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

shell/common/animator_unittests.cc

@@ -89,7 +89,7 @@ TEST_F(ShellTest, VSyncTargetTime) {
         [vsync_clock, &create_vsync_waiter](Shell& shell) {
           return ShellTestPlatformView::Create(
               shell, shell.GetTaskRunners(), vsync_clock, create_vsync_waiter,
-              ShellTestPlatformView::BackendType::kDefaultBackend, nullptr,
+              ShellTestPlatformView::DefaultBackendType(), nullptr,
               shell.GetIsGpuDisabledSyncSwitch());
         },
         [](Shell& shell) { return std::make_unique<Rasterizer>(shell); });

shell/common/shell_test_platform_view.cc

@@ -31,29 +31,33 @@ std::unique_ptr<ShellTestPlatformView> ShellTestPlatformView::Create(
   // 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
   }
 }
 

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"
@@ -15,12 +17,24 @@ 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::kMetalBackend;
+#else
+    FML_LOG(FATAL) << "No backend is enabled in this build.";
+    std::terminate();
+#endif
+  }
+
   static std::unique_ptr<ShellTestPlatformView> Create(
       PlatformView::Delegate& delegate,
       const TaskRunners& task_runners,
@@ -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

@@ -505,7 +505,7 @@ TEST_F(ShellTest,
               return static_cast<std::unique_ptr<VsyncWaiter>>(
                   std::make_unique<VsyncWaiterFallback>(task_runners));
             },
-            ShellTestPlatformView::BackendType::kDefaultBackend, nullptr,
+            ShellTestPlatformView::DefaultBackendType(), nullptr,
             shell.GetIsGpuDisabledSyncSwitch());
       },
       [](Shell& shell) { return std::make_unique<Rasterizer>(shell); });