Eliminate ShellTestPlatformView::BackendType::kDefaultBackendType

`kDefaultBackendType` is intended to make life easier for authors of
tests, but in any switch statement where it's used (currently just a
single location), we rely on ordering it first and `#ifdef`ing out all
backends that aren't available.

Instead, we define a static function that returns the default that
callers can invoke instead. This avoids relinace on case ordering and
fallthrough.

In flutter#56722 we split backends out
into separate translation units, and remove the `#ifdef`s which means we
can't rely on this trick anymore.

Author: cbracken

shell/common/animator_unittests.cc

@@ -88,8 +88,8 @@ TEST_F(ShellTest, VSyncTargetTime) {
         flutter::PlatformData(), task_runners, settings,
         [vsync_clock, &create_vsync_waiter](Shell& shell) {
           return ShellTestPlatformView::Create(
-              shell, shell.GetTaskRunners(), vsync_clock, create_vsync_waiter,
-              ShellTestPlatformView::BackendType::kDefaultBackend, nullptr,
+              ShellTestPlatformView::DefaultBackendType(), shell,
+              shell.GetTaskRunners(), vsync_clock, create_vsync_waiter, nullptr,
               shell.GetIsGpuDisabledSyncSwitch());
         },
         [](Shell& shell) { return std::make_unique<Rasterizer>(shell); });

shell/common/shell_test_platform_view.cc

@@ -20,40 +20,44 @@ namespace flutter {
 namespace testing {
 
 std::unique_ptr<ShellTestPlatformView> ShellTestPlatformView::Create(
+    BackendType backend,
     PlatformView::Delegate& delegate,
     const TaskRunners& task_runners,
     const std::shared_ptr<ShellTestVsyncClock>& vsync_clock,
     const CreateVsyncWaiter& create_vsync_waiter,
-    BackendType backend,
     const std::shared_ptr<ShellTestExternalViewEmbedder>&
         shell_test_external_view_embedder,
     const std::shared_ptr<const fml::SyncSwitch>& is_gpu_disabled_sync_switch) {
   // TODO(gw280): https://github.com/flutter/flutter/issues/50298
   // Make this fully runtime configurable
   switch (backend) {
-    case BackendType::kDefaultBackend:
-#ifdef SHELL_ENABLE_GL
     case BackendType::kGLBackend:
+#ifdef SHELL_ENABLE_GL
       return std::make_unique<ShellTestPlatformViewGL>(
           delegate, task_runners, vsync_clock, create_vsync_waiter,
           shell_test_external_view_embedder);
+#else
+      FML_LOG(FATAL) << "OpenGL not enabled in this build";
+      return nullptr;
 #endif  // SHELL_ENABLE_GL
-#ifdef SHELL_ENABLE_VULKAN
     case BackendType::kVulkanBackend:
+#ifdef SHELL_ENABLE_VULKAN
       return std::make_unique<ShellTestPlatformViewVulkan>(
           delegate, task_runners, vsync_clock, create_vsync_waiter,
           shell_test_external_view_embedder);
+#else
+      FML_LOG(FATAL) << "Vulkan not enabled in this build";
+      return nullptr;
 #endif  // SHELL_ENABLE_VULKAN
-#ifdef SHELL_ENABLE_METAL
     case BackendType::kMetalBackend:
+#ifdef SHELL_ENABLE_METAL
       return std::make_unique<ShellTestPlatformViewMetal>(
           delegate, task_runners, vsync_clock, create_vsync_waiter,
           shell_test_external_view_embedder, is_gpu_disabled_sync_switch);
-#endif  // SHELL_ENABLE_METAL
-
-    default:
-      FML_LOG(FATAL) << "No backends supported for ShellTestPlatformView";
+#else
+      FML_LOG(FATAL) << "Metal not enabled in this build";
       return nullptr;
+#endif  // SHELL_ENABLE_METAL
   }
 }
 
@@ -76,11 +80,11 @@ std::unique_ptr<PlatformView> ShellTestPlatformViewBuilder::operator()(
     }
   };
   return ShellTestPlatformView::Create(
+      config_.rendering_backend,                  //
       shell,                                      //
       task_runners,                               //
       vsync_clock,                                //
       create_vsync_waiter,                        //
-      config_.rendering_backend,                  //
       config_.shell_test_external_view_embedder,  //
       shell.GetIsGpuDisabledSyncSwitch()          //
   );

shell/common/shell_test_platform_view.h

@@ -5,6 +5,8 @@
 #ifndef FLUTTER_SHELL_COMMON_SHELL_TEST_PLATFORM_VIEW_H_
 #define FLUTTER_SHELL_COMMON_SHELL_TEST_PLATFORM_VIEW_H_
 
+#include <exception>
+
 #include "flutter/shell/common/platform_view.h"
 #include "flutter/shell/common/shell_test_external_view_embedder.h"
 #include "flutter/shell/common/vsync_waiters_test.h"
@@ -15,18 +17,30 @@ namespace testing {
 class ShellTestPlatformView : public PlatformView {
  public:
   enum class BackendType {
-    kDefaultBackend = 0,
     kGLBackend,
     kVulkanBackend,
     kMetalBackend,
   };
 
+  static BackendType DefaultBackendType() {
+#if defined(SHELL_ENABLE_GL)
+    return BackendType::kGLBackend;
+#elif defined(SHELL_ENABLE_METAL)
+    return BackendType::kMetalBackend;
+#elif defined(SHELL_ENABLE_VULKAN)
+    return BackendType::kVulkanBackend;
+#else
+    FML_LOG(FATAL) << "No backend is enabled in this build.";
+    std::terminate();
+#endif
+  }
+
   static std::unique_ptr<ShellTestPlatformView> Create(
+      BackendType backend,
       PlatformView::Delegate& delegate,
       const TaskRunners& task_runners,
       const std::shared_ptr<ShellTestVsyncClock>& vsync_clock,
       const CreateVsyncWaiter& create_vsync_waiter,
-      BackendType backend,
       const std::shared_ptr<ShellTestExternalViewEmbedder>&
           shell_test_external_view_embedder,
       const std::shared_ptr<const fml::SyncSwitch>&
@@ -50,7 +64,7 @@ class ShellTestPlatformViewBuilder {
     std::shared_ptr<ShellTestExternalViewEmbedder>
         shell_test_external_view_embedder = nullptr;
     ShellTestPlatformView::BackendType rendering_backend =
-        ShellTestPlatformView::BackendType::kDefaultBackend;
+        ShellTestPlatformView::DefaultBackendType();
   };
 
   explicit ShellTestPlatformViewBuilder(Config config);

shell/common/shell_unittests.cc

@@ -500,13 +500,13 @@ TEST_F(ShellTest,
         // vsync mechanism. We should have better DI in the tests.
         const auto vsync_clock = std::make_shared<ShellTestVsyncClock>();
         return ShellTestPlatformView::Create(
-            shell, shell.GetTaskRunners(), vsync_clock,
+            ShellTestPlatformView::DefaultBackendType(), shell,
+            shell.GetTaskRunners(), vsync_clock,
             [task_runners = shell.GetTaskRunners()]() {
               return static_cast<std::unique_ptr<VsyncWaiter>>(
                   std::make_unique<VsyncWaiterFallback>(task_runners));
             },
-            ShellTestPlatformView::BackendType::kDefaultBackend, nullptr,
-            shell.GetIsGpuDisabledSyncSwitch());
+            nullptr, shell.GetIsGpuDisabledSyncSwitch());
       },
       [](Shell& shell) { return std::make_unique<Rasterizer>(shell); });
   ASSERT_TRUE(ValidateShell(shell.get()));