[node] Support mbgl::MapObserver

platform/node/src/node_map.cpp

@@ -39,6 +39,13 @@ static const char* releasedMessage() {
     return "Map resources have already been released";
 }
 
+NodeBackend::NodeBackend()
+    : HeadlessBackend(sharedDisplay()) {}
+
+void NodeBackend::onDidFailLoadingMap() {
+    throw std::runtime_error("Requires a map style to be a valid style JSON");
+}
+
 void NodeMap::Init(v8::Local<v8::Object> target) {
     v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
 
@@ -953,7 +960,6 @@ NodeMap::NodeMap(v8::Local<v8::Object> options)
                            ->NumberValue()
                      : 1.0;
       }()),
-      backend(sharedDisplay()),
       map(std::make_unique<mbgl::Map>(backend,
                                       mbgl::Size{ 256, 256 },
                                       pixelRatio,
@@ -962,12 +968,6 @@ NodeMap::NodeMap(v8::Local<v8::Object> options)
                                       mbgl::MapMode::Still)),
       async(new uv_async_t) {
 
-    backend.setMapChangeCallback([&](mbgl::MapChange change) {
-        if (change == mbgl::MapChangeDidFailLoadingMap) {
-            throw std::runtime_error("Requires a map style to be a valid style JSON");
-        }
-    });
-
     async->data = this;
     uv_async_init(uv_default_loop(), async, [](uv_async_t* h) {
         reinterpret_cast<NodeMap *>(h->data)->renderFinished();

platform/node/src/node_map.hpp

@@ -15,6 +15,12 @@
 
 namespace node_mbgl {
 
+class NodeBackend : public mbgl::HeadlessBackend {
+public:
+    NodeBackend();
+    void onDidFailLoadingMap() final;
+};
+
 class NodeMap : public Nan::ObjectWrap,
                 public mbgl::FileSource {
 public:
@@ -59,7 +65,7 @@ public:
     std::unique_ptr<mbgl::AsyncRequest> request(const mbgl::Resource&, mbgl::FileSource::Callback);
 
     const float pixelRatio;
-    mbgl::HeadlessBackend backend;
+    NodeBackend backend;
     std::unique_ptr<mbgl::OffscreenView> view;
     NodeThreadPool threadpool;
     std::unique_ptr<mbgl::Map> map;