A few things

Make child entites work.

z-sort entity list before rendering.

entites have a creation timestamp.

src/Engine/Entity/Entity.cpp

@@ -59,8 +59,11 @@ Vector2 Engine::Entity::GetPosition() const {
 
 bool Engine::Entity::AppendChild(Entity* entity) {
     bool success = false;
-    if (!Globals::CurrentScene->EntityListContains(entity))
-        children.push_back(entity), success = true;
+    if (!Globals::CurrentScene->EntityListContains(entity)) {
+        entity->parent = this;
+        children.push_back(entity);
+        success = true;
+    }
     return success;
 }
 
@@ -88,3 +91,19 @@ void Engine::Entity::RemoveChild(Entity* entity) {
     if (it != children.end())
         children.erase(it);
 }
+
+long Engine::Entity::GetCreationTime() const {
+    return creation_time;
+}
+
+long Engine::Entity::GetAge() const {
+    return std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count() - creation_time;
+}
+
+Engine::Entity* Engine::Entity::GetParent() const {
+    return parent;
+}
+
+std::vector<Engine::Entity*> Engine::Entity::GetChildren() const {
+    return children;
+}

src/Engine/Level/Scene.cpp

@@ -1,5 +1,6 @@
 #include <Engine/Level/Scene.h>
 #include <Engine/Entity/Camera.h>
+#include <algorithm>
 
 bool Engine::Scene::EntityListContains(const Entity* entity) const {
     for (auto* e : entity_list)
@@ -20,7 +21,7 @@ size_t Engine::Scene::FixedCount() const {
 }
 
 size_t Engine::Scene::EntityCount() const {
-    return entity_list.size();
+    return GetFlatEntityList(entity_list).size();
 }
 
 void Engine::Scene::Update() {
@@ -33,24 +34,33 @@ void Engine::Scene::Update() {
 }
 
 void Engine::Scene::Render(RenderTarget* render_target) {
-    for (auto& f : fixed_list)
-        if (f->Enabled())
-            f->Render();
+    // The Camera and the HUD are ignored here because they're special cases.
+    std::vector<Renderable*> display_list{};
+    for (auto* e : GetFlatEntityList(entity_list))
+        if (auto* r = dynamic_cast<Renderable*>(e))
+            if (auto* c = dynamic_cast<Camera*>(r); c == nullptr)
+                if (auto* h = dynamic_cast<Hud*>(c); h == nullptr)
+                    display_list.push_back(r);
 
-    // TODO Render order. In this system it's not possible for child entities to be rendered before the parent.
+    std::sort(display_list.begin(), display_list.end(), [](Renderable* a, Renderable* b) {
+        return a->GetDepth() > b->GetDepth();
+    });
 
-    J2D::Begin(render_target, true);
+    J2D::Begin( render_target, true);
 
     if (active_camera)
         active_camera->Render();
 
-    for (auto& e : entity_list)
-        if (auto* r = dynamic_cast<Renderable*>(e))
-            if (auto* c = dynamic_cast<Camera*>(r); c == nullptr)
-                r->Render();
+    for (auto& f : fixed_list)
+        if (f->Enabled())
+            f->Render();
+
+    for (auto& r : display_list)
+        r->Render();
 
     if (heads_up_display)
         heads_up_display->Render();
+
     J2D::End();
 }
 
@@ -118,3 +128,15 @@ Engine::Camera* Engine::Scene::GetActiveCamera() const {
     return active_camera;
 }
 
+std::vector<Engine::Entity*> Engine::Scene::GetFlatEntityList(const std::vector<Entity*>& ent_list) const {
+    std::vector<Entity*> result{};
+
+    for (auto* e : ent_list) {
+        auto children = GetFlatEntityList(e->GetChildren());
+        for (auto* c : children)
+            result.push_back(c);
+        result.push_back(e);
+    }
+    return result;
+}
+

src/Game/Scene/ControllableBox.cpp

@@ -4,7 +4,7 @@
 
 void ControllableBox::Init() {
     auto* hud = new Game::DemoGameHud();
-    auto* b = new Game::Box({0, 0});
-    heads_up_display = hud;
+    auto *b = new Game::Box({0, 0});
     AppendEntity(b);
+    heads_up_display = hud;
 }