Make the animator host an implicit fallback VSync provider in case platforms...

Make the animator host an implicit fallback VSync provider in case platforms cant provide their own. (#2953)

services/vsync/BUILD.gn

@@ -4,8 +4,22 @@
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
+source_set("vsync_lib_fallback") {
+  sources = [
+    "fallback/vsync_provider_fallback_impl.h",
+    "fallback/vsync_provider_fallback_impl.cc",
+  ]
+
+  deps = [
+    "//base:base",
+    "//mojo/public/cpp/application",
+    "//mojo/public/cpp/bindings:utility",
+    "//mojo/services/vsync/interfaces",
+  ]
+}
+
 group("vsync") {
-  deps = [ ":vsync_lib" ]
+  deps = [ ":vsync_lib", ":vsync_lib_fallback" ]
 }
 
 if (is_android) {
@@ -54,18 +68,7 @@ if (is_android) {
     ]
   }
 } else {
-  # Fallback VSync (avoid if at all possible)
-  source_set("vsync_lib") {
-    sources = [
-      "fallback/vsync_provider_fallback_impl.h",
-      "fallback/vsync_provider_fallback_impl.cc",
-    ]
-
-    deps = [
-      "//base:base",
-      "//mojo/public/cpp/application",
-      "//mojo/public/cpp/bindings:utility",
-      "//mojo/services/vsync/interfaces",
-    ]
+  group("vsync_lib") {
+    # Empty, will use fallbacks by default.
   }
 }

sky/shell/BUILD.gn

@@ -46,9 +46,8 @@ source_set("common") {
   ]
 
   public_deps = [
-    ":generate_sky_embedder_diagnostic_server_resources_cc",
-    "//base:i18n",
     "//base",
+    "//base:i18n",
     "//build/config/sanitizers:deps",
     "//dart/runtime:libdart",
     "//flutter/assets",
@@ -57,16 +56,17 @@ source_set("common") {
     "//flutter/glue",
     "//flutter/lib/ui",
     "//flutter/runtime",
-    "//flutter/synchronization",
     "//flutter/services/editing:interfaces",
     "//flutter/services/engine:interfaces",
     "//flutter/services/platform",
     "//flutter/services/pointer:interfaces",
     "//flutter/services/semantics:interfaces",
+    "//flutter/services/vsync:vsync_lib_fallback",
     "//flutter/skia",
     "//flutter/sky/engine",
     "//flutter/sky/engine/core:core",
     "//flutter/sky/engine/wtf",
+    "//flutter/synchronization",
     "//lib/ftl",
     "//lib/tonic",
     "//mojo/common",
@@ -79,6 +79,7 @@ source_set("common") {
     "//mojo/services/gfx/composition/interfaces",
     "//mojo/services/navigation/interfaces",
     "//mojo/services/vsync/interfaces",
+    ":generate_sky_embedder_diagnostic_server_resources_cc",
   ]
 }
 

sky/shell/ui/animator.cc

@@ -18,7 +18,11 @@ Animator::Animator(Rasterizer* rasterizer, Engine* engine)
       layer_tree_pipeline_(ftl::MakeRefCounted<LayerTreePipeline>(3)),
       pending_frame_semaphore_(1),
       paused_(false),
-      weak_factory_(this) {}
+      weak_factory_(this) {
+  new services::vsync::VsyncProviderFallbackImpl(
+      mojo::InterfaceRequest<::vsync::VSyncProvider>(
+          mojo::GetProxy(&fallback_vsync_provider_)));
+}
 
 Animator::~Animator() = default;
 
@@ -96,7 +100,7 @@ void Animator::RequestFrame() {
     DCHECK(weak->vsync_provider_)
         << "A VSync provider must be present to schedule a frame.";
 
-    weak->vsync_provider_->AwaitVSync(base::Bind(&Animator::BeginFrame, weak));
+    weak->AwaitVSync(base::Bind(&Animator::BeginFrame, weak));
   });
 }
 
@@ -105,5 +109,18 @@ void Animator::set_vsync_provider(vsync::VSyncProviderPtr vsync_provider) {
   RequestFrame();
 }
 
+void Animator::AwaitVSync(
+    const vsync::VSyncProvider::AwaitVSyncCallback& callback) {
+  // First, try the platform provided VSync provider.
+  if (vsync_provider_) {
+    vsync_provider_->AwaitVSync(callback);
+    return;
+  }
+
+  // Then, use the fallback provider if the platform cannot reliably supply
+  // VSync signals to us.
+  return fallback_vsync_provider_->AwaitVSync(callback);
+}
+
 }  // namespace shell
 }  // namespace sky

sky/shell/ui/animator.h

@@ -13,6 +13,7 @@
 #include "lib/ftl/memory/ref_ptr.h"
 #include "lib/ftl/time/time_point.h"
 #include "mojo/services/vsync/interfaces/vsync.mojom.h"
+#include "flutter/services/vsync/fallback/vsync_provider_fallback_impl.h"
 
 namespace sky {
 namespace shell {
@@ -38,9 +39,12 @@ class Animator {
 
   void BeginFrame(int64_t time_stamp);
 
+  void AwaitVSync(const vsync::VSyncProvider::AwaitVSyncCallback& callback);
+
   Rasterizer* rasterizer_;
   Engine* engine_;
   vsync::VSyncProviderPtr vsync_provider_;
+  vsync::VSyncProviderPtr fallback_vsync_provider_;
   ftl::RefPtr<LayerTreePipeline> layer_tree_pipeline_;
   flutter::Semaphore pending_frame_semaphore_;
   std::unique_ptr<flow::LayerTree> renderable_tree_;