基于ArcGIS .NET实现Xamarin Forms应用离线地图瓦片缓存方案问询

核心逻辑 #

基于ArcGIS Runtime SDK for .NET的TileCache和ExportTileCacheTask，实现本地缓存优先加载+动态区域瓦片下载的功能：通过用户绘制的矩形范围确定下载区域，批量拉取指定层级的瓦片到本地沙盒；自定义瓦片图层优先读取本地缓存，缺失时再请求在线服务。

分步实现 #

1. 地图区域选择

使用ArcGIS内置的Draw工具让用户绘制矩形区域，获取地理范围Envelope：

// 初始化矩形绘制工具
var drawTool = new Draw(mapView)
{
    GeometryType = GeometryType.Envelope,
    Symbol = new SimpleFillSymbol(SimpleFillSymbolStyle.Null, Colors.Transparent, new SimpleLineSymbol(SimpleLineSymbolStyle.Solid, Colors.Blue, 2))
};

// 绘制完成触发下载
drawTool.DrawComplete += async (sender, args) =>
{
    var downloadExtent = args.Geometry as Envelope;
    // 替换为你需要的层级范围
    await DownloadOfflineTiles(downloadExtent, minZoomLevel: 10, maxZoomLevel: 16);
};

// 启动绘制
await drawTool.StartAsync();

2. 批量下载瓦片到本地

通过ExportTileCacheTask创建下载任务，将瓦片保存到移动端沙盒路径：

private async Task DownloadOfflineTiles(Envelope extent, int minZoomLevel, int maxZoomLevel)
{
    var onlineTileServiceUrl = "https://services.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer";
    var exportTask = await ExportTileCacheTask.CreateAsync(new Uri(onlineTileServiceUrl));

    // 配置下载参数：范围、层级、缓存格式
    var exportParams = new ExportTileCacheParameters(extent, minZoomLevel, maxZoomLevel)
    {
        TileFormat = ExportTileCacheTileFormat.Png
    };

    // 本地缓存路径（跨平台适配）
    var localCachePath = Path.Combine(FileSystem.AppDataDirectory, "OfflineMapCache.tpkx");

    // 执行下载任务
    var downloadJob = exportTask.ExportTileCache(exportParams, localCachePath);
    // 可选：监听进度更新UI
    downloadJob.ProgressChanged += (s, e) =>
    {
        MainThread.BeginInvokeOnMainThread(() =>
        {
            progressBar.Progress = e.Progress / 100.0;
        });
    };

    await downloadJob.StartAsync();

    if (downloadJob.Status == JobStatus.Succeeded)
    {
        // 加载本地缓存图层
        SwitchToLocalFirstLayer(localCachePath, onlineTileServiceUrl);
    }
}

3. 自定义本地优先瓦片图层

继承ArcGISTiledLayer，重写瓦片请求逻辑，优先读取本地缓存：

public class LocalFirstTiledLayer : ArcGISTiledLayer
{
    private readonly TileCache _localTileCache;
    private readonly ArcGISTiledLayer _onlineBackupLayer;

    public LocalFirstTiledLayer(string localCachePath, string onlineServiceUrl)
    {
        _localTileCache = new TileCache(localCachePath);
        _onlineBackupLayer = new ArcGISTiledLayer(new Uri(onlineServiceUrl));
        InitializeLayer();
    }

    private async void InitializeLayer()
    {
        try
        {
            // 先尝试加载本地缓存
            var localLayer = new ArcGISTiledLayer(_localTileCache);
            await localLayer.LoadAsync();

            if (localLayer.LoadStatus == LoadStatus.Loaded)
            {
                Source = localLayer.Source;
                return;
            }
        }
        catch { }

        // 本地缓存失效则加载在线图层
        await _onlineBackupLayer.LoadAsync();
        Source = _onlineBackupLayer.Source;
    }

    // 重写瓦片获取逻辑：本地优先
    protected override async Task<ImageTile> GetTileAsync(int level, int row, int column, CancellationToken cancellationToken)
    {
        // 检查本地是否存在该瓦片
        var localTile = await _localTileCache.GetTileAsync(level, row, column, cancellationToken);
        if (localTile != null)
        {
            return localTile;
        }

        // 本地无瓦片时请求在线
        return await _onlineBackupLayer.GetTileAsync(level, row, column, cancellationToken);
    }
}

4. 切换到本地优先图层

将自定义图层设置为地图底图：

private void SwitchToLocalFirstLayer(string cachePath, string onlineUrl)
{
    var localFirstLayer = new LocalFirstTiledLayer(cachePath, onlineUrl);
    mapView.Map = new Map(Basemap.Create(localFirstLayer));
}

关键注意事项 #

• SDK版本要求：使用ArcGIS Runtime SDK for .NET 100.12及以上版本，确保ExportTileCacheTask支持移动端动态下载
• 缓存格式：推荐使用.tpkx格式（紧凑瓦片包），比传统.tpk占用更少存储空间
• 权限配置：Android需添加INTERNET、WRITE_EXTERNAL_STORAGE权限；iOS需开启文件访问权限
• 性能优化：下载前可检查本地缓存是否已存在对应区域瓦片，避免重复下载

内容的提问来源于stack exchange，提问作者JoeTaicoon