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Animate a line

Animate a line by updating a GeoJSON source on each frame.

const map = new maplibregl.Map({
    container: 'map',
    style: 'https://demotiles.maplibre.org/style.json',
    center: [0, 0],
    zoom: 0.5
});

// Create a GeoJSON source with an empty lineString.
const geojson = {
    'type': 'FeatureCollection',
    'features': [
        {
            'type': 'Feature',
            'geometry': {
                'type': 'LineString',
                'coordinates': [[0, 0]]
            }
        }
    ]
};

const speedFactor = 30; // number of frames per longitude degree
let animation; // to store and cancel the animation
let startTime = 0;
let progress = 0; // progress = timestamp - startTime
let resetTime = false; // indicator of whether time reset is needed for the animation
const pauseButton = document.getElementById('pause');

map.on('load', () => {
    map.addSource('line', {
        'type': 'geojson',
        'data': geojson
    });

    // add the line which will be modified in the animation
    map.addLayer({
        'id': 'line-animation',
        'type': 'line',
        'source': 'line',
        'layout': {
            'line-cap': 'round',
            'line-join': 'round'
        },
        'paint': {
            'line-color': '#ed6498',
            'line-width': 5,
            'line-opacity': 0.8
        }
    });

    startTime = performance.now();

    animateLine();

    // click the button to pause or play
    pauseButton.addEventListener('click', () => {
        pauseButton.classList.toggle('pause');
        if (pauseButton.classList.contains('pause')) {
            cancelAnimationFrame(animation);
        } else {
            resetTime = true;
            animateLine();
        }
    });

    // reset startTime and progress once the tab loses or gains focus
    // requestAnimationFrame also pauses on hidden tabs by default
    document.addEventListener('visibilitychange', () => {
        resetTime = true;
    });

    // animated in a circle as a sine wave along the map.
    function animateLine(timestamp) {
        if (resetTime) {
            // resume previous progress
            startTime = performance.now() - progress;
            resetTime = false;
        } else {
            progress = timestamp - startTime;
        }

        // restart if it finishes a loop
        if (progress > speedFactor * 360) {
            startTime = timestamp;
            geojson.features[0].geometry.coordinates = [];
        } else {
            const x = progress / speedFactor;
            // draw a sine wave with some math.
            const y = Math.sin((x * Math.PI) / 90) * 40;
            // append new coordinates to the lineString
            geojson.features[0].geometry.coordinates.push([x, y]);
            // then update the map
            map.getSource('line').setData(geojson);
        }
        // Request the next frame of the animation.
        animation = requestAnimationFrame(animateLine);
    }
});

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Animate a line</title>
    <meta property="og:description" content="Animate a line by updating a GeoJSON source on each frame." />
    <meta property="og:created" content="2023-06-27" />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/maplibre-gl@5.24.0/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/maplibre-gl@5.24.0/dist/maplibre-gl.js'></script>
    <style>
        body { margin: 0; padding: 0; }
        html, body, #map { height: 100%; }
    </style>
</head>
<body>
<style>
    button {
        position: absolute;
        top: 0;
        margin: 20px;
    }

    #pause::after {
        content: 'Pause';
    }

    #pause.pause::after {
        content: 'Play';
    }
</style>
<div id="map"></div>
<button id="pause"></button>
<script>
    const map = new maplibregl.Map({
        container: 'map',
        style: 'https://demotiles.maplibre.org/style.json',
        center: [0, 0],
        zoom: 0.5
    });

    // Create a GeoJSON source with an empty lineString.
    const geojson = {
        'type': 'FeatureCollection',
        'features': [
            {
                'type': 'Feature',
                'geometry': {
                    'type': 'LineString',
                    'coordinates': [[0, 0]]
                }
            }
        ]
    };

    const speedFactor = 30; // number of frames per longitude degree
    let animation; // to store and cancel the animation
    let startTime = 0;
    let progress = 0; // progress = timestamp - startTime
    let resetTime = false; // indicator of whether time reset is needed for the animation
    const pauseButton = document.getElementById('pause');

    map.on('load', () => {
        map.addSource('line', {
            'type': 'geojson',
            'data': geojson
        });

        // add the line which will be modified in the animation
        map.addLayer({
            'id': 'line-animation',
            'type': 'line',
            'source': 'line',
            'layout': {
                'line-cap': 'round',
                'line-join': 'round'
            },
            'paint': {
                'line-color': '#ed6498',
                'line-width': 5,
                'line-opacity': 0.8
            }
        });

        startTime = performance.now();

        animateLine();

        // click the button to pause or play
        pauseButton.addEventListener('click', () => {
            pauseButton.classList.toggle('pause');
            if (pauseButton.classList.contains('pause')) {
                cancelAnimationFrame(animation);
            } else {
                resetTime = true;
                animateLine();
            }
        });

        // reset startTime and progress once the tab loses or gains focus
        // requestAnimationFrame also pauses on hidden tabs by default
        document.addEventListener('visibilitychange', () => {
            resetTime = true;
        });

        // animated in a circle as a sine wave along the map.
        function animateLine(timestamp) {
            if (resetTime) {
                // resume previous progress
                startTime = performance.now() - progress;
                resetTime = false;
            } else {
                progress = timestamp - startTime;
            }

            // restart if it finishes a loop
            if (progress > speedFactor * 360) {
                startTime = timestamp;
                geojson.features[0].geometry.coordinates = [];
            } else {
                const x = progress / speedFactor;
                // draw a sine wave with some math.
                const y = Math.sin((x * Math.PI) / 90) * 40;
                // append new coordinates to the lineString
                geojson.features[0].geometry.coordinates.push([x, y]);
                // then update the map
                map.getSource('line').setData(geojson);
            }
            // Request the next frame of the animation.
            animation = requestAnimationFrame(animateLine);
        }
    });
</script>
</body>
</html>