Create a scanline decoder without creating a codec

include/codec/SkScanlineDecoder.h

Index: include/codec/SkScanlineDecoder.h
diff --git a/include/codec/SkScanlineDecoder.h b/include/codec/SkScanlineDecoder.h
index 8376e57c09d84bb48c1d7ae295ccd441a7feab48..c233663dbdb2d216349eed791744e20eec060f3f 100644
--- a/include/codec/SkScanlineDecoder.h
+++ b/include/codec/SkScanlineDecoder.h
@@ -16,6 +16,27 @@
 class SkScanlineDecoder : public SkNoncopyable {
 public:
     /**
+     *  If this stream represents an encoded image that we know how to decode
+     *  in scanlines, return an SkScanlineDecoder that can decode it. Otherwise
+     *  return NULL.
+     *
+     *  start() must be called in order to decode any scanlines.
+     *
+     *  If NULL is returned, the stream is deleted immediately. Otherwise, the
+     *  SkScanlineDecoder takes ownership of it, and will delete it when done
+     *  with it.
+     */
+    static SkScanlineDecoder* NewFromStream(SkStream*);
+
+    /**
+     *  Similar to NewFromStream, but reads from an SkData.
+     *
+     *  Will take a ref if it returns a scanline decoder, else will not affect
+     *  the data.
+     */
+    static SkScanlineDecoder* NewFromData(SkData*);
+
+    /**
      *  Clean up after reading/skipping scanlines.
      *
      *  It is possible that not all scanlines will have been read/skipped.  In
@@ -25,8 +46,43 @@ public:
     virtual ~SkScanlineDecoder() {}
 
     /**
+     *  Returns the default info, corresponding to the encoded data.
+     */
+    const SkImageInfo& getInfo() { return fSrcInfo; }
+
+    /**
+     *  Initialize on the first scanline, with the specified options.
+     *
+     *  This must be called in order to call getScanlnies or skipScanlines. If
+     *  it has been called before, this will require rewinding the stream.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo, this implies a scale.
+     *  @param options Contains decoding options, including if memory is zero
+     *      initialized.
+     *  @param ctable A pointer to a color table.  When dstInfo.colorType() is
+     *      kIndex8, this should be non-NULL and have enough storage for 256
+     *      colors.  The color table will be populated after decoding the palette.
+     *  @param ctableCount A pointer to the size of the color table.  When
+     *      dstInfo.colorType() is kIndex8, this should be non-NULL.  It will
+     *      be modified to the true size of the color table (<= 256) after
+     *      decoding the palette.
+     *  @return Enum representing success or reason for failure.
+     */
+    SkCodec::Result start(const SkImageInfo& dstInfo, const SkCodec::Options* options,
+                          SkPMColor ctable[], int* ctableCount);
+
+    /**
+     *  Simplified version of start() that asserts that info is NOT
+     *  kIndex8_SkColorType and uses the default Options.
+     */
+    SkCodec::Result start(const SkImageInfo& dstInfo);
+
+    /**
      *  Write the next countLines scanlines into dst.
      *
+     *  Not valid to call before calling start().
+     *
      *  @param dst Must be non-null, and large enough to hold countLines
      *      scanlines of size rowBytes.
      *  @param countLines Number of lines to write.
@@ -34,6 +90,7 @@ public:
      *      a scanline based on the SkImageInfo used to create this object.
      */
     SkCodec::Result getScanlines(void* dst, int countLines, size_t rowBytes) {
+        SkASSERT(!fDstInfo.isEmpty());
         if ((rowBytes < fDstInfo.minRowBytes() && countLines > 1 ) || countLines <= 0
                 || fCurrScanline + countLines > fDstInfo.height()) {
             return SkCodec::kInvalidParameters;
@@ -46,12 +103,15 @@ public:
     /**
      *  Skip count scanlines.
      *
+     *  Not valid to call before calling start().
+     *
      *  The default version just calls onGetScanlines and discards the dst.
      *  NOTE: If skipped lines are the only lines with alpha, this default
      *  will make reallyHasAlpha return true, when it could have returned
      *  false.
      */
     SkCodec::Result skipScanlines(int countLines) {
+        SkASSERT(!fDstInfo.isEmpty());
         if (fCurrScanline + countLines > fDstInfo.height()) {
             // Arguably, we could just skip the scanlines which are remaining,
             // and return kSuccess. We choose to return invalid so the client
@@ -76,8 +136,9 @@ public:
     }
 
 protected:
-    SkScanlineDecoder(const SkImageInfo& requested)
-        : fDstInfo(requested)
+    SkScanlineDecoder(const SkImageInfo& srcInfo)
+        : fSrcInfo(srcInfo)
+        , fDstInfo()
         , fCurrScanline(0) {}
 
     virtual bool onReallyHasAlpha() const { return false; }
@@ -85,9 +146,14 @@ protected:
     const SkImageInfo& dstInfo() const { return fDstInfo; }
 
 private:
-    const SkImageInfo   fDstInfo;
+    const SkImageInfo   fSrcInfo;
+    SkImageInfo         fDstInfo;
     int                 fCurrScanline;
 
+    virtual SkCodec::Result onStart(const SkImageInfo& dstInfo,
+                                    const SkCodec::Options& options,
+                                    SkPMColor ctable[], int* ctableCount) = 0;
+
     // Naive default version just calls onGetScanlines on temp memory.
     virtual SkCodec::Result onSkipScanlines(int countLines) {
         SkAutoMalloc storage(fDstInfo.minRowBytes());