第20章 - 实战案例与生产实践

#include <mln/storage/offline.hpp>

// 区域定义：style、bounds、minZoom、maxZoom、pixelRatio、是否包含 CJK 字形
mln::OfflineTilePyramidRegionDefinition region(
    "https://example.com/style.json",   // styleURL
    mln::LatLngBounds::hull({39.86, 116.28}, {39.96, 116.46}),  // 北京城区
    10.0,                                // minZoom
    16.0,                                // maxZoom
    2.0f,                                // pixelRatio
    true                                 // includeIdeographs
);

#include <mln/storage/offline.hpp>

class ProgressObserver final : public mln::OfflineRegionObserver {
public:
    void statusChanged(mln::OfflineRegionStatus status) override {
        if (status.complete()) {
            onDone(status.completedResourceSize);
            return;
        }

        // requiredResourceCountIsPrecise 为 false 时总数只是下界，进度条会跳
        const double ratio = status.requiredResourceCountIsPrecise
            ? static_cast<double>(status.completedResourceCount) / status.requiredResourceCount
            : -1.0;  // 未知，显示不确定态

        onProgress(ratio, status.completedTileCount, status.requiredTileCount,
                   status.completedResourceSize);
    }

    void responseError(mln::Response::Error error) override {
        // 多数错误会自动退避重试，这里只做上报，不要立刻放弃整个区域
        onError(static_cast<int>(error.reason));
    }
};

#include <mln/storage/database_file_source.hpp>
#include <mln/storage/file_source_manager.hpp>
#include <mln/storage/resource_options.hpp>

auto options = mln::ResourceOptions()
    .withCachePath("/data/data/com.example.app/files/cache.db")  // 缓存数据库
    .withAssetPath("/data/data/com.example.app/files/assets/")   // asset:// 根目录
    .withMaximumCacheSize(512ull * 1024 * 1024);                 // 512 MB 上限

auto* manager = mln::FileSourceManager::get();
auto fs = manager->getFileSource(mln::FileSourceType::Database, options);

#include <mln/gfx/headless_frontend.hpp>
#include <mln/map/map.hpp>
#include <mln/map/map_options.hpp>
#include <mln/storage/resource_options.hpp>
#include <mln/util/run_loop.hpp>

mln::PremultipliedImage renderOne(const std::string& styleURL,
                                  const mln::CameraOptions& camera,
                                  uint32_t width, uint32_t height) {
    mln::util::RunLoop loop;                       // 每线程一个
    mln::HeadlessFrontend frontend({width, height}, 1.0f);

    mln::Map map(frontend,
                 mln::MapObserver::nullObserver(),
                 mln::MapOptions()
                     .withMapMode(mln::MapMode::Static)
                     .withSize(frontend.getSize())
                     .withPixelRatio(1.0f),
                 mln::ResourceOptions()
                     .withCachePath("/var/cache/maplibre/render-cache.sqlite")
                     .withAssetPath("/srv/maplibre/assets"));

    map.loadStyleURL(styleURL);
    map.jumpTo(camera);
    return frontend.render(map).image;             // 阻塞直到一帧完成
}