Notify data changes using Observer pattern

This pattern, in contrast with callbacks, allows us to clearly see
the dependency and hierarchy between the objects. It will also help
on writing unit tests as we can count how many times a notification
was triggered.

src/mbgl/map/resource_loader.cpp

@@ -5,7 +5,6 @@
 #include <mbgl/style/style.hpp>
 
 #include <cassert>
-#include <functional>
 
 namespace mbgl {
 
@@ -15,6 +14,10 @@ ResourceLoader::ResourceLoader() {
 
 ResourceLoader::~ResourceLoader() {
     assert(Environment::currentlyOn(ThreadType::Map));
+
+    for (const auto& source : style_->sources) {
+        source->setObserver(nullptr);
+    }
 }
 
 void ResourceLoader::setObserver(Observer* observer) {
@@ -29,7 +32,8 @@ void ResourceLoader::setStyle(Style* style) {
 
     Environment& env = Environment::Get();
     for (const auto& source : style->sources) {
-        source->load(accessToken_, env, std::bind(&ResourceLoader::emitTileDataChanged, this));
+        source->setObserver(this);
+        source->load(accessToken_, env);
     }
 }
 
@@ -37,6 +41,14 @@ void ResourceLoader::setAccessToken(const std::string& accessToken) {
     accessToken_ = accessToken;
 }
 
+void ResourceLoader::onSourceLoaded() {
+    emitTileDataChanged();
+}
+
+void ResourceLoader::onTileLoaded() {
+    emitTileDataChanged();
+}
+
 void ResourceLoader::emitTileDataChanged() {
     assert(Environment::currentlyOn(ThreadType::Map));