[core][tile|static mode] Introduce MapOptions::keepRenderData() flag

Render orchestrator stores the render data between render frames if this flag is set.

include/mbgl/map/map_options.hpp

@@ -88,6 +88,28 @@ public:
      */
     bool crossSourceCollisions() const;
 
+    /**
+     * @brief Specify whether render data for layers, sources and images should be kept between renderStill() calls.
+     *
+     * This flag is ignored in Continuous mode. In Static mode and Tile mode, if this flag is set to false, all the data
+     * are created from scratch for every renderStill() call, which guaranties that no extra memory is used, however it
+     * might cause higher CPU load and network traffic.
+     *
+     * By default, it is set to true.
+     *
+     * @param keepRenderData true to enable, false to disable
+     * @return MapOptions for chaining options together.
+     */
+    MapOptions& withKeepRenderData(bool keepRenderData);
+
+    /**
+     * @brief Gets the previously set (or default) keepRenderData value.
+     *
+     * @return true if render data is kept between renderStill() calls,
+     * false otherwise.
+     */
+    bool keepRenderData() const;
+
     /**
      * @brief Sets the orientation of the Map. By default, it is set to
      * Upwards.

src/mbgl/map/map_impl.cpp

@@ -17,6 +17,7 @@ Map::Impl::Impl(RendererFrontend& frontend_,
       mode(mapOptions.mapMode()),
       pixelRatio(mapOptions.pixelRatio()),
       crossSourceCollisions(mapOptions.crossSourceCollisions()),
+      keepRenderData(mapOptions.keepRenderData()),
       fileSource(std::move(fileSource_)),
       style(std::make_unique<style::Style>(fileSource, pixelRatio)),
       annotationManager(*style) {
@@ -64,8 +65,9 @@ void Map::Impl::onUpdate() {
                                annotationManager.makeWeakPtr(),
                                fileSource,
                                prefetchZoomDelta,
-                               bool(stillImageRequest),
-                               crossSourceCollisions};
+                               stillImageRequest.get(),
+                               crossSourceCollisions,
+                               keepRenderData};
 
     rendererFrontend.update(std::make_shared<UpdateParameters>(std::move(params)));
 }

src/mbgl/map/map_impl.hpp

@@ -59,6 +59,7 @@ public:
     const MapMode mode;
     const float pixelRatio;
     const bool crossSourceCollisions;
+    const bool keepRenderData;
 
     MapDebugOptions debugOptions { MapDebugOptions::NoDebug };
 

src/mbgl/map/map_options.cpp

@@ -9,6 +9,7 @@ public:
     ViewportMode viewportMode = ViewportMode::Default;
     NorthOrientation orientation = NorthOrientation::Upwards;
     bool crossSourceCollisions = true;
+    bool keepRenderData = true;
     Size size = { 64, 64 };
     float pixelRatio = 1.0;
 };
@@ -54,6 +55,15 @@ bool MapOptions::crossSourceCollisions() const {
     return impl_->crossSourceCollisions;
 }
 
+MapOptions& MapOptions::withKeepRenderData(bool keepRenderData_) {
+    impl_->keepRenderData = keepRenderData_;
+    return *this;
+}
+
+bool MapOptions::keepRenderData() const {
+    return impl_->keepRenderData;
+}
+
 MapOptions& MapOptions::withNorthOrientation(NorthOrientation orientation) {
     impl_->orientation = orientation;
     return *this;

src/mbgl/renderer/render_orchestrator.cpp

@@ -141,6 +141,13 @@ std::unique_ptr<RenderTree> RenderOrchestrator::createRenderTree(
     if (!isMapModeContinuous) {
         // Reset zoom history state.
         zoomHistory.first = true;
+        if (!updateParameters->keepRenderData && stillImageRequest != updateParameters->stillImageRequest) {
+            sourceImpls = makeMutable<std::vector<Immutable<style::Source::Impl>>>();
+            layerImpls = makeMutable<std::vector<Immutable<style::Layer::Impl>>>();
+            renderSources.clear();
+            renderLayers.clear();
+            stillImageRequest = updateParameters->stillImageRequest;
+        }
     }
 
     if (LayerManager::annotationsEnabled) {

src/mbgl/renderer/render_orchestrator.hpp

@@ -126,6 +126,7 @@ private:
 
     const bool backgroundLayerAsColor;
     bool contextLost = false;
+    const void* stillImageRequest = nullptr;
 
     // Vectors with reserved capacity of layerImpls->size() to avoid reallocation
     // on each frame.

src/mbgl/renderer/update_parameters.hpp

@@ -41,9 +41,12 @@ public:
     const uint8_t prefetchZoomDelta;
     
     // For still image requests, render requested
-    const bool stillImageRequest;
-    
+    const void* stillImageRequest;
+
     const bool crossSourceCollisions;
+
+    // If set, the render data from the previous render calls is kept.
+    const bool keepRenderData;
 };
 
 } // namespace mbgl