Emit the signal on the MapThread

We were previously emitting the signal on the thread parsing the
glyph, but signals can only be trigger from the MapThread
because the observers are living there.

src/mbgl/map/map_context.cpp

@@ -36,7 +36,7 @@ MapContext::MapContext(uv_loop_t* loop, View& view_, FileSource& fileSource, Map
       envScope(env, ThreadType::Map, "Map"),
       updated(static_cast<UpdateType>(Update::Nothing)),
       asyncUpdate(util::make_unique<uv::async>(loop, [this] { update(); })),
-      glyphStore(util::make_unique<GlyphStore>(env)),
+      glyphStore(util::make_unique<GlyphStore>(loop, env)),
       glyphAtlas(util::make_unique<GlyphAtlas>(1024, 1024)),
       spriteAtlas(util::make_unique<SpriteAtlas>(512, 512)),
       lineAtlas(util::make_unique<LineAtlas>(512, 512)),

src/mbgl/text/glyph_store.cpp

@@ -9,6 +9,7 @@
 #include <mbgl/util/constants.hpp>
 #include <mbgl/util/token.hpp>
 #include <mbgl/util/math.hpp>
+#include <mbgl/util/uv_detail.hpp>
 #include <mbgl/storage/file_source.hpp>
 #include <mbgl/platform/log.hpp>
 #include <mbgl/platform/platform.hpp>
@@ -236,8 +237,11 @@ bool GlyphPBF::isParsed() const {
     return parsed;
 }
 
-GlyphStore::GlyphStore(Environment& env_)
-    : env(env_), observer(nullptr) {
+GlyphStore::GlyphStore(uv_loop_t* loop, Environment& env_)
+    : env(env_),
+      asyncEmitGlyphRangeLoaded(util::make_unique<uv::async>(loop, [this] { emitGlyphRangeLoaded(); })),
+      observer(nullptr) {
+    asyncEmitGlyphRangeLoaded->unref();
 }
 
 GlyphStore::~GlyphStore() {
@@ -258,7 +262,7 @@ bool GlyphStore::requestGlyphRangesIfNeeded(const std::string& fontStack,
 
     auto callback = [this, fontStack](GlyphPBF* glyph) {
         glyph->parse(*createFontStack(fontStack));
-        emitGlyphRangeLoaded();
+        asyncEmitGlyphRangeLoaded->send();
     };
 
     std::lock_guard<std::mutex> lock(rangesMutex);

src/mbgl/text/glyph_store.hpp

@@ -13,6 +13,14 @@
 #include <string>
 #include <unordered_map>
 
+typedef struct uv_loop_s uv_loop_t;
+
+namespace uv {
+
+class async;
+
+}
+
 namespace mbgl {
 
 class FileSource;
@@ -87,7 +95,7 @@ public:
         virtual void onGlyphRangeLoaded() = 0;
     };
 
-    GlyphStore(Environment &);
+    GlyphStore(uv_loop_t* loop, Environment &);
     ~GlyphStore();
 
     // Asynchronously request for GlyphRanges and when it gets loaded, notifies the
@@ -116,6 +124,8 @@ private:
     std::unordered_map<std::string, std::unique_ptr<FontStack>> stacks;
     std::mutex stacksMutex;
 
+    std::unique_ptr<uv::async> asyncEmitGlyphRangeLoaded;
+
     Observer* observer;
 };