@Override
 public void setClosedCallback(CompletedCallback handler) {
   mSink.setClosedCallback(handler);
 }
 @Override
 public CompletedCallback getClosedCallback() {
   return mSink.getClosedCallback();
 }
 @Override
 public void close() {
   mSink.close();
 }
 @Override
 public boolean isOpen() {
   return mSink.isOpen();
 }
 @Override
 public WritableCallback getWriteableCallback() {
   return mSink.getWriteableCallback();
 }
 @Override
 public void setWriteableCallback(WritableCallback handler) {
   mSink.setWriteableCallback(handler);
 }
 @Override
 public void write(ByteBufferList bb) {
   assertContent();
   mSink.write(bb);
 }