Make OffscreenCanvas Transferable

third_party/WebKit/Source/bindings/core/v8/TransferableExtractor.h

Index: third_party/WebKit/Source/bindings/core/v8/TransferableExtractor.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/TransferableExtractor.h b/third_party/WebKit/Source/bindings/core/v8/TransferableExtractor.h
new file mode 100644
index 0000000000000000000000000000000000000000..6438e300268d46b3cb5858369f651e9a405781fc
--- /dev/null
+++ b/third_party/WebKit/Source/bindings/core/v8/TransferableExtractor.h
@@ -0,0 +1,26 @@
+// Copyright 2016 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 TransferableExtractor_h
+#define TransferableExtractor_h
+
+#include "bindings/core/v8/ExceptionState.h"
+#include "bindings/core/v8/SerializedScriptValue.h"
+#include "core/CoreExport.h"
+#include "wtf/PassOwnPtr.h"
+#include <v8.h>
+
+namespace blink {
+
+class CORE_EXPORT TransferableExtractor {

[haraken, 2016/04/19 00:48:10]

Do you have a plan to add more extractors? It looks a bit overkilling to
introduce this class only for TransferableOffscreenCanvasExtractor.

If you only need the TransferableOffscreenCanvasExtractor, can you register the
extractor on SerializedScriptValueForModules (and use the extractor in
SerializedScriptValueForModules)?

[xidachen, 2016/04/19 15:29:50]

The reason to have this class here is that we cannot foresee the future. Just
like when we add the ArrayBuffer and ImageBitmap to be transferable, we should
have created a Transferables class at that time.

As worker gets more and more popular, I certainly believe that there will be
other transferable types in the future because transfer an object involves zero
copy. These transferable types could live under modules/. So this
TransferableExtractor provides a place holder for all types in modules that are
to be "transferable" in the future.

[haraken, 2016/04/20 01:18:24]

If you don't have a concrete plan now, I'd prefer keeping the code as simple as
possible. We can generalize the mechanism when we actually need it.

+public:
+    virtual ~TransferableExtractor() { }
+    static void registerTransferableExtractor(PassOwnPtr<TransferableExtractor>);
+    static const Vector<OwnPtr<TransferableExtractor>>& getExtractors();
+    virtual bool extract(v8::Local<v8::Value>, v8::Isolate*, Transferables&, ExceptionState&, const unsigned) = 0;
+};
+
+} // namespace blink
+
+#endif // TransferableExtractor_h