allow registerFont after a canvas has been created

Fixes #1921

Author: chearon

CHANGELOG.md

@@ -10,6 +10,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 * Replaced `simple-get ` with ` Node.js builtin` `fetch` (#2309)
 * `ctx.font` has a new C++ parser and is 2x-400x faster. Please file an issue if you experience different results, as caching has been removed.
+* The restriction of registering fonts before a canvas is created has been removed. You can now register a font as late as right before the `fillText` call ([#1921](https://github.com/Automattic/node-canvas/issues/1921))
 
 ### Added
 * Support for accessibility and links in PDFs

src/Canvas.cc

@@ -38,7 +38,10 @@
 
 using namespace std;
 
-std::vector<FontFace> font_face_list;
+std::vector<FontFace> Canvas::font_face_list;
+
+// Increases each time a font is (de)registered
+int Canvas::fontSerial = 1;
 
 /*
  * Initialize Canvas.
@@ -734,6 +737,7 @@ Canvas::RegisterFont(const Napi::CallbackInfo& info) {
   free(family);
   free(weight);
   free(style);
+  fontSerial++;
 }
 
 void
@@ -749,6 +753,7 @@ Canvas::DeregisterAllFonts(const Napi::CallbackInfo& info) {
   });
 
   font_face_list.clear();
+  fontSerial++;
   if (!success) Napi::Error::New(env, "Could not deregister one or more fonts").ThrowAsJavaScriptException();
 }
 

src/Canvas.h

@@ -91,9 +91,11 @@ class Canvas : public Napi::ObjectWrap<Canvas> {
     void resurface(Napi::Object This);
 
     Napi::Env env;
+    static int fontSerial;
 
   private:
     Backend* _backend;
     Napi::ObjectReference _jsBackend;
     Napi::FunctionReference ctor;
+    static std::vector<FontFace> font_face_list;
 };

src/CanvasRenderingContext2d.cc

@@ -2472,6 +2472,16 @@ Context2d::paintText(const Napi::CallbackInfo&info, bool stroke) {
 
   PangoLayout *layout = this->layout();
 
+  // If fonts have been registered, the PangoContext is using an outdated FontMap
+  if (canvas()->fontSerial != fontSerial) {
+    pango_context_set_font_map(
+      pango_layout_get_context(layout),
+      pango_cairo_font_map_get_default()
+    );
+
+    fontSerial = canvas()->fontSerial;
+  }
+
   pango_layout_set_text(layout, str.c_str(), -1);
   pango_cairo_update_layout(context(), layout);
 

src/CanvasRenderingContext2d.h

@@ -227,4 +227,5 @@ class Context2d : public Napi::ObjectWrap<Context2d> {
     cairo_t *_context = nullptr;
     cairo_path_t *_path;
     PangoLayout *_layout = nullptr;
+    int fontSerial = 0;
 };