[core] LayerManager can disable annotations

At the moment, the annotations implementation in the `mapbox-gl-native`
core is creating concrete layer instances apart from `LayerManager/LayerFactory`
code path.

So, annotations must be disabled if the `LayerManager` implementation does
not provide line, fill or symbol layers (those, used by the annotations).

Note: in future, annotations implementation will be moved from the core to the
platform SDK level(see https://github.com/mapbox/mapbox-plugins-android/tree/master/plugin-annotation)
and `LayerManager` won't need to disable it.

1 files changed, 22 insertions, 0 deletions

diff --git a/include/mbgl/style/layer.hpp b/include/mbgl/style/layer.hpp
index 9e73e994e6..d85f459815 100644
--- a/include/mbgl/style/layer.hpp
+++ b/include/mbgl/style/layer.hpp
@@ -152,6 +152,12 @@ protected:
 
 /**
  * @brief A singleton class responsible for creating layer instances.
+ * 
+ * The LayerManager has implementation per platform. The LayerManager implementation
+ * defines what layer types are available and it can also disable annotations.
+ * 
+ * Linker excludes the unreachable code for the disabled annotations and layers
+ * from the binaries, significantly reducing their size.
  */
 class LayerManager {
 public:
@@ -168,6 +174,22 @@ public:
     /// Returns a new RenderLayer instance on success call; returns `nulltptr` otherwise.
     std::unique_ptr<RenderLayer> createRenderLayer(Immutable<style::Layer::Impl>) noexcept;
 
+    /**
+     * @brief a build-time flag to enable/disable annotations in mapbox-gl-native core.
+     * 
+     * At the moment, the annotations implementation in core is creating concrete
+     * layer instances apart from LayerManager/LayerFactory code path.
+     * 
+     * So, annotations must be disabled if the LayerManager implementation does
+     * not provide line, fill or symbol layers (those, used by the annotations 
+     * implementation).
+     * 
+     * Note: in future, annotations implemantation will be moved from the core to platform 
+     * SDK (see https://github.com/mapbox/mapbox-plugins-android/tree/master/plugin-annotation)
+     * and this flag won't be needed any more.
+     */
+    static const bool annotationsEnabled;
+
 protected:
     virtual ~LayerManager() = default;
     virtual LayerFactory* getFactory(const std::string& type) noexcept = 0;