Add scanline decoding to SkCodec.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index ec4b3e1cc80832090c1389823b7e206ab6e97783..3442e64c92a0b983421699a9d9f55921b23e87fb 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -11,6 +11,7 @@
 #include "SkEncodedFormat.h"
 #include "SkImageGenerator.h"
 #include "SkImageInfo.h"
+#include "SkScanlineDecoder.h"
 #include "SkSize.h"
 #include "SkStream.h"
 #include "SkTemplates.h"
@@ -56,6 +57,34 @@ public:
      */
     SkEncodedFormat getEncodedFormat() const { return this->onGetEncodedFormat(); }
 
+    /**
+     *  Create a new scanline decoder.
+     *
+     *  Create a new object which can be used to decode individual scanlines.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo, this implies a scale.
+     *  @return New SkScanlineDecoder, or NULL on failure.
+     *
+     *  NOTE: If any rows were previously decoded, this requires rewinding the
+     *  SkStream.
+     *
+     *  NOTE: The scanline decoder is owned by the SkCodec and will delete it
+     *  when the SkCodec is deleted.
+     */
+    SkScanlineDecoder* getScanlineDecoder(const SkImageInfo& dstInfo);

[reed1, 2015/03/24 15:45:20]

1. Create a new scanline decoder
2. The decoder is owned by the codec

These two seem a little at odds. I was imagining that I could create multiple
scanliners, but now I'm not sure.

[scroggo, 2015/03/24 16:55:47]

I have updated the comment. I think it's more clear now.

+
+    /**
+     *  Some images may initially report that they have alpha due to the format
+     *  of the encoded data, but then never use any colors which have alpha
+     *  less than 100%. This function can be called *after* decoding to
+     *  determine if such an image truly had alpha. Calling it before decoding
+     *  is undefined.
+     */
+    bool reallyHasAlpha() const {
+        return this->onReallyHasAlpha();
+    }
+
 protected:
     SkCodec(const SkImageInfo&, SkStream*);
 
@@ -71,9 +100,6 @@ protected:
     }
 #endif
 
-    // Helper for subclasses.
-    const SkImageInfo& getOriginalInfo() { return fInfo; }
-
     virtual SkISize onGetScaledDimensions(float /* desiredScale */) const {
         // By default, scaling is not supported.
         return fInfo.dimensions();
@@ -82,6 +108,23 @@ protected:
     virtual SkEncodedFormat onGetEncodedFormat() const = 0;
 
     /**
+     *  Override if your codec supports scanline decoding.
+     *
+     *  No need to call rewindIfNeeded(), which will have already been called
+     *  by the base class.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo, this implies a scale.
+     *  @return New SkScanlineDecoder on success, NULL otherwise. The SkCodec
+     *      will take ownership of the returned scanline decoder.
+     */
+    virtual SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo) {
+        return NULL;
+    }
+
+    virtual bool onReallyHasAlpha() const { return false; }
+
+    /**
      *  If the stream was previously read, attempt to rewind.
      *  @returns:
      *      true
@@ -106,9 +149,10 @@ protected:
     }
 
 private:
-    const SkImageInfo       fInfo;
-    SkAutoTDelete<SkStream> fStream;
-    bool                    fNeedsRewind;
+    const SkImageInfo                   fInfo;
+    SkAutoTDelete<SkStream>             fStream;
+    bool                                fNeedsRewind;
+    SkAutoTDelete<SkScanlineDecoder>    fScanlineDecoder;
 
     typedef SkImageGenerator INHERITED;
 };