[Impeller] Reland: Added a switch to turn on vulkan (flutter#42669)

relands flutter#42585

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

common/settings.h

@@ -217,6 +217,9 @@ struct Settings {
   bool enable_impeller = false;
 #endif
 
+  // Requests a particular backend to be used (ex "opengles" or "vulkan")
+  std::optional<std::string> impeller_backend;
+
   // Enable Vulkan validation on backends that support it. The validation layers
   // must be available to the application.
   bool enable_vulkan_validation = false;

shell/common/switches.cc

@@ -443,11 +443,23 @@ Settings SettingsFromCommandLine(const fml::CommandLine& command_line) {
   settings.use_asset_fonts =
       !command_line.HasOption(FlagForSwitch(Switch::DisableAssetFonts));
 
-  std::string enable_impeller_value;
-  if (command_line.GetOptionValue(FlagForSwitch(Switch::EnableImpeller),
-                                  &enable_impeller_value)) {
-    settings.enable_impeller =
-        enable_impeller_value.empty() || "true" == enable_impeller_value;
+  {
+    std::string enable_impeller_value;
+    if (command_line.GetOptionValue(FlagForSwitch(Switch::EnableImpeller),
+                                    &enable_impeller_value)) {
+      settings.enable_impeller =
+          enable_impeller_value.empty() || "true" == enable_impeller_value;
+    }
+  }
+
+  {
+    std::string impeller_backend_value;
+    if (command_line.GetOptionValue(FlagForSwitch(Switch::ImpellerBackend),
+                                    &impeller_backend_value)) {
+      if (!impeller_backend_value.empty()) {
+        settings.impeller_backend = impeller_backend_value;
+      }
+    }
   }
 
   settings.enable_vulkan_validation =

shell/common/switches.h

@@ -261,6 +261,10 @@ DEF_SWITCH(EnableImpeller,
            "enable-impeller",
            "Enable the Impeller renderer on supported platforms. Ignored if "
            "Impeller is not supported on the platform.")
+DEF_SWITCH(ImpellerBackend,
+           "impeller-backend",
+           "Requests a particular Impeller backend on platforms that support "
+           "multiple backends. (ex `opengles` or `vulkan`)")
 DEF_SWITCH(EnableVulkanValidation,
            "enable-vulkan-validation",
            "Enable loading Vulkan validation layers. The layers must be "

shell/platform/android/android_context_gl_impeller.cc

@@ -74,7 +74,7 @@ static std::shared_ptr<impeller::Context> CreateImpellerContext(
     FML_LOG(ERROR) << "Could not add reactor worker.";
     return nullptr;
   }
-  FML_LOG(ERROR) << "Using the Impeller rendering backend.";
+  FML_LOG(ERROR) << "Using the Impeller rendering backend (OpenGLES).";
   return context;
 }
 

shell/platform/android/android_context_vulkan_impeller.cc

@@ -34,6 +34,9 @@ static std::shared_ptr<impeller::Context> CreateImpellerContext(
   settings.cache_directory = fml::paths::GetCachesDirectory();
   settings.worker_task_runner = std::move(worker_task_runner);
   settings.enable_validation = enable_vulkan_validation;
+
+  FML_LOG(ERROR) << "Using the Impeller rendering backend (Vulkan).";
+
   return impeller::ContextVK::Create(std::move(settings));
 }
 

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

@@ -43,6 +43,8 @@ public class FlutterLoader {
       "io.flutter.embedding.android.EnableImpeller";
   private static final String ENABLE_VULKAN_VALIDATION_META_DATA_KEY =
       "io.flutter.embedding.android.EnableVulkanValidation";
+  private static final String IMPELLER_BACKEND_META_DATA_KEY =
+      "io.flutter.embedding.android.ImpellerBackend";
 
