[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, 28 insertions, 0 deletions

diff --git a/src/mbgl/util/thread_pool.hpp b/src/mbgl/util/thread_pool.hpp
new file mode 100644
index 0000000000..509fd06061
--- /dev/null
+++ b/src/mbgl/util/thread_pool.hpp
@@ -0,0 +1,28 @@
+#pragma once
+
+#include <mbgl/actor/mailbox.hpp>
+#include <mbgl/actor/scheduler.hpp>
+
+#include <condition_variable>
+#include <mutex>
+#include <queue>
+#include <thread>
+
+namespace mbgl {
+
+class ThreadPool final : public Scheduler {
+public:
+    explicit ThreadPool(std::size_t count);
+    ~ThreadPool() override;
+
+    void schedule(std::weak_ptr<Mailbox>) override;
+
+private:
+    std::vector<std::thread> threads;
+    std::queue<std::weak_ptr<Mailbox>> queue;
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool terminate{ false };
+};
+
+} // namespace mbgl