动画线条

通过在每一帧更新GeoJSON源来制作线条动画;

<!DOCTYPE html>
<html lang="en">
<head>
    <title>动画线条</title>
    <meta property="og:description" content="通过在每一帧更新GeoJSON源来制作线条动画" />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/maplibre-gl@5.5.0/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/maplibre-gl@5.5.0/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: '暂停';
    }

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

    // 创建一个带有空LineString的GeoJSON源
    const geojson = {
        'type': 'FeatureCollection',
        'features': [
            {
                'type': 'Feature',
                'geometry': {
                    'type': 'LineString',
                    'coordinates': [[0, 0]]
                }
            }
        ]
    };

    const speedFactor = 30; // 每经度的帧数
    let animation; // 存储和取消动画
    let startTime = 0;
    let progress = 0; // progress = timestamp - startTime
    let resetTime = false; // 是否需要重置时间的指示器
    const pauseButton = document.getElementById('pause');

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

        // 添加将在动画中修改的线条
        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();

        // 点击按钮暂停或播放
        pauseButton.addEventListener('click', () => {
            pauseButton.classList.toggle('pause');
            if (pauseButton.classList.contains('pause')) {
                cancelAnimationFrame(animation);
            } else {
                resetTime = true;
                animateLine();
            }
        });

        // 当标签失去或获得焦点时重置startTime和progress
        // requestAnimationFrame默认情况下也会在隐藏标签上暂停
        document.addEventListener('visibilitychange', () => {
            resetTime = true;
        });

        // 沿着地图以正弦波的形式在圆圈中动画
        function animateLine(timestamp) {
            if (resetTime) {
                // 恢复之前的进度
                startTime = performance.now() - progress;
                resetTime = false;
            } else {
                progress = timestamp - startTime;
            }

            // 完成一个循环后重新开始
            if (progress > speedFactor * 360) {
                startTime = timestamp;
                geojson.features[0].geometry.coordinates = [];
            } else {
                const x = progress / speedFactor;
                // 用一些数学方法绘制正弦波
                const y = Math.sin((x * Math.PI) / 90) * 40;
                // 将新坐标附加到LineString
                geojson.features[0].geometry.coordinates.push([x, y]);
                // 然后更新地图
                map.getSource('line').setData(geojson);
            }
            // 请求动画的下一帧
            animation = requestAnimationFrame(animateLine);
        }
    });
</script>
</body>
</html>