WIP RasterCanvasLayerAllocator

include/core/SkRasterCanvasLayerAllocator.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkRasterCanvasLayerAllocator_DEFINED
+#define SkRasterCanvasLayerAllocator_DEFINED
+
+#include "SkImageInfo.h"
+#include "SkRefCnt.h"
+
+class SkCanvas;
+class SkPaint;
+
+/**
+ * Base class to encapsulate platform-specific bitmap allocations.
+ *
+ * TBD: in subclass destructors do we expect them to tear down
+ * all the bitmaps and contexts they've set up that haven't already
+ * been freed?
+ *
+ * Implementations must track the pairings between the buffer allocated
+ * and the matching native context.
+ *
+ * For example, on Windows one might create an HBITMAP (using
+ * CreateDIBSection() with a BITMAPINFOHEADER) and call
+ * SkBitmap::installPixels() to point to the same pixels (ppvBits).
+ * Keeps a map from ppvBits -> HBITMAP.
+ *
+ * On Mac, we might allocate space ourselves, passing it to both
+ * bitmap.setPixels() and CGBitmapContextCreate(). Keeps a map from
+ * data -> CGContextRef.
+ */
+
+class SkRasterCanvasLayerAllocator : public SkRefCnt {
+public:
+    SkRasterCanvasLayerAllocator();
+    virtual ~SkRasterCanvasLayerAllocator();
+
+    /**
+     * Attempt to allocate a raster canvas.
+     * Returns nullptr on failure.
+     */
+    SkCanvas* CreateCanvas(const SkImageInfo& info);

[tomhudson, 2016/05/26 16:36:17]

If |this| becomes unshareable, probably move CreateCanvas() to a free function?

+
+    /**
+     * Given the buffer returned by allocateLayer(),
+     * which should be accessible via SkBitmap::getPixels()
+     * returns the native context.
+     * Should return nullptr if passed nullptr.
+     */
+    virtual void* getNativeContext(void* buffer) = 0;

[tomhudson, 2016/05/26 16:36:17]

If |this| becomes unshareable, can replace with getTopNativeContext() and not
require a parameter; internal data-keeping is probably just a stack of contexts?

+
+    /**
+     * Perform any OS-dependent synchronization necessary to
+     * guarantee that both Skia and the backing agree on the
+     * state of all pixels.
+     */
+    virtual void flush() = 0;
+
+    /**
+     * Cleans up as necessary an allocated buffer and its matching
+     * native context.
+     */
+    virtual void free(void* buffer, void* nativeContext) = 0;
+
+    void free(void* buffer) { free(buffer, this->getNativeContext(buffer)); }

[tomhudson, 2016/05/26 16:36:17]

If |this| becomes unshareable, can replace with freeTop() requiring no
parameters?

+
+protected:
+    /**
+     * Allocates a buffer with native-context-specific methods.
+     * Sets rowBytes; returns nullptr on failure.
+     */
+    virtual void* allocateLayer(const SkImageInfo& info, size_t* rowBytes) = 0;
+
+    friend class SkBitmapDevice;
+};
+
+#endif