svgwidget: greatly improve performance by caching render result

src/SvgWidget.cpp

@@ -1,4 +1,7 @@
 #include "SvgWidget.hpp"
+#include "cairomm/context.h"
+#include "cairomm/enums.h"
+#include "cairomm/surface.h"
 
 void SvgWidget::on_init() {
     m_file = g_file_new_for_path(m_path.c_str());
@@ -6,8 +9,7 @@ void SvgWidget::on_init() {
 
     set_style(&Raven::clear_widget_style);
 
-    if (!m_handle)
-    {
+    if (!m_handle) {
         std::cerr << "could not load svg file: " << m_path << std::endl;
         exit(EXIT_FAILURE);
     }
@@ -15,15 +17,36 @@ void SvgWidget::on_init() {
     set_did_init(true);
 }
 
-void SvgWidget::on_paint() {
+void SvgWidget::on_after_layout() {
+    auto width = rect().width();
+    auto height = rect().height();
+
+    if (width == m_known_width && height == m_known_height) {
+        return;
+    }
+
+    m_known_width = width;
+    m_known_height = height;
+
     RsvgRectangle viewport = {
         .x = 0.0,
         .y = 0.0,
-        .width = rect().width(),
-        .height = rect().height(),
+        .width = width,
+        .height = height,
     };
 
-    rsvg_handle_render_document(m_handle, painter()->cairo()->cobj(), &viewport, NULL);
+    m_image_surface = Cairo::ImageSurface::create(Cairo::Format::FORMAT_ARGB32, width, height);
+    m_image_context = Cairo::Context::create(m_image_surface);
+
+    rsvg_handle_render_document(m_handle, m_image_context->cobj(), &viewport, NULL);
+}
+
+void SvgWidget::on_paint() {
+    if (!m_image_surface)
+        return;
+
+    painter()->cairo()->set_source(m_image_surface, 0, 0);
+    painter()->cairo()->paint();
 }
 
 SvgWidget::~SvgWidget() {

src/SvgWidget.hpp

@@ -1,6 +1,10 @@
 #pragma once
 
 #include <string>
+#include "cairomm/context.h"
+#include "cairomm/enums.h"
+#include "cairomm/refptr.h"
+#include "cairomm/surface.h"
 #include "raven/Widget.hpp"
 #include "librsvg/rsvg.h"
 
@@ -17,9 +21,14 @@ public:
     void set_path(std::string path) { m_path = path; repaint(); }
 protected:
     void on_init();
+    void on_after_layout();
     void on_paint();
 private:
     std::string m_path;
     RsvgHandle *m_handle;
     GFile *m_file;
+    Cairo::RefPtr<Cairo::Context> m_image_context;
+    Cairo::RefPtr<Cairo::ImageSurface> m_image_surface;
+    double m_known_width { 0.0 };
+    double m_known_height { 0.0 };
 };