Add support for multiple frames in SkCodec

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index 664dc561080bb39a76c8d09a35b6cb19fad8a0a7..b991ecab3aada96c94afab94ed294430b7bf6e48 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -18,6 +18,8 @@
 #include "SkTypes.h"
 #include "SkYUVSizeInfo.h"
 
+#include <vector>
+
 class SkColorSpace;
 class SkData;
 class SkPngChunkReader;
@@ -606,38 +608,38 @@ public:
      */
     int outputScanline(int inputScanline) const;
 
-    /**
-     *  Return the number of frames in the image.
-     *
-     *  May require reading through the stream to determine the number of
-     *  frames.
-     *
-     *  As such, future decoding calls may require a rewind.
-     */
-    size_t getFrameCount() {
-        return this->onGetFrameCount();
-    }
-
     // The required frame for an independent frame is marked as
     // kIndependentFrame.
     static constexpr size_t kIndependentFrame = static_cast<size_t>(-1);
 
     /**
-     *  For a multiframed image, return the image that frame |index| needs to
-     *  be blended with.
-     *
-     *  If the frame needs no blending, return kIndependentFrame.
+     *  Information about individual frames in a multi-framed image.
      */
-    size_t getRequiredFrame(size_t index) {
-        return this->onGetRequiredFrame(index);
-    }
+    struct FrameInfo {
+        /**
+         *  The frame that this frame needs to be blended with, or
+         *  kIndependentFrame.
+         */
+        size_t fRequiredFrame;
+
+        /**
+         *  Number of 1/100 seconds to show this frame.
+         */
+        size_t fDuration;
+    };
 
     /**
-     *  For a multiframed image, return the number of 1/100 seconds to
-     *  show frame |index|.
+     *  Return info about the frames in the image.
+     *
+     *  May require reading through the stream to determine the number of
+     *  frames.
+     *
+     *  As such, future decoding calls may require a rewind.
+     *
+     *  For single-frame images, this will return an empty vector.
      */
-    size_t getFrameDuration(size_t index) {
-        return this->onGetFrameDuration(index);
+    std::vector<FrameInfo> getFrameInfo() {

[scroggo, 2016/09/26 15:51:43]

sgtm
I guess there's not a huge downside - I don't want the caller to think they need
to animate or to think e.g. fDuration means something when it doesn't.

But as you say below, a GIF with only one frame received so far should have some
information about fDuration that will give us a hint that it is animated.
It looks like Chrome defaults to 0 delay time [1], keep disposal method [2], and
loop once [4]

[1]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/image...
[2]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/image...
(ImageFrame::DisposeNotSpecified is treated the same as keep [3])
[3]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/image...
[4]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/image...
Each time the client calls getFrameInfo(), we're going to have to create a new
vector (or copy one, if we have it internally) with this API. This is true even
if nothing has changed.

That seems like unnecessary work/memory use, but that's just me prematurely
optimizing...

+        return this->onGetFrameInfo();
     }
 
 protected:
@@ -780,16 +782,9 @@ protected:
 
     virtual int onOutputScanline(int inputScanline) const;
 
-    virtual size_t onGetFrameCount() {
-        return 1;
-    }
-
-    virtual size_t onGetRequiredFrame(size_t) {
-        return kIndependentFrame;
-    }
-
-    virtual size_t onGetFrameDuration(size_t) {
-        return 0;
+    virtual std::vector<FrameInfo> onGetFrameInfo() {
+        // empty vector - this is not animated.
+        return {};
     }
 
     /**