[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`.

1 files changed, 19 insertions, 1 deletions

diff --git a/src/mbgl/actor/scheduler.cpp b/src/mbgl/actor/scheduler.cpp
index d7cdb2737b..cb0c7728ec 100644
--- a/src/mbgl/actor/scheduler.cpp
+++ b/src/mbgl/actor/scheduler.cpp
@@ -1,8 +1,11 @@
 #include <mbgl/actor/scheduler.hpp>
 #include <mbgl/util/thread_local.hpp>
+#include <mbgl/util/thread_pool.hpp>
 
 namespace mbgl {
-    
+
+util::ThreadLocal<Scheduler> g_currentScheduler;
+
 static auto& current() {
     static util::ThreadLocal<Scheduler> scheduler;
     return scheduler;
@@ -16,4 +19,19 @@ Scheduler* Scheduler::GetCurrent() {
     return current().get();
 }
 
+// static
+std::shared_ptr<Scheduler> Scheduler::GetBackground() {
+    static std::weak_ptr<Scheduler> weak;
+    static std::mutex mtx;
+
+    std::lock_guard<std::mutex> lock(mtx);
+    std::shared_ptr<Scheduler> scheduler = weak.lock();
+
+    if (!scheduler) {
+        weak = scheduler = std::make_shared<ThreadPool>(4);
+    }
+
+    return scheduler;
+}
+
 } //namespace mbgl