[core] Merge LiveTileData into VectorTileData

To encapsulate the differences, this introduces an abstract
GeometryTileMonitor class, with concrete VectorTileMonitor and
AnnotationTileMonitor implementations, and makes pbf parsing for
VectorTile data happen lazily in getLayer, since we want that to
happen on the worker thread, rather than when VectorTile is created.

Fixes #2792

src/mbgl/map/vector_tile.cpp

@@ -1,4 +1,12 @@
 #include <mbgl/map/vector_tile.hpp>
+#include <mbgl/map/source.hpp>
+#include <mbgl/storage/resource.hpp>
+#include <mbgl/storage/response.hpp>
+#include <mbgl/storage/file_source.hpp>
+#include <mbgl/util/thread_context.hpp>
+#include <mbgl/util/run_loop.hpp>
+
+#include <sstream>
 
 namespace mbgl {
 
@@ -122,22 +130,29 @@ GeometryCollection VectorTileFeature::getGeometries() const {
     return lines;
 }
 
-VectorTile::VectorTile(pbf tile_pbf) {
-    while (tile_pbf.next()) {
-        if (tile_pbf.tag == 3) { // layer
-            util::ptr<VectorTileLayer> layer = std::make_shared<VectorTileLayer>(tile_pbf.message());
-            layers.emplace(layer->name, layer);
-        } else {
-            tile_pbf.skip();
-        }
-    }
+VectorTile::VectorTile(std::shared_ptr<const std::string> data_)
+    : data(data_) {
 }
 
 util::ptr<GeometryTileLayer> VectorTile::getLayer(const std::string& name) const {
+    if (!parsed) {
+        parsed = true;
+        pbf tile_pbf(reinterpret_cast<const unsigned char *>(data->c_str()), data->size());
+        while (tile_pbf.next()) {
+            if (tile_pbf.tag == 3) { // layer
+                util::ptr<VectorTileLayer> layer = std::make_shared<VectorTileLayer>(tile_pbf.message());
+                layers.emplace(layer->name, layer);
+            } else {
+                tile_pbf.skip();
+            }
+        }
+    }
+
     auto layer_it = layers.find(name);
     if (layer_it != layers.end()) {
         return layer_it->second;
     }
+
     return nullptr;
 }
 
@@ -163,4 +178,32 @@ util::ptr<const GeometryTileFeature> VectorTileLayer::getFeature(std::size_t i)
     return std::make_shared<VectorTileFeature>(features.at(i), *this);
 }
 
+VectorTileMonitor::VectorTileMonitor(const SourceInfo& source, const TileID& id, float pixelRatio)
+    : url(source.tileURL(id, pixelRatio)) {
+}
+
+Request* VectorTileMonitor::monitorTile(std::function<void (std::exception_ptr, std::unique_ptr<GeometryTile>)> callback) {
+    return util::ThreadContext::getFileSource()->request({ Resource::Kind::Tile, url }, util::RunLoop::getLoop(), [callback, this](const Response& res) {
+        if (res.data && data == res.data) {
+            // We got the same data again. Abort early.
+            return;
+        }
+
+        if (res.status == Response::NotFound) {
+            callback(nullptr, nullptr);
+            return;
+        }
+
+        if (res.status != Response::Successful) {
+            std::stringstream message;
+            message << "Failed to load [" << url << "]: " << res.message;
+            callback(std::make_exception_ptr(std::runtime_error(message.str())), nullptr);
+            return;
+        }
+
+        data = res.data;
+        callback(nullptr, std::make_unique<VectorTile>(data));
+    });
+}
+
 }