Mapbox 多色折线答案

【问题标题】：Mapbox multicolored polylineMapbox 多色折线
【发布时间】：2019-10-27 19:14:11
【问题描述】：

我正在将我的应用程序移至 Mapbox，而我坚持的一件事是创建一条多色折线，其中线段颜色由速度动态设置。我能做的最接近的事情是创建一组不同颜色的线，并将它们添加到地图中，就像许多单独的线一样，但是处理时间和帧速率下降太多了，更不用说你可以看到不同的部分了。这很容易通过 MapKit 或谷歌地图实现，但据我所知，折线在 Mapbox 中只能有一种颜色。

Here 提出了一个非常相似的问题，但那是 3 年前的事了，答案似乎与使用路线 API 有关。

基本函数如下所示，我创建了一个自定义 MGLMulticolorPolyline 类，以便我可以设置颜色

    func locationPolyLine(locations : [Location], averageSpeed: Double, topSpeed: Double) -> [MGLMulticolorPolyline] {




    var coordinates: [(CLLocation, CLLocation)] = []
    var speeds: [Double] = []
    let smallerLocationArray = locations.enumerated().filter({ index, _ in
        index % 2 != 0
    }).map { $0.1 }




    for (first, second) in zip(smallerLocationArray, smallerLocationArray.dropFirst()) {
        //create an array of coordinates
        let start = CLLocation(latitude: first.locLatitude, longitude: first.locLongitude)
        let end = CLLocation(latitude: second.locLatitude, longitude: second.locLongitude)
        coordinates.append((start, end))


        let distance = end.distance(from: start)
        guard let firstTimestamp = first.locTimestamp,
            let secondTimestamp = second.locTimestamp else {continue}
        let time = secondTimestamp.timeIntervalSince(firstTimestamp as Date)
        let speed = time > 0 ? distance / time : 0
        speeds.append(speed)

    }




    var segments: [MGLMulticolorPolyline] = []
    var index = 0
    for ((start, end), speed) in zip(coordinates, speeds) {
        let coords = [start.coordinate, end.coordinate]
        let segment = MGLMulticolorPolyline(coordinates: coords, count: 2)

        if smallerLocationArray[index].lift == true{
            segment.color = UIColor.blue
        } else{
            segment.color = segmentColor(speed: speed,
                                         midSpeed: averageSpeed,
                                         slowestSpeed: 0.0,
                                         fastestSpeed: topSpeed)
        }
        segments.append(segment)

        index += 1
    }
    return segments

}
}

它像这样被添加到地图中

    let multiColoredPolyline = MapboxHelper().locationPolyLine(locations: locationsArray, averageSpeed: avgSpeed, topSpeed: maxSpeed)
    mapView.addAnnotations(multiColoredPolyline)

【问题讨论】：

标签： swift mapbox mapbox-ios

【解决方案1】：

往后一点，Mapbox 介绍了the line-gradient property。

https://docs.mapbox.com/mapbox-gl-js/example/line-gradient/ 给出了一个如何使用它的例子。

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Create a gradient line using an expression</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no">
<link href="https://api.mapbox.com/mapbox-gl-js/v2.3.0/mapbox-gl.css" rel="stylesheet">
<script src="https://api.mapbox.com/mapbox-gl-js/v2.3.0/mapbox-gl.js"></script>
<style>
body { margin: 0; padding: 0; }
#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<div id="map"></div>

<script>
    // TO MAKE THE MAP APPEAR YOU MUST
    // ADD YOUR ACCESS TOKEN FROM
    // https://account.mapbox.com
    mapboxgl.accessToken = '<your access token here>';
    var map = (window.map = new mapboxgl.Map({
        container: 'map',
        style: 'mapbox://styles/mapbox/light-v10',
        center: [-77.035, 38.875],
        zoom: 12
    }));

    var geojson = {
        'type': 'FeatureCollection',
        'features': [
            {
                'type': 'Feature',
                'properties': {},
                'geometry': {
                    'coordinates': [
                        [-77.044211, 38.852924],
                        [-77.045659, 38.860158],
                        [-77.044232, 38.862326],
                        [-77.040879, 38.865454],
                        [-77.039936, 38.867698],
                        [-77.040338, 38.86943],
                        [-77.04264, 38.872528],
                        [-77.03696, 38.878424],
                        [-77.032309, 38.87937],
                        [-77.030056, 38.880945],
                        [-77.027645, 38.881779],
                        [-77.026946, 38.882645],
                        [-77.026942, 38.885502],
                        [-77.028054, 38.887449],
                        [-77.02806, 38.892088],
                        [-77.03364, 38.892108],
                        [-77.033643, 38.899926]
                    ],
                    'type': 'LineString'
                }
            }
        ]
    };

    map.on('load', function () {
        // 'line-gradient' can only be used with GeoJSON sources
        // and the source must have the 'lineMetrics' option set to true
        map.addSource('line', {
            type: 'geojson',
            lineMetrics: true,
            data: geojson
        });

        // the layer must be of type 'line'
        map.addLayer({
            type: 'line',
            source: 'line',
            id: 'line',
            paint: {
                'line-color': 'red',
                'line-width': 14,
                // 'line-gradient' must be specified using an expression
                // with the special 'line-progress' property
                'line-gradient': [
                    'interpolate',
                    ['linear'],
                    ['line-progress'],
                    0,
                    'blue',
                    0.1,
                    'royalblue',
                    0.3,
                    'cyan',
                    0.5,
                    'lime',
                    0.7,
                    'yellow',
                    1,
                    'red'
                ]
            },
            layout: {
                'line-cap': 'round',
                'line-join': 'round'
            }
        });
    });
</script>

</body>
</html>

还有一个 pathgradient 库，记录在 https://docs.mapbox.com/help/tutorials/add-gradient-route-line-video/。

在代码中有点难看，他们在文档中根本没有解释，但是 https://github.com/mapbox/path-gradients/blob/master/main.js#L22 行是你可以设置任何你想要的颜色的地方，所以不要选择依赖的颜色在ith 元素的位置上，您可以根据ith 元素的速度设置颜色。

【讨论】：