[core] Make the BackgroundScheduler a singleton

- Do not carry it over everywhere as parameter, it is a shared
  instance anyway and the lifecycle is pretty much the app lifecycle
  from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
  do tasks in the background, we don't make assumptions if it is a
  thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.

5 files changed, 12 insertions, 6 deletions

diff --git a/include/mbgl/actor/actor.hpp b/include/mbgl/actor/actor.hpp
index 0052fad242..97964ae531 100644
--- a/include/mbgl/actor/actor.hpp
+++ b/include/mbgl/actor/actor.hpp
@@ -55,6 +55,10 @@ public:
     Actor(Scheduler& scheduler, Args&&... args)
         : target(scheduler, parent, std::forward<Args>(args)...) {}
 
+    template <class... Args>
+    Actor(std::shared_ptr<Scheduler> scheduler, Args&&... args)
+        : retainer(std::move(scheduler)), target(*retainer, parent, std::forward<Args>(args)...) {}
+
     Actor(const Actor&) = delete;
 
     ActorRef<std::decay_t<Object>> self() {
@@ -62,6 +66,7 @@ public:
     }
 
 private:
+    std::shared_ptr<Scheduler> retainer;
     AspiringActor<Object> parent;
     EstablishedActor<Object> target;
 };
diff --git a/include/mbgl/actor/scheduler.hpp b/include/mbgl/actor/scheduler.hpp
index 75ead29f0a..6470ab1245 100644
--- a/include/mbgl/actor/scheduler.hpp
+++ b/include/mbgl/actor/scheduler.hpp
@@ -41,6 +41,11 @@ public:
     // Set/Get the current Scheduler for this thread
     static Scheduler* GetCurrent();
     static void SetCurrent(Scheduler*);
+
+    // Get the scheduler for asynchronous tasks. This method
+    // will lazily initialize a shared worker pool when ran
+    // from the first time.
+    static std::shared_ptr<Scheduler> GetBackground();
 };
 
 } // namespace mbgl
diff --git a/include/mbgl/map/map.hpp b/include/mbgl/map/map.hpp
index 2fdd72dcb8..b1d4b14e4f 100644
--- a/include/mbgl/map/map.hpp
+++ b/include/mbgl/map/map.hpp
@@ -22,7 +22,6 @@
 
 namespace mbgl {
 
-class Scheduler;
 class RendererFrontend;
 
 namespace style {
@@ -34,7 +33,6 @@ class Map : private util::noncopyable {
 public:
     explicit Map(RendererFrontend&,
                  MapObserver&,
-                 Scheduler&,
                  const MapOptions&,
                  const ResourceOptions&);
     ~Map();
diff --git a/include/mbgl/renderer/renderer.hpp b/include/mbgl/renderer/renderer.hpp
index 50483fcf4b..787f9dceee 100644
--- a/include/mbgl/renderer/renderer.hpp
+++ b/include/mbgl/renderer/renderer.hpp
@@ -14,7 +14,6 @@ namespace mbgl {
 
 class RendererObserver;
 class RenderedQueryOptions;
-class Scheduler;
 class SourceQueryOptions;
 class UpdateParameters;
 
@@ -24,7 +23,7 @@ class RendererBackend;
 
 class Renderer {
 public:
-    Renderer(gfx::RendererBackend&, float pixelRatio_, Scheduler&,
+    Renderer(gfx::RendererBackend&, float pixelRatio_,
              const optional<std::string> programCacheDir = {},
              const optional<std::string> localFontFamily = {});
     ~Renderer();
diff --git a/include/mbgl/style/style.hpp b/include/mbgl/style/style.hpp
index d6fdbd8f2c..4a6a542b88 100644
--- a/include/mbgl/style/style.hpp
+++ b/include/mbgl/style/style.hpp
@@ -11,7 +11,6 @@
 namespace mbgl {
 
 class FileSource;
-class Scheduler;
 
 namespace style {
 
@@ -22,7 +21,7 @@ class Layer;
 
 class Style {
 public:
-    Style(Scheduler&, FileSource&, float pixelRatio);
+    Style(FileSource&, float pixelRatio);
     ~Style();
 
     void loadJSON(const std::string&);