[core] Refactor HeadlessFrontend/Backend: GL separation and factory (#14692)

Refactor out HeadlessFrontend and HeadlessBackend gl independent code to gfx.
Define gl::HeadlessBackend as subclass, instantiated by gfx::HeadlessBackend static factory method.
GL dependent tests are still using gl::HeadlessBackend directly (not through gfx).

3 files changed, 36 insertions, 18 deletions

diff --git a/platform/default/include/mbgl/gfx/headless_backend.hpp b/platform/default/include/mbgl/gfx/headless_backend.hpp
new file mode 100644
index 0000000000..de9283dcdf
--- /dev/null
+++ b/platform/default/include/mbgl/gfx/headless_backend.hpp
@@ -0,0 +1,29 @@
+#pragma once
+
+#include <mbgl/gfx/renderable.hpp>
+#include <mbgl/gfx/renderer_backend.hpp>
+#include <mbgl/util/image.hpp>
+
+#include <memory>
+
+namespace mbgl {
+namespace gfx {
+
+// Common headless backend interface, provides HeadlessBackend backend factory
+// and enables extending gfx::Renderable with platform specific implementation
+// of readStillImage.
+class HeadlessBackend : public gfx::Renderable {
+public:
+    // Factory.
+    static std::unique_ptr<HeadlessBackend> make(Size = { 256, 256 }, gfx::ContextMode = gfx::ContextMode::Unique);
+    
+    virtual PremultipliedImage readStillImage() = 0;
+    virtual RendererBackend* getRendererBackend() = 0;
+    void setSize(Size);
+
+protected:
+    HeadlessBackend(Size);
+};
+
+} // namespace gfx
+} // namespace mbgl
diff --git a/platform/default/include/mbgl/gl/headless_frontend.hpp b/platform/default/include/mbgl/gfx/headless_frontend.hpp
index 1e693bdef1..7f937b34b8 100644
--- a/platform/default/include/mbgl/gl/headless_frontend.hpp
+++ b/platform/default/include/mbgl/gfx/headless_frontend.hpp
@@ -2,7 +2,7 @@
 
 #include <mbgl/map/camera.hpp>
 #include <mbgl/renderer/renderer_frontend.hpp>
-#include <mbgl/gl/headless_backend.hpp>
+#include <mbgl/gfx/headless_backend.hpp>
 #include <mbgl/util/async_task.hpp>
 #include <mbgl/util/optional.hpp>
 
@@ -14,10 +14,6 @@ class Renderer;
 class Map;
 class TransformState;
 
-namespace gfx {
-class RendererBackend;
-} // namespace gfx
-
 class HeadlessFrontend : public RendererFrontend {
 public:
     HeadlessFrontend(float pixelRatio_,
@@ -58,7 +54,7 @@ private:
     Size size;
     float pixelRatio;
 
-    gl::HeadlessBackend backend;
+    std::unique_ptr<gfx::HeadlessBackend> backend;
     util::AsyncTask asyncInvalidate;
 
     std::unique_ptr<Renderer> renderer;
diff --git a/platform/default/include/mbgl/gl/headless_backend.hpp b/platform/default/include/mbgl/gl/headless_backend.hpp
index b2141e9540..8aefb5ff6c 100644
--- a/platform/default/include/mbgl/gl/headless_backend.hpp
+++ b/platform/default/include/mbgl/gl/headless_backend.hpp
@@ -1,26 +1,21 @@
 #pragma once
 
-#include <mbgl/gfx/renderable.hpp>
+#include <mbgl/gfx/headless_backend.hpp>
 #include <mbgl/gl/renderer_backend.hpp>
-
 #include <memory>
 #include <functional>
 
 namespace mbgl {
 namespace gl {
 
-class HeadlessBackend final : public gl::RendererBackend, public gfx::Renderable {
+class HeadlessBackend final : public gl::RendererBackend, public gfx::HeadlessBackend {
 public:
     HeadlessBackend(Size = { 256, 256 }, gfx::ContextMode = gfx::ContextMode::Unique);
     ~HeadlessBackend() override;
-
-    gfx::Renderable& getDefaultRenderable() override;
-
-    Size getFramebufferSize() const;
     void updateAssumedState() override;
-
-    void setSize(Size);
-    PremultipliedImage readStillImage();
+    gfx::Renderable& getDefaultRenderable() override;
+    PremultipliedImage readStillImage() override;
+    RendererBackend* getRendererBackend() override;
 
     class Impl {
     public:
@@ -41,8 +36,6 @@ private:
 
 private:
     std::unique_ptr<Impl> impl;
-
-    float pixelRatio;
     bool active = false;
 };