[core] Add runtime API for setting tile prefetch delta for Source

New setPrefetchZoomDelta(optional<uint8_t> delta) method allow overriding
default tile prefetch setting that is defined by the Map instance. The
method can be moved to generic style specification if found to be useful
for gl-js engine.

include/mbgl/style/source.hpp

@@ -73,6 +73,8 @@ public:
     SourceObserver* observer = nullptr;
 
     virtual void loadDescription(FileSource&) = 0;
+    void setPrefetchZoomDelta(optional<uint8_t> delta) noexcept;
+    optional<uint8_t> getPrefetchZoomDelta() const noexcept;
     void dumpDebugLogs() const;
 
     virtual bool supportsLayerType(const mbgl::style::LayerTypeInfo*) const = 0;
@@ -85,6 +87,9 @@ public:
     mapbox::base::TypeWrapper peer;
 
     virtual mapbox::base::WeakPtr<Source> makeWeakPtr() = 0;
+
+protected:
+    virtual Mutable<Impl> createMutable() const noexcept = 0;
 };
 
 } // namespace style

include/mbgl/style/sources/custom_geometry_source.hpp

@@ -50,6 +50,10 @@ public:
     mapbox::base::WeakPtr<Source> makeWeakPtr() override {
         return weakFactory.makeWeakPtr();
     }
+
+protected:
+    Mutable<Source::Impl> createMutable() const noexcept final;
+
 private:
     std::shared_ptr<ThreadPool> threadPool;
     std::unique_ptr<Actor<CustomTileLoader>> loader;

include/mbgl/style/sources/geojson_source.hpp

@@ -81,6 +81,9 @@ public:
         return weakFactory.makeWeakPtr();
     }
 
+protected:
+    Mutable<Source::Impl> createMutable() const noexcept final;
+
 private:
     optional<std::string> url;
     std::unique_ptr<AsyncRequest> req;

include/mbgl/style/sources/image_source.hpp

@@ -33,6 +33,10 @@ public:
     mapbox::base::WeakPtr<Source> makeWeakPtr() override {
         return weakFactory.makeWeakPtr();
     }
+
+protected:
+    Mutable<Source::Impl> createMutable() const noexcept final;
+
 private:
     optional<std::string> url;
     std::unique_ptr<AsyncRequest> req;

include/mbgl/style/sources/raster_source.hpp

@@ -31,6 +31,9 @@ public:
         return weakFactory.makeWeakPtr();
     }
 
+protected:
+    Mutable<Source::Impl> createMutable() const noexcept final;
+
 private:
     const variant<std::string, Tileset> urlOrTileset;
     std::unique_ptr<AsyncRequest> req;

include/mbgl/style/sources/vector_source.hpp

@@ -30,6 +30,9 @@ public:
         return weakFactory.makeWeakPtr();
     }
 
+protected:
+    Mutable<Source::Impl> createMutable() const noexcept final;
+
 private:
     const variant<std::string, Tileset> urlOrTileset;
     std::unique_ptr<AsyncRequest> req;

src/mbgl/annotation/annotation_source.cpp

@@ -14,6 +14,10 @@ AnnotationSource::Impl::Impl()
     : Source::Impl(SourceType::Annotations, AnnotationManager::SourceID) {
 }
 
+const AnnotationSource::Impl& AnnotationSource::impl() const {
+    return static_cast<const Impl&>(*baseImpl);
+}
+
 void AnnotationSource::loadDescription(FileSource&) {
     loaded = true;
 }
@@ -26,4 +30,8 @@ bool AnnotationSource::supportsLayerType(const mbgl::style::LayerTypeInfo* info)
     return !std::strcmp(info->type, "line") || !std::strcmp(info->type, "symbol") || !std::strcmp(info->type, "fill");
 }
 
+Mutable<Source::Impl> AnnotationSource::createMutable() const noexcept {
+    return staticMutableCast<Source::Impl>(makeMutable<Impl>(impl()));
+}
+
 } // namespace mbgl

src/mbgl/annotation/annotation_source.hpp

@@ -12,13 +12,15 @@ public:
     class Impl;
     const Impl& impl() const;
 
+protected:
+    Mutable<Source::Impl> createMutable() const noexcept final;
+
 private:
     void loadDescription(FileSource&) final;
     bool supportsLayerType(const mbgl::style::LayerTypeInfo*) const override;
     mapbox::base::WeakPtr<Source> makeWeakPtr() override {
         return weakFactory.makeWeakPtr();
     }
-    Mutable<Impl> mutableImpl() const;
     mapbox::base::WeakPtrFactory<Source> weakFactory {this};
 };
 

src/mbgl/style/source.cpp

@@ -31,6 +31,18 @@ void Source::setObserver(SourceObserver* observer_) {
     observer = observer_ ? observer_ : &nullObserver;
 }
 
