[SkParagraph] Cache the font collection created for use by the Skia text shaper library (flutter#15317)

Author: jason-simmons

third_party/txt/src/txt/font_collection.cc

@@ -315,19 +315,20 @@ void FontCollection::ClearFontFamilyCache() {
 
 sk_sp<skia::textlayout::FontCollection>
 FontCollection::CreateSktFontCollection() {
-  sk_sp<skia::textlayout::FontCollection> skt_collection =
-      sk_make_sp<skia::textlayout::FontCollection>();
-
-  skt_collection->setDefaultFontManager(default_font_manager_,
-                                        GetDefaultFontFamily().c_str());
-  skt_collection->setAssetFontManager(asset_font_manager_);
-  skt_collection->setDynamicFontManager(dynamic_font_manager_);
-  skt_collection->setTestFontManager(test_font_manager_);
-  if (!enable_font_fallback_) {
-    skt_collection->disableFontFallback();
+  if (!skt_collection_) {
+    skt_collection_ = sk_make_sp<skia::textlayout::FontCollection>();
+
+    skt_collection_->setDefaultFontManager(default_font_manager_,
+                                           GetDefaultFontFamily().c_str());
+    skt_collection_->setAssetFontManager(asset_font_manager_);
+    skt_collection_->setDynamicFontManager(dynamic_font_manager_);
+    skt_collection_->setTestFontManager(test_font_manager_);
+    if (!enable_font_fallback_) {
+      skt_collection_->disableFontFallback();
+    }
   }
 
-  return skt_collection;
+  return skt_collection_;
 }
 
 #endif  // FLUTTER_ENABLE_SKSHAPER

third_party/txt/src/txt/font_collection.h

@@ -107,6 +107,11 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
       fallback_fonts_for_locale_;
   bool enable_font_fallback_;
 
+#if FLUTTER_ENABLE_SKSHAPER
+  // An equivalent font collection usable by the Skia text shaper library.
+  sk_sp<skia::textlayout::FontCollection> skt_collection_;
+#endif
+
   // Performs the actual work of MatchFallbackFont. The result is cached in
   // fallback_match_cache_.
   const std::shared_ptr<minikin::FontFamily>& DoMatchFallbackFont(