mediaview: Add a typemap for arc::mojom::ChangeType.

components/arc/file_system/file_system_struct_traits.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_ARC_FILE_SYSTEM_FILE_SYSTEM_STRUCT_TRAITS_H_
+#define COMPONENTS_ARC_FILE_SYSTEM_FILE_SYSTEM_STRUCT_TRAITS_H_
+
+#include "base/logging.h"
+#include "components/arc/common/file_system.mojom.h"
+#include "storage/browser/fileapi/watcher_manager.h"
+
+namespace mojo {
+
+template <>
+struct EnumTraits<arc::mojom::ChangeType, storage::WatcherManager::ChangeType> {
+  static arc::mojom::ChangeType ToMojom(
+      storage::WatcherManager::ChangeType type) {
+    switch (type) {
+      case storage::WatcherManager::CHANGED:
+        return arc::mojom::ChangeType::CHANGED;
+      case storage::WatcherManager::DELETED:
+        return arc::mojom::ChangeType::DELETED;

[dcheng, 2017/03/02 07:26:36]

In general, we prefer to out-of-line not-obviously trivial methods. In this
case, it should optimize pretty well since the enum values on both sides should
match, but it's not obvious from reading the code.

If you want to keep inline, I guess one option is to static_cast and then have
static_assert that the enum values match. Up to you.

[Shuhei Takahashi, 2017/03/02 07:45:44]

Hmm, OK, please let me move this definition to out-of-line. I'm hesitant to use
static_cast because new enum values might be added to
storage::WatcherManager::ChangeType without noticing.

+    }
+    NOTREACHED();
+    return arc::mojom::ChangeType::CHANGED;
+  }
+
+  static bool FromMojom(arc::mojom::ChangeType mojom_type,
+                        storage::WatcherManager::ChangeType* type) {
+    switch (mojom_type) {
+      case arc::mojom::ChangeType::CHANGED:
+        *type = storage::WatcherManager::CHANGED;
+        return true;
+      case arc::mojom::ChangeType::DELETED:
+        *type = storage::WatcherManager::DELETED;
+        return true;
+    }
+    NOTREACHED();
+    return false;
+  }
+};
+
+}  // namespace mojo
+
+#endif  // COMPONENTS_ARC_FILE_SYSTEM_FILE_SYSTEM_STRUCT_TRAITS_H_