Add SkCodec, including PNG implementation.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
new file mode 100644
index 0000000000000000000000000000000000000000..5d5727e626693b3b2b3ac4ae869b18bd1a8802e4
--- /dev/null
+++ b/include/codec/SkCodec.h
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+/*
+ * 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 SkCodec_DEFINED
+#define SkCodec_DEFINED
+
+#include "SkImageGenerator.h"
+#include "SkImageInfo.h"
+#include "SkSize.h"
+#include "SkTemplates.h"
+#include "SkTypes.h"
+
+class SkData;
+class SkStream;
+
+/**
+ *  Abstraction layer directly on top of a codec.

[djsollen, 2015/02/25 16:02:19]

Seems ambiguous as to the type of codec.  how about: top of an image codec

[scroggo, 2015/02/25 18:00:54]

Done.

+ */
+class SkCodec : public SkImageGenerator {
+public:
+    /**
+     *  If this stream represents an encoded image that we know how to decode,
+     *  return an SkCodec that can decode it. Otherwise return NULL.
+     *
+     *  If NULL is returned, the stream is deleted immediately. Otherwise, the
+     *  SkCodec takes ownership of it, and will delete it when done with it.
+     */
+    static SkCodec* NewFromStream(SkStream*);
+
+    /**
+     *  If this data represents an encoded image that we know how to decode,
+     *  return an SkCodec that can decode it. Otherwise return NULL.
+     *
+     *  Will take a ref if it returns a codec, else will not affect the data.
+     */
+    static SkCodec* NewFromData(SkData*);
+
+    /**
+     *  Return a size that approximately supports the desired scale factor.
+     *  The codec may not be able to scale efficiently to the exact scale
+     *  factor requested, so return a size that approximates that scale.
+     *
+     *  FIXME: Move to SkImageGenerator?
+     */
+    SkISize getScaledDimensions(float desiredScale) const;
+
+protected:
+    SkCodec(const SkImageInfo&, SkStream*);
+
+    /**
+     *  The SkAlphaType is a conservative answer. i.e. it is possible that it
+     *  initially returns a non-opaque answer, but completing the decode
+     *  reveals that the image is actually opaque.
+     */
+    bool onGetInfo(SkImageInfo* info) SK_OVERRIDE {
+        *info = fInfo;
+        return true;
+    }
+
+    // Helper for subclasses.
+    const SkImageInfo& getOriginalInfo() { return fInfo; }
+
+    virtual SkISize onGetScaledDimensions(float /* desiredScale */) const {
+        // By default, scaling is not supported.
+        return fInfo.dimensions();
+    }
+
+    /**
+     *  If the stream was previously read, attempt to rewind.
+     *  @returns:
+     *      true
+     *       - if the stream needed to be rewound, and the rewind
+     *         succeeded.
+     *       - if the stream did not need to be rewound.
+     *      false
+     *       - if the stream needed to be rewound, and rewind failed.
+     *  Subclasses MUST call this function before reading the stream (e.g. in
+     *  onGetPixels). If it returns false, onGetPixels should return
+     *  kCouldNotRewind.
+     */
+    bool couldRewindIfNeeded();

[djsollen, 2015/02/25 16:02:19]

call it rewindIfNeeded as the "could" makes it seem like a query that doesn't
actually rewind.

[scroggo, 2015/02/25 18:00:54]

Naming is hard...

This function does two things:

1) Rewinds if needed.
2) Returns whether it's okay to continue.

I thought that "could" helped to clarify (2) (e.g. the fact that there is a
return value and what it means). But I see your point that it makes (1) unclear.

Maybe a better name is "atBeginningOfStream". That doesn't let the reader know
that it rewinds if necessary though. Some other ideas:

rewindIfNeededOrDie()
rewindIfNeededAndReportFailure()

Had we not merged this class with SkImageGenerator, I would have put this inside
getPixels, so the subclass would not need to worry about it (and I wouldn't have
had to name this method!). (I toyed with the idea of making subclasses just
override a *different* method, which is called by onGetPixels. I don't like that
idea though...)

[djsollen, 2015/02/25 20:52:15]

I see your dilemma, but would personally call it rewindIfNeeded() and just warn
if the return type is unused. The fact that this returns a bool, while a weak
signal, implies that you should check for failure.

[scroggo, 2015/02/25 21:33:52]

Done.

+
+private:
+    const SkImageInfo fInfo;
+    SkAutoTDelete<SkStream> fStream;
+    bool fNeedsRewind;
+};
+#endif // SkCodec_DEFINED