Make SkCodec support peek() and read()

include/codec/SkCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodec_DEFINED
 #define SkCodec_DEFINED
 
 #include "../private/SkTemplates.h"
 #include "SkColor.h"
 #include "SkEncodedFormat.h"
 #include "SkImageInfo.h"
 #include "SkSize.h"
 #include "SkStream.h"
 #include "SkTypes.h"
 
 class SkData;
 class SkPngChunkReader;
 class SkSampler;
 
 /**
  *  Abstraction layer directly on top of an image codec.
  */
 class SkCodec : SkNoncopyable {
 public:
     /**
+     *  Number of bytes that must be buffered in SkStream input.
+     *
+     *  An SkStream passed to NewFromStream must be able to use this many
+     *  bytes to determine the image type. Then the same SkStream must be
+     *  passed to the correct decoder to read from the beginning.
+     *
+     *  This can be accomplished by implementing peek() to support peeking
+     *  this many bytes, or by implementing rewind() to be able to rewind()
+     *  after reading this many bytes.
+     */
+    static size_t BufferedBytesNeeded();

[reed1, 2015/12/08 15:04:57]

MinBufferedBytesNeeded?

[scroggo, 2015/12/08 18:35:44]

Done.

+
+    /**
      *  If this stream represents an encoded image that we know how to decode,
      *  return an SkCodec that can decode it. Otherwise return NULL.
      *
+     *  As stated above, this call must be able to peek or read
+     *  BufferedBytesNeeded to determine the correct format, and then start
+     *  reading from the beginning. First it will attempt to peek, and it
+     *  assumes that if less than BufferedBytesNeeded bytes (but more than
+     *  zero) are returned, this is because the stream is shorter than this,
+     *  so falling back to reading would not provide more data. If peek()
+     *  returns zero bytes, this call will instead attempt to read(). This
+     *  will require that the stream can be rewind()ed.
+     *
      *  If SkPngChunkReader is not NULL, take a ref and pass it to libpng if
      *  the image is a png.
      *
      *  If NULL is returned, the stream is deleted immediately. Otherwise, the
      *  SkCodec takes ownership of it, and will delete it when done with it.
      */
     static SkCodec* NewFromStream(SkStream*, SkPngChunkReader* = NULL);
 
     /**
      *  If this data represents an encoded image that we know how to decode,
@@ skipping 545 matching lines @@
      *  May create a sampler, if one is not currently being used. Otherwise, does
      *  not affect ownership.
      *
      *  Only valid during scanline decoding.
      */
     virtual SkSampler* getSampler(bool /*createIfNecessary*/) { return nullptr; }
 
     friend class SkSampledCodec;
 };
 #endif // SkCodec_DEFINED