Eliminate copies of encoded image data

third_party/WebKit/Source/platform/image-decoders/SegmentReader.h

+// 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 SegmentReader_h
+#define SegmentReader_h
+
+#include "platform/SharedBuffer.h"
+#include "third_party/skia/include/core/SkData.h"
+#include "third_party/skia/src/core/SkRWBuffer.h"
+#include "wtf/Noncopyable.h"
+#include "wtf/PassRefPtr.h"
+#include "wtf/RefCounted.h"
+
+namespace blink {
+
+// Interface that looks like SharedBuffer. Used by ImageDecoders to use various
+// sources of input including:
+// - SharedBuffer
+//   - for when the caller already has a SharedBuffer
+// - SkData
+//   - for when the caller already has an SkData
+// - SkROBuffer
+//   - for when the caller wants to read/write in different threads
+//
+// Unlike SharedBuffer, this is a read-only interface. There is no way to
+// modify the underlying data source.
+class SegmentReader : public RefCounted<SegmentReader> {
+    WTF_MAKE_NONCOPYABLE(SegmentReader);
+public:
+    static PassRefPtr<SegmentReader> createFromSharedBuffer(PassRefPtr<SharedBuffer>);
+    static PassRefPtr<SegmentReader> createFromSkData(PassRefPtr<SkData>);
+    static PassRefPtr<SegmentReader> createFromSkROBuffer(PassRefPtr<SkROBuffer>);
+
+    SegmentReader() {}
+    virtual ~SegmentReader() {}
+    virtual size_t size() const = 0;
+    virtual size_t getSomeData(const char*& data, size_t position) const = 0;
+    virtual PassRefPtr<SkData> getAsSkData() const = 0;

[Peter Kasting, 2016/03/25 06:24:37]

Given this interface, I think it would be a little less confusing to read to
merge the three .cpp files you have (for the three different implementations of
this) into one SegmentReader.cpp.

Then within that file, you can first define each of the classes, with dividers,
and then finally define the createXXX functions for SegmentReader in a bottom
section:

// SharedBufferSegmentReader ---------------------------------------------------

class SharedBufferSegmentReader : public SegmentReader {
...
};


// DataSegmentReader -----------------------------------------------------------

...

...


// SegmentReader ---------------------------------------------------------------

// static
PassRefPtr<SegmentReader>
SegmentReader::createFromSharedBuffer(PassRefPtr<SharedBuffer>) {
  ...
}

...

This ensures the implementations of concrete SegmentReader functions are all
together and in the place one would most likely look; there aren't .cpp files
with no corresponding .h files to confuse people looking at directory file
lists; makes it clear all these other classes are implementation details of this
particular public class; and puts those other classes near each other so readers
can see the parallelism in their implementations.

[scroggo, 2016/03/25 15:49:53]

Done in patch set 15.

+};
+
+} // namespace blink
+#endif // SegmentReader_h