[core] Introduce the ResourceTransform

Wrapper for transforming URLs on the OnlineFileSource.

cmake/core-files.cmake

@@ -310,6 +310,7 @@ set(MBGL_CORE_FILES
     include/mbgl/storage/offline.hpp
     include/mbgl/storage/online_file_source.hpp
     include/mbgl/storage/resource.hpp
+    include/mbgl/storage/resource_transform.hpp
     include/mbgl/storage/response.hpp
     src/mbgl/storage/asset_file_source.hpp
     src/mbgl/storage/file_source_request.cpp
@@ -318,6 +319,7 @@ set(MBGL_CORE_FILES
     src/mbgl/storage/local_file_source.hpp
     src/mbgl/storage/network_status.cpp
     src/mbgl/storage/resource.cpp
+    src/mbgl/storage/resource_transform.cpp
     src/mbgl/storage/response.cpp
 
     # style

include/mbgl/storage/resource_transform.hpp

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <mbgl/actor/actor_ref.hpp>
+#include <mbgl/storage/resource.hpp>
+
+#include <functional>
+#include <string>
+
+namespace mbgl {
+
+class Mailbox;
+
+class ResourceTransform {
+public:
+    using TransformCallback = std::function<std::string(Resource::Kind kind, const std::string&& url)>;
+    using FinishedCallback = std::function<void(const std::string&&)>;
+
+    ResourceTransform(ActorRef<ResourceTransform>, TransformCallback&&);
+
+    void transform(Resource::Kind, const std::string&& url, FinishedCallback&&);
+
+private:
+    TransformCallback transformCallback;
+};
+
+} // namespace mbgl

src/mbgl/storage/resource_transform.cpp

@@ -0,0 +1,13 @@
+#include <mbgl/storage/resource_transform.hpp>
+
+namespace mbgl {
+
+ResourceTransform::ResourceTransform(ActorRef<ResourceTransform>, TransformCallback&& callback)
+    : transformCallback(std::move(callback)) {
+}
+
+void ResourceTransform::transform(Resource::Kind kind, const std::string&& url, FinishedCallback&& finished) {
+    finished(transformCallback(kind, std::move(url)));
+}
+
+} // namespace mbgl