom” is how close you look at the chosen destination. The greater the number, the closer the zoom.

var hoveredStateId =  null;

map.on('load', function () {
    map.addSource("states", {
        "type": "geojson",
        "data": "https://www.mapbox.com/mapbox-gl-js/assets/us_states.geojson"
    });

HoveredStateID is a placeholder variable that will be used later for hover effect. The following code block is to load the base map. Just follow the templates. Three things to note here:

  • “state” refers to the object’s name that contains the GEOJSON data. You can name whatever you want
  • “GEOJSON” refers to to the style of the file. Mapping requires GEOJSON files, whether you load it from an external source, like we do in this case, or from a hardcoded file
  • The link that goes with “data” is where the author stores the data.

One note here: if you use Github or any cloud platform to store and source your file, be careful. For instance, let’s look at a file I have on github.

Github_Link

Just copying the usual link when you access your file on Github like that won’t work. To get the link that works, click on “Raw” and here is how it shows on the screen

Github_Content_Link

Copy the link in the browser. It should work.

Back to the HTML. Add the two “map.addLayer” code sections to what you already have. It should look like the below

map.on('load', function () {
    map.addSource("states", {
        "type": "geojson",
        "data": "https://www.mapbox.com/mapbox-gl-js/assets/us_states.geojson"
    });

    map.addLayer({
        "id": "state-fills",
        "type": "fill",
        "source": "states",
        "layout": {},
        "paint": {
            "fill-color": "#627BC1",
            "fill-opacity": ["case",
                ["boolean", ["feature-state", "hover"], false],
                1,
                0.5
            ]
        }
    });

    map.addLayer({
        "id": "state-borders",
        "type": "line",
        "source": "states",
        "layout": {},
        "paint": {
            "line-color": "#627BC1",
            "line-width": 2
        }
    });

The first addLayer is for the polygon itself while the second one is for the lines between the states. “id” refers to the name of the layer for future reference. Remember to tie the “source” value back to the name of map.addSource. In this case, it’s “states”. The rest is a Mapbox standard template for hover effect. You can change the color whenever you feel like.

The next step is to work on “hover effect”. Add the following code to the end of the previous block

    map.on("mousemove", "state-fills", function(e) {
        if (e.features.length > 0) {
            if (hoveredStateId) {
                map.setFeatureState({source: 'states', id: hoveredStateId}, { hover: false});
            }
            hoveredStateId = e.features[0].id;
            map.setFeatureState({source: 'states', id: hoveredStateId}, { hover: true});
        }
    });

    // When the mouse leaves the state-fill layer, update the feature state of the
    // previously hovered feature.
    map.on("mouseleave", "state-fills", function() {
        if (hoveredStateId) {
            map.setFeatureState({source: 'states', id: hoveredStateId}, { hover: false});
        }
        hoveredStateId =  null;
    });

The first thing to notice is here:  map.on(“mousemove”, “state-fills”, function(e) {

“State-fills” is the “id” of the polygon layer mentioned previously. So whatever name is chosen for that addLayer, it should be used here.

source: ‘states’

In this case, ‘states’ refers to the source of the data in the map.addSource section above. Remember to use the same reference name for the source. The rest is just a standard template. If you have time, feel free to explore. I am under pressure to deliver features for my Capstone, so I just prefer not touching or changing any of it.

Here is an important note. If you don’t follow, the hover effect won’t work. I use the same code as Mapbox’s example, just changing the GEOJSON source. The hover effect doesn’t work as you can see below:

The key is the data source. Let’s look at the data that Mapbox uses. Here is the tree view of the first item in the polygon array, just to show its structure

GEOJSON_2

Here is the structure of the data I used that led to the unsuccessful “hover effect”

GEOJSON_3

Notice the difference? As far as I am concerned, the hover template in question needs the data to have a certain structure. Otherwise, the code won’t work. Now, there should be other ways to go around this, but if you don’t have time, I’d suggest modifying the data to mirror Mapbox’s example. Here is the structure of my modified data

GEOJSON_4

Does the code work? You bet!

GEOJSON_5

Hopefully this post will be useful to starters like I am.

 

 

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