+void Source::setPrefetchZoomDelta(optional<uint8_t> delta) noexcept {
+    if (getPrefetchZoomDelta() == delta) return;
+    auto newImpl = createMutable();
+    newImpl->setPrefetchZoomDelta(std::move(delta));
+    baseImpl = std::move(newImpl);
+    observer->onSourceChanged(*this);
+}
+
+optional<uint8_t> Source::getPrefetchZoomDelta() const noexcept {
+    return baseImpl->getPrefetchZoomDelta();
+}
+
 void Source::dumpDebugLogs() const {
     Log::Info(Event::General, "Source::id: %s", getID().c_str());
     Log::Info(Event::General, "Source::loaded: %d", loaded);

src/mbgl/style/source_impl.cpp

@@ -8,5 +8,13 @@ Source::Impl::Impl(SourceType type_, std::string id_)
       id(std::move(id_)) {
 }
 
+void Source::Impl::setPrefetchZoomDelta(optional<uint8_t> delta) noexcept {
+    prefetchZoomDelta = std::move(delta);
+}
+
+optional<uint8_t> Source::Impl::getPrefetchZoomDelta() const noexcept {
+    return prefetchZoomDelta;
+}
+
 } // namespace style
 } // namespace mbgl

src/mbgl/style/source_impl.hpp

@@ -20,9 +20,12 @@ public:
     Impl& operator=(const Impl&) = delete;
 
     virtual optional<std::string> getAttribution() const = 0;
+    void setPrefetchZoomDelta(optional<uint8_t> delta) noexcept;
+    optional<uint8_t> getPrefetchZoomDelta() const noexcept;
 
     const SourceType type;
     const std::string id;
+    optional<uint8_t> prefetchZoomDelta;
 
 protected:
     Impl(SourceType, std::string);

src/mbgl/style/sources/custom_geometry_source.cpp

@@ -27,7 +27,7 @@ const CustomGeometrySource::Impl& CustomGeometrySource::impl() const {
 }
 
 void CustomGeometrySource::loadDescription(FileSource&) {
-    baseImpl = makeMutable<CustomGeometrySource::Impl>(impl(), loader->self());
+    baseImpl = makeMutable<Impl>(impl(), loader->self());
     loaded = true;
     observer->onSourceLoaded(*this);
 }
@@ -49,5 +49,9 @@ void CustomGeometrySource::invalidateRegion(const LatLngBounds& bounds) {
     loader->self().invoke(&CustomTileLoader::invalidateRegion, bounds, impl().getZoomRange());
 }
 
+Mutable<Source::Impl> CustomGeometrySource::createMutable() const noexcept {
+    return staticMutableCast<Source::Impl>(makeMutable<Impl>(impl()));
+}
+
 } // namespace style
 } // namespace mbgl

src/mbgl/style/sources/geojson_source.cpp

@@ -120,5 +120,9 @@ bool GeoJSONSource::supportsLayerType(const mbgl::style::LayerTypeInfo* info) co
     return mbgl::underlying_type(Tile::Kind::Geometry) == mbgl::underlying_type(info->tileKind);
 }
 
+Mutable<Source::Impl> GeoJSONSource::createMutable() const noexcept {
+    return staticMutableCast<Source::Impl>(makeMutable<Impl>(impl()));
+}
+
 } // namespace style
 } // namespace mbgl

src/mbgl/style/sources/image_source.cpp

@@ -87,5 +87,9 @@ bool ImageSource::supportsLayerType(const mbgl::style::LayerTypeInfo* info) cons
     return mbgl::underlying_type(Tile::Kind::Raster) == mbgl::underlying_type(info->tileKind);
 }
 
+Mutable<Source::Impl> ImageSource::createMutable() const noexcept {
+    return staticMutableCast<Source::Impl>(makeMutable<Impl>(impl()));
+}
+
 } // namespace style
 } // namespace mbgl

src/mbgl/style/sources/raster_source.cpp

@@ -87,5 +87,9 @@ bool RasterSource::supportsLayerType(const mbgl::style::LayerTypeInfo* info) con
     return mbgl::underlying_type(Tile::Kind::Raster) == mbgl::underlying_type(info->tileKind);
 }
 
+Mutable<Source::Impl> RasterSource::createMutable() const noexcept {
+    return staticMutableCast<Source::Impl>(makeMutable<Impl>(impl()));
+}
+
 } // namespace style
 } // namespace mbgl

src/mbgl/style/sources/vector_source.cpp

@@ -91,5 +91,9 @@ bool VectorSource::supportsLayerType(const mbgl::style::LayerTypeInfo* info) con
     return mbgl::underlying_type(Tile::Kind::Geometry) == mbgl::underlying_type(info->tileKind);
 }
 
+Mutable<Source::Impl> VectorSource::createMutable() const noexcept {
+    return staticMutableCast<Source::Impl>(makeMutable<Impl>(impl()));
+}
+
 } // namespace style
 } // namespace mbgl