   /**
    * Set whether leave or clean up the VM after the last shell shuts down. It can be set from app's
@@ -325,6 +327,8 @@ public void ensureInitializationComplete(
         if (metaData.getBoolean(ENABLE_VULKAN_VALIDATION_META_DATA_KEY, false)) {
           shellArgs.add("--enable-vulkan-validation");
         }
+        String backend = metaData.getString(IMPELLER_BACKEND_META_DATA_KEY, "opengles");
+        shellArgs.add("--impeller-backend=" + backend);
       }
 
       final String leakVM = isLeakVM(metaData) ? "true" : "false";

shell/platform/android/platform_view_android.cc

@@ -51,16 +51,9 @@ std::unique_ptr<AndroidSurface> AndroidSurfaceFactoryImpl::CreateSurface() {
       }
     case AndroidRenderingAPI::kVulkan:
       FML_DCHECK(enable_impeller_);
-      // TODO(kaushikiska@): Enable this after wiring a preference for Vulkan
-      // backend.
-#if false
-    return std::make_unique<AndroidSurfaceVulkanImpeller>(
+      return std::make_unique<AndroidSurfaceVulkanImpeller>(
           std::static_pointer_cast<AndroidContextVulkanImpeller>(
               android_context_));
-#else
-      return std::make_unique<AndroidSurfaceGLImpeller>(
-          std::static_pointer_cast<AndroidContextGLImpeller>(android_context_));
-#endif
     default:
       FML_DCHECK(false);
       return nullptr;
@@ -73,19 +66,36 @@ static std::shared_ptr<flutter::AndroidContext> CreateAndroidContext(
     const std::shared_ptr<fml::ConcurrentTaskRunner>& worker_task_runner,
     uint8_t msaa_samples,
     bool enable_impeller,
+    const std::optional<std::string>& impeller_backend,
     bool enable_vulkan_validation) {
   if (use_software_rendering) {
     return std::make_shared<AndroidContext>(AndroidRenderingAPI::kSoftware);
   }
   if (enable_impeller) {
-    // TODO(kaushikiska@): Enable this after wiring a preference for Vulkan
-    // backend.
-#if false
-    return std::make_unique<AndroidContextVulkanImpeller>(enable_vulkan_validation, std::move(worker_task_runner));
-#else
-    return std::make_unique<AndroidContextGLImpeller>(
-        std::make_unique<impeller::egl::Display>());
-#endif
+    // TODO(gaaclarke): We need to devise a more complete heuristic about what
+    //                  backend to use by default.
+    // Default value is OpenGLES.
+    AndroidRenderingAPI backend = AndroidRenderingAPI::kOpenGLES;
+    if (impeller_backend.has_value()) {
+      if (impeller_backend.value() == "opengles") {
+        backend = AndroidRenderingAPI::kOpenGLES;
+      } else if (impeller_backend.value() == "vulkan") {
+        backend = AndroidRenderingAPI::kVulkan;
+      } else {
+        FML_CHECK(impeller_backend.value() == "vulkan" ||
+                  impeller_backend.value() == "opengles");
+      }
+    }
+    switch (backend) {
+      case AndroidRenderingAPI::kOpenGLES:
+        return std::make_unique<AndroidContextGLImpeller>(
+            std::make_unique<impeller::egl::Display>());
+      case AndroidRenderingAPI::kVulkan:
+        return std::make_unique<AndroidContextVulkanImpeller>(
+            enable_vulkan_validation, worker_task_runner);
+      default:
+        FML_UNREACHABLE();
+    }
   }
   return std::make_unique<AndroidContextGLSkia>(
       AndroidRenderingAPI::kOpenGLES,               //
@@ -112,6 +122,7 @@ PlatformViewAndroid::PlatformViewAndroid(
               worker_task_runner,
               msaa_samples,
               delegate.OnPlatformViewGetSettings().enable_impeller,
+              delegate.OnPlatformViewGetSettings().impeller_backend,
               delegate.OnPlatformViewGetSettings().enable_vulkan_validation)) {}
 
 PlatformViewAndroid::PlatformViewAndroid(