Add scanline decoding to SkCodec.

include/codec/SkScanlineDecoder.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 SkScanlineDecoder_DEFINED
+#define SkScanlineDecoder_DEFINED
+
+#include "SkTypes.h"
+#include "SkTemplates.h"
+#include "SkImageGenerator.h"
+#include "SkImageInfo.h"
+
+class SkScanlineDecoder : public SkNoncopyable {
+public:
+    // Note for implementations: An SkScanlineDecoder will be deleted by (and
+    // therefore *before*) its associated SkCodec, in case the order matters.
+    virtual ~SkScanlineDecoder() {}
+
+    /**
+     *  Write the next countLines scanlines into dst.
+     *
+     *  @param dst Must be non-null, and large enough to hold countLines
+     *      scanlines of size rowBytes.
+     *  @param countLines Number of lines to write.
+     *  @param rowBytes Number of bytes per row. Must be large enough to hold
+     *      a scanline based on the SkImageInfo used to create this object.
+     */
+    SkImageGenerator::Result getScanlines(void* dst, int countLines, size_t rowBytes) {
+        if ((rowBytes < fDstInfo.minRowBytes() && countLines > 1 ) || countLines <= 0
+                || fCurrScanline + countLines > fDstInfo.height()) {
+            return SkImageGenerator::kInvalidParameters;
+        }
+        const SkImageGenerator::Result result = this->onGetScanlines(dst, countLines, rowBytes);
+        this->checkForFinish(countLines);
+        return result;
+    }
+
+    /**
+     *  Skip count scanlines.
+     *
+     *  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.
+     */
+    SkImageGenerator::Result skipScanlines(int countLines) {
+        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
+            // can catch their bug.
+            return SkImageGenerator::kInvalidParameters;
+        }
+        const SkImageGenerator::Result result = this->onSkipScanlines(countLines);
+        this->checkForFinish(countLines);
+        return result;
+    }
+
+    /**
+     *  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 {
+        return this->onReallyHasAlpha();
+    }
+
+protected:
+    SkScanlineDecoder(const SkImageInfo& requested)
+        : fDstInfo(requested)
+        , fCurrScanline(0) {}
+
+    virtual bool onReallyHasAlpha() const { return false; }
+
+private:
+    const SkImageInfo   fDstInfo;
+    int                 fCurrScanline;
+
+    // Naive default version just calls onGetScanlines on temp memory.
+    virtual SkImageGenerator::Result onSkipScanlines(int countLines) {
+        SkAutoMalloc storage(fDstInfo.minRowBytes());
+        // Note that we pass 0 to rowBytes so we continue to use the same memory.
+        // Also note that while getScanlines checks that rowBytes is big enough,
+        // onGetScanlines bypasses that check.
+        // Calling the virtual method also means we do not double count
+        // countLines.
+        return this->onGetScanlines(storage.get(), countLines, 0);
+    }
+
+    virtual SkImageGenerator::Result onGetScanlines(void* dst, int countLines,
+                                                    size_t rowBytes) = 0;
+
+    /**
+     *  Called after any set of scanlines read/skipped. Updates fCurrScanline,
+     *  and, if we are at the end, calls onFinish().
+     */
+    void checkForFinish(int countLines) {
+        fCurrScanline += countLines;
+        if (fCurrScanline >= fDstInfo.height()) {
+            this->onFinish();
+        }
+    }
+
+    /**
+     *  This function will be called after reading/skipping all scanlines to do
+     *  any necessary cleanups.
+     */
+    virtual void onFinish() {} // Default does nothing.
+};
+#endif // SkScanlineDecoder_DEFINED