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) {
+        // Should we keep track of how many scanlines have been read, at
+        // least in debug mode? Then we can make an even smarter check
+        // regarding countLines.
+        if ((rowBytes < fDstInfo.minRowBytes() && countLines > 1 ) || countLines <= 0
+                || countLines > fDstInfo.height()) {
+            return SkImageGenerator::kInvalidParameters;
+        }
+        return this->onGetScanlines(dst, countLines, rowBytes);
+    }
+
+    /**
+     *  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 count) {
+        return this->onSkipScanlines(count);
+    }
+
+    /**
+     *  Call this function after reading/skipping all scanlines.
+     */
+    SkImageGenerator::Result finish() {
+        return this->onFinish();
+    }
+
+    /**
+     *  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:
+    SkScanlineDecoder(const SkImageInfo& requested)
+        : fDstInfo(requested) {}
+
+    virtual bool onReallyHasAlpha() const { return false; }
+
+private:
+    const SkImageInfo   fDstInfo;
+
+    // Naive default version just calls onGetScanlines on temp memory.
+    virtual SkImageGenerator::Result onSkipScanlines(int count) {
+        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.
+        return this->onGetScanlines(storage.get(), count, 0);
+    }
+
+    virtual SkImageGenerator::Result onGetScanlines(void* dst, int countLines,
+                                                    size_t rowBytes) = 0;
+
+    virtual SkImageGenerator::Result onFinish() {
+        // Return success by default, since this may not be necessary for all codecs.
+        return SkImageGenerator::kSuccess;
+    }
+};
+#endif // SkScanlineDecoder_DEFINED