Work in progress to make SkShader immutable.

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 108c6b0829b22ddb764a1aba14417b52f0a4f781..6a25f53101ce5819ef0554f676965bf3401e4358 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -128,59 +128,130 @@ public:
      */
     virtual bool isOpaque() const { return false; }
 
+    // Moved to public so it could on a Context object. May not be the right place for it.
+    enum MatrixClass {
+        kLinear_MatrixClass,            // no perspective
+        kFixedStepInX_MatrixClass,      // fast perspective, need to call fixedStepInX() each scanline
+        kPerspective_MatrixClass        // slow perspective, need to mappoints each pixel
+    };
+
     /**
-     *  Return the alpha associated with the data returned by shadeSpan16(). If
-     *  kHasSpan16_Flag is not set, this value is meaningless.
+     *  Total number of bytes needed to store subclass specific Context information.
+     *  Each class will override this by including the macro DEFINE_CONTEXT_RETRIEVAL_FUNCTIONS()
+     *  in its private section, and defining getMySpaceNeededForContext(), which returns the
+     *  number of bytes required by that specific subclass (not including its parents).
+     */
+    virtual size_t getTotalSpaceNeededForContext() const { return 0; }
+
+    /**
+     *  Called to get a pointer to subclass-specific information.
      */
-    virtual uint8_t getSpan16Alpha() const { return fPaintAlpha; }
+    virtual void* getMyContext(Context* c) const = 0;
+
+    class Context : public SkNoncopyable {
+    public:
+        Context(const SkMatrix& totalInverse, uint8_t paintAlpha,
+                SkBitmap::Config deviceConfig);
+
+        uint8_t             getPaintAlpha() const { return fPaintAlpha; }
+        SkBitmap::Config    getDeviceConfig() const { return (SkBitmap::Config)fDeviceConfig; }
+        const SkMatrix&     getTotalInverse() const { return fTotalInverse; }
+        MatrixClass         getInverseClass() const { return (MatrixClass)fTotalInverseClass; }
+
+        /**
+         *  Should not be called directly. Instead, the subclass should call getMyContext.
+         */
+        void*               getExtraStorage() {
+            SkAssert(fStorageAllocated);
+            return fExtraStorage;
+        }
+
+        void                reset(const SkShader& shader) {
+            // This can only be called once
+            SkAssert(!fStorageAllocated);
+            fExtraStorage = fStorage.reset(shader.getTotalSpaceNeededForContext());
+            // What should we do if this fails?
+            SkASSERT(fExtraStorage != NULL);
+            fStorageAllocated = true;
+        }
+
+    private:
+        SkMatrix            fTotalInverse;
+        uint8_t             fPaintAlpha;
+        uint8_t             fDeviceConfig;
+        uint8_t             fTotalInverseClass;
+        // This size is arbitrary, for now, but probably should be related to the size of the
+        // subclasses needed.
+        SkAutoSMalloc<1024> fStorage;
+        // Should be debugcode
+        bool                fStorageAllocated;
+        // Space for the subclasses' Context information. Will pint to fStorage.
+        void*               fExtraStorage;
+    };
 
     /**
      *  Called once before drawing, with the current paint and device matrix.
-     *  Return true if your shader supports these parameters, or false if not.
-     *  If false is returned, nothing will be drawn. If true is returned, then
+     *  Return a new Context object if your shader supports these parameters,
+     *  or NULL if not. If NULL is returned, