iOS端如何解码Mapbox生成的encoded polyline编码折线？

问题根因 #

MGLShape(data:encoding:) 方法仅支持传入标准GeoJSON格式的二进制数据，你传入的是Encoded Polyline编码字符串的二进制数据，格式不匹配因此返回nil。

实现方案 #

Mapbox的Encoded Polyline编码规则和通用的polyline编码规则完全一致，你可以通过以下两步实现需求：

1. 实现Encoded Polyline解码扩展 #

给String加扩展实现解码逻辑，直接输出坐标数组：

import CoreLocation
import Mapbox

extension String {
    func decodePolyline() -> [CLLocationCoordinate2D] {
        var coordinates = [CLLocationCoordinate2D]()
        var index = startIndex
        var lat = 0.0
        var lon = 0.0

        while index < endIndex {
            var byte = 0
            var shift = 0
            var result = 0
            repeat {
                guard let asciiValue = self[index].asciiValue else { return coordinates }
                byte = Int(asciiValue) - 63
                result |= (byte & 0x1F) << shift
                shift += 5
                index = self.index(after: index)
            } while byte >= 0x20
            let deltaLat = (result & 1) != 0 ? ~(result >> 1) : (result >> 1)
            lat += Double(deltaLat)

            shift = 0
            result = 0
            repeat {
                guard let asciiValue = self[index].asciiValue else { return coordinates }
                byte = Int(asciiValue) - 63
                result |= (byte & 0x1F) << shift
                shift += 5
                index = self.index(after: index)
            } while byte >= 0x20
            let deltaLon = (result & 1) != 0 ? ~(result >> 1) : (result >> 1)
            lon += Double(deltaLon)

            coordinates.append(CLLocationCoordinate2D(latitude: lat / 1e5, longitude: lon / 1e5))
        }
        return coordinates
    }
}

2. 生成MGLShape实例 #

解码得到坐标数组后，直接生成MGLPolyline即可使用：

let encodedPolyline = "_nwfFteckVd@rIvEhEaB~Ht@lIxD~EvE~DfB|HdCeHrBwHnGT|ExDbGzAjGAyC{GjG[fGbA~@iICuIjG@lFqCrBpHj@lIaClHmGOiEbFVvIkCdHgEdFkGImGQoFbDgGgAmGl@iG_@}F_CmCgHZuI`@wIy@oIk@oI|BkH}EgE}BmH"
let coordinates = encodedPolyline.decodePolyline()
guard !coordinates.isEmpty else {
    // 解码失败逻辑处理
    return
}
let polyline = MGLPolyline(coordinates: coordinates, count: UInt(coordinates.count))
let shapeSource = MGLShapeSource(identifier: "polyLineSource", shape: polyline, options: nil)

内容的提问来源于stack exchange，提问作者teradyl