Add reallyHasAlpha() to SkAndroidCodec

include/codec/SkAndroidCodec.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 SkAndroidCodec_DEFINED
 #define SkAndroidCodec_DEFINED
 
@@ skipping 190 matching lines @@
             const AndroidOptions* options);
 
     /**
      *  Simplified version of getAndroidPixels() where we supply the default AndroidOptions.
      *
      *  This will return an error if the info is kIndex_8_SkColorType and also will not perform
      *  any scaling or subsetting.
      */
     SkCodec::Result getAndroidPixels(const SkImageInfo& info, void* pixels, size_t rowBytes);
 
+    /**
+     *  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.
+     *  FIXME: see skbug.com/3582.
+     */
+    bool reallyHasAlpha() const {

[scroggo, 2015/12/10 17:02:32]

With some changes, this doesn't even need to be virtual.

It seems that both subclasses (SkSampledCodec and SkWebpAdapterCodec) have an
fCodec member. We could move that into this class (like we did with fStream) and
then call that directly.

Combining with an idea we discussed in person: SkAndroidCodec needs a lot less
duplicated virtuals if we instead provide an accessor for the codec:

public class SkAndroidCodec{
public:
  ...
  // I'm not tied to this name...
  const SkCodec* actualCodec() const { return fCodec.get(); }
protected:
  SkCodec* codec() { return fCodec.get(); }
private:
  SkAutoTDelete<SkCodec> fCodec;
}

Then the calling class can call these const methods directly:

SkCodec* codec = androidCodec.actualCodec();
codec->getEncodedFormat()
codec->reallyHasAlpha()
...

[msarett, 2015/12/10 17:41:18]

I went halfway there.  Can we save the actualCodec() part for later?

[scroggo, 2015/12/10 18:29:42]

fine with me

+        return this->onReallyHasAlpha();
+    }
+
 protected:
 
     SkAndroidCodec(const SkImageInfo&);
 
     virtual SkEncodedFormat onGetEncodedFormat() const = 0;
 
     virtual SkISize onGetSampledDimensions(int sampleSize) const = 0;
 
     virtual bool onGetSupportedSubset(SkIRect* desiredSubset) const = 0;
 
     virtual SkCodec::Result onGetAndroidPixels(const SkImageInfo& info, void* pixels,
             size_t rowBytes, const AndroidOptions& options) = 0;
 
+    virtual bool onReallyHasAlpha() const = 0;
+
 private:
 
     // This will always be a reference to the info that is contained by the
     // embedded SkCodec.
     const SkImageInfo& fInfo;
 };
 #endif // SkAndroidCodec_DEFINED