[core] Schedule invocation of onStyleImageMissing completion callback on the same thread

Before this change, ImageManger's 'done' callback for onStyleImageMissing observer
notification that was created on renderer thread, could be called from different thread,
therefore, is not thread safe. For example, on Android platform, callback was invoked
from UI thread. This change makes callback to be scheduled on originating thread.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/mbgl/renderer/image_manager.hpp b/src/mbgl/renderer/image_manager.hpp
index 61f3f3c276..c2d6466b3a 100644
--- a/src/mbgl/renderer/image_manager.hpp
+++ b/src/mbgl/renderer/image_manager.hpp
@@ -15,6 +15,9 @@
 
 namespace mbgl {
 
+template <class T>
+class Actor;
+
 namespace gfx {
 class Context;
 } // namespace gfx
@@ -64,9 +67,11 @@ private:
     bool loaded = false;
 
     std::map<ImageRequestor*, ImageRequestPair> requestors;
+    using Callback = std::function<void()>;
+    using ActorCallback = Actor<Callback>;
     struct MissingImageRequestPair {
         ImageRequestPair pair;
-        unsigned int callbacksRemaining;
+        std::map<std::string, std::unique_ptr<ActorCallback>> callbacks;
     };
     std::map<ImageRequestor*, MissingImageRequestPair> missingImageRequestors;
     std::map<std::string, std::set<ImageRequestor*>> requestedImages;