Migrate FlBinaryMessenger using embedder API instead of mock engine.

This allows us to remove most of the remaining mock engine code.

shell/platform/linux/fl_binary_messenger.cc

@@ -155,21 +155,9 @@ static gboolean fl_binary_messenger_platform_message_cb(
     GBytes* message,
     const FlutterPlatformMessageResponseHandle* response_handle,
     void* user_data) {
-  FlBinaryMessengerImpl* self = FL_BINARY_MESSENGER_IMPL(user_data);
-
-  PlatformMessageHandler* handler = static_cast<PlatformMessageHandler*>(
-      g_hash_table_lookup(self->platform_message_handlers, channel));
-  if (handler == nullptr) {
-    return FALSE;
-  }
-
-  g_autoptr(FlBinaryMessengerResponseHandleImpl) handle =
-      fl_binary_messenger_response_handle_impl_new(self, response_handle);
-  handler->message_handler(FL_BINARY_MESSENGER(self), channel, message,
-                           FL_BINARY_MESSENGER_RESPONSE_HANDLE(handle),
-                           handler->message_handler_data);
-
-  return TRUE;
+  FlBinaryMessenger* self = FL_BINARY_MESSENGER(user_data);
+  return fl_binary_messenger_handle_message(self, channel, message,
+                                            response_handle);
 }
 
 static void fl_binary_messenger_impl_dispose(GObject* object) {
@@ -487,6 +475,28 @@ G_MODULE_EXPORT void fl_binary_messenger_set_warns_on_channel_overflow(
       self, channel, warns);
 }
 
+gboolean fl_binary_messenger_handle_message(
+    FlBinaryMessenger* messenger,
+    const gchar* channel,
+    GBytes* message,
+    const FlutterPlatformMessageResponseHandle* response_handle) {
+  FlBinaryMessengerImpl* self = FL_BINARY_MESSENGER_IMPL(messenger);
+
+  PlatformMessageHandler* handler = static_cast<PlatformMessageHandler*>(
+      g_hash_table_lookup(self->platform_message_handlers, channel));
+  if (handler == nullptr) {
+    return FALSE;
+  }
+
+  g_autoptr(FlBinaryMessengerResponseHandleImpl) handle =
+      fl_binary_messenger_response_handle_impl_new(self, response_handle);
+  handler->message_handler(FL_BINARY_MESSENGER(self), channel, message,
+                           FL_BINARY_MESSENGER_RESPONSE_HANDLE(handle),
+                           handler->message_handler_data);
+
+  return TRUE;
+}
+
 void fl_binary_messenger_shutdown(FlBinaryMessenger* self) {
   g_return_if_fail(FL_IS_BINARY_MESSENGER(self));
 

shell/platform/linux/fl_binary_messenger_private.h

@@ -7,6 +7,7 @@
 
 #include <glib-object.h>
 
+#include "flutter/shell/platform/embedder/embedder.h"
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_engine.h"
 
 G_BEGIN_DECLS
@@ -22,6 +23,24 @@ G_BEGIN_DECLS
  */
 FlBinaryMessenger* fl_binary_messenger_new(FlEngine* engine);
 
+/**
+ * fl_binary_messenger_handle_message:
+ * @messenger: an #FlBinaryMessenger.
+ * @channel: channel message received on.
+ * @message: message data.
+ * @response_handle: handle to provide to
+ * fl_engine_send_platform_message_response().
+ *
+ * Handles a message received from the engine. Available for testing purposes.
+ *
+ * Returns: %TRUE if the message is handled.
+ */
+gboolean fl_binary_messenger_handle_message(
+    FlBinaryMessenger* messenger,
+    const gchar* channel,
+    GBytes* message,
+    const FlutterPlatformMessageResponseHandle* response_handle);
+
 /**
  * fl_binary_messenger_shutdown:
  * @messenger: an #FlBinaryMessenger.