Make SkPngCodec decode progressively.

src/codec/SkPngCodec.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.
  */
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
 #include "SkPngChunkReader.h"
 #include "SkEncodedFormat.h"
 #include "SkImageInfo.h"
 #include "SkRefCnt.h"
 #include "SkSwizzler.h"
 
 #include "png.h"
 
 class SkStream;
 
 class SkPngCodec : public SkCodec {
 public:
     static bool IsPng(const char*, size_t);
 
     // Assume IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*, SkPngChunkReader* = NULL);
 
     virtual ~SkPngCodec();
 
 protected:
+    SkPngCodec(int width, int height, const SkEncodedInfo&, SkStream*, SkPngChunkReader*,
+            png_structp, png_infop, int bitDepth, sk_sp<SkColorSpace>);
+
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     bool onRewind() override;
     uint32_t onGetFillValue(SkColorType) const override;
 
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
-    Result initializeSwizzler(const SkImageInfo& requestedInfo, const Options&,
-                              SkPMColor*, int* ctableCount);
+    bool initializeSwizzler(const SkImageInfo& requestedInfo, const Options&,
+                            SkPMColor*, int* ctableCount);
     SkSampler* getSampler(bool createIfNecessary) override {
         SkASSERT(fSwizzler);
         return fSwizzler;
     }
 
-    SkPngCodec(int width, int height, const SkEncodedInfo&, SkStream*, SkPngChunkReader*,
-            png_structp, png_infop, int, int, sk_sp<SkColorSpace>);
+    png_structp png_ptr() { return fPng_ptr; }
+    png_infop info_ptr() { return fInfo_ptr; }
+    SkSwizzler* swizzler() { return fSwizzler; }
 
-    png_structp png_ptr() { return fPng_ptr; }
-    SkSwizzler* swizzler() { return fSwizzler; }
-    int numberPasses() const { return fNumberPasses; }
+    /**
+     *  Pass available input to libpng to process it.
+     *
+     *  libpng will call any relevant callbacks installed. This will continue decoding
+     *  until it reaches the end of the file, or until a callback tells libpng to stop.
+     */
+    void processData();
+
+    Result onStartIncrementalDecode(const SkImageInfo& dstInfo, const SkCodec::Options&,
+            SkPMColor* ctable, int* ctableCount) override;
+    Result onIncrementalDecode(std::function<void*(int)> callback, int*) override;
 
 private:
     SkAutoTUnref<SkPngChunkReader>  fPngChunkReader;
     png_structp                     fPng_ptr;
     png_infop                       fInfo_ptr;
 
-    // These are stored here so they can be used both by normal decoding and scanline decoding.
     SkAutoTUnref<SkColorTable>      fColorTable;    // May be unpremul.
     SkAutoTDelete<SkSwizzler>       fSwizzler;
 
-    const int                       fNumberPasses;
-    int                             fBitDepth;
+    const int                       fBitDepth;
 
     bool createColorTable(SkColorType dstColorType, bool premultiply, int* ctableCount);
     void destroyReadStruct();
 
+    virtual Result decodeAllRows(void* dst, size_t rowBytes, int* rowsDecoded) = 0;
+    virtual void setRange(int firstRow, int lastRow) = 0;
+    virtual Result decode(std::function<void*(int)> callback, int* rowsDecoded) = 0;
+
     typedef SkCodec INHERITED;
 };