[tests] Add unit test for tile prefetching

cmake/test-files.cmake

@@ -28,6 +28,7 @@ set(MBGL_TEST_FILES
 
     # map
     test/map/map.test.cpp
+    test/map/prefetch.test.cpp
     test/map/transform.test.cpp
 
     # math

test/fixtures/map/prefetch/empty.json

@@ -0,0 +1,5 @@
+{
+  "version": 8,
+  "sources": {},
+  "layers": []
+}

test/fixtures/map/prefetch/style.json

@@ -0,0 +1,24 @@
+{
+  "version": 8,
+  "name": "Test",
+  "sources": {
+    "raster": {
+      "type": "raster",
+      "tiles": [ "{z}" ],
+      "tileSize": 256,
+      "maxzoom": 20,
+      "minzoom": 0
+    }
+  },
+  "layers": [{
+    "id": "background",
+    "type": "background",
+    "paint": {
+      "background-color": "blue"
+    }
+  }, {
+    "id": "raster",
+    "type": "raster",
+    "source": "raster"
+  }]
+}

test/map/prefetch.test.cpp

@@ -0,0 +1,93 @@
+#include <mbgl/test/util.hpp>
+#include <mbgl/test/stub_file_source.hpp>
+
+#include <mbgl/gl/headless_backend.hpp>
+#include <mbgl/gl/offscreen_view.hpp>
+#include <mbgl/map/backend_scope.hpp>
+#include <mbgl/map/map.hpp>
+#include <mbgl/storage/default_file_source.hpp>
+#include <mbgl/style/style.hpp>
+#include <mbgl/util/default_thread_pool.hpp>
+#include <mbgl/util/image.hpp>
+#include <mbgl/util/io.hpp>
+#include <mbgl/util/run_loop.hpp>
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+using namespace mbgl;
+using namespace mbgl::style;
+using namespace std::literals::string_literals;
+
+TEST(Map, PrefetchTiles) {
+    util::RunLoop runLoop;
+    HeadlessBackend backend(test::sharedDisplay());
+    BackendScope scope(backend);
+    OffscreenView view(backend.getContext(), { 512, 512 });
+    ThreadPool threadPool(4);
+    StubFileSource fileSource;
+    Map map(backend, view.getSize(), 1, fileSource, threadPool, MapMode::Still);
+
+    std::vector<int> tiles;
+
+    fileSource.response = [&] (const Resource& res) -> optional<Response> {
+        Response response;
+
+        auto zoom = std::stoi(res.url);
+        tiles.push_back(zoom);
+
+        // Return a red tile for prefetched tiles or green to the actual tile.
+        // The end rendering result should be all green because the map is only
+        // considered fully rendered when only ideal tiles are shown.
+        if (zoom == int(map.getZoom()) + 1) {
+            response.data = std::make_shared<std::string>(
+                util::read_file("test/fixtures/map/prefetch/tile_green.png"));
+        } else {
+            response.data = std::make_shared<std::string>(
+                util::read_file("test/fixtures/map/prefetch/tile_red.png"));
+        }
+
+        return { std::move(response) };
+    };
+
+    auto checkTilesForZoom = [&](int zoom, const std::vector<int>& expected) {
+        tiles.clear();
+
+        // Force tile reloading.
+        map.getStyle().loadJSON(util::read_file("test/fixtures/map/prefetch/empty.json"));
+        map.getStyle().loadJSON(util::read_file("test/fixtures/map/prefetch/style.json"));
+
+        map.setLatLngZoom({ 40.726989, -73.992857 }, zoom); // Manhattan
+
+        // Should always render the ideal tiles (i.e. a green map)
+        test::checkImage("test/fixtures/map/prefetch", test::render(map, view));
+
+        ASSERT_TRUE(std::is_permutation(tiles.begin(), tiles.end(), expected.begin()));
+        ASSERT_FALSE(tiles.empty());
+    };
+
+    // Check defaults, should be 4.
+    ASSERT_EQ(map.getPrefetchZoomDelta(), 4);
+    checkTilesForZoom(12, { 13, 13, 13, 13, 13, 13, 13, 13, 13, 9 });
+
+    // Setting it to 0 disables prefetching.
+    map.setPrefetchZoomDelta(0);
+
+    // No prefetching, raster tiles will use ideal
+    // tiles instead of the actual zoom level, that is
+    // why the zoom levels for non-prefetched tiles are
+    // not the same.
+    checkTilesForZoom(10, { 11, 11, 11, 11, 11, 11, 11, 11, 11 });
+
+    map.setPrefetchZoomDelta(5);
+    checkTilesForZoom(12, { 13, 13, 13, 13, 13, 13, 13, 13, 13, 8 });
+
+    // Should clamp at `minzoom`.
+    map.setPrefetchZoomDelta(20);
+    checkTilesForZoom(10, { 11, 11, 11, 11, 11, 11, 11, 11, 11, 0 });
+
+    // Disabled again.
+    map.setPrefetchZoomDelta(0);
+    checkTilesForZoom(13, { 14, 14, 14, 14, 14, 14, 14, 14, 14 });
+}