Pre-warm default font manager, to reduce time cost of engine setup

Author: scutlight

shell/platform/android/flutter_main.cc

@@ -21,6 +21,7 @@
 #include "flutter/shell/common/shell.h"
 #include "flutter/shell/common/switches.h"
 #include "third_party/dart/runtime/include/dart_tools_api.h"
+#include "third_party/skia/include/core/SkFontMgr.h"
 
 namespace flutter {
 
@@ -155,6 +156,10 @@ void FlutterMain::SetupObservatoryUriCallback(JNIEnv* env) {
       });
 }
 
+static void CreateDefaultFontManager(JNIEnv* env, jclass jcaller) {
+  sk_sp<SkFontMgr> font_mgr(SkFontMgr::RefDefault());
+}
+
 bool FlutterMain::Register(JNIEnv* env) {
   static const JNINativeMethod methods[] = {
       {
@@ -163,6 +168,11 @@ bool FlutterMain::Register(JNIEnv* env) {
                        "lang/String;Ljava/lang/String;Ljava/lang/String;J)V",
           .fnPtr = reinterpret_cast<void*>(&Init),
       },
+      {
+          .name = "nativeCreateDefaultFontManager",
+          .signature = "()V",
+          .fnPtr = reinterpret_cast<void*>(&CreateDefaultFontManager),
+      },
   };
 
   jclass clazz = env->FindClass("io/flutter/embedding/engine/FlutterJNI");

shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

@@ -106,6 +106,13 @@ public static native void nativeInit(
       @NonNull String engineCachesPath,
       long initTimeMillis);
 
+  /**
+   * Create the default font manager provided by SkFontMgr::RefDefault() which is a process-wide
+   * singleton owned by Skia. Note that, the first call to SkFontMgr::RefDefault() will take
+   * noticeable time, but later calls will return a reference to the preexisting font manager.
+   */
+  public static native void nativeCreateDefaultFontManager();
+
   // TODO(mattcarroll): add javadocs
   @UiThread
   public native boolean nativeGetIsSoftwareRenderingEnabled();

shell/platform/android/io/flutter/embedding/engine/loader/FlutterLoader.java

@@ -144,6 +144,17 @@ public InitResult call() {
 
             System.loadLibrary("flutter");
 
+            // Pre-warm the default font manager as soon as possible on a background thread.
+            // It helps to reduce time cost of engine setup that blocks the platform thread.
+            new Thread(
+                    new Runnable() {
+                      @Override
+                      public void run() {
+                        FlutterJNI.nativeCreateDefaultFontManager();
+                      }
+                    })
+                .start();
+
             if (resourceExtractor != null) {
               resourceExtractor.waitForCompletion();
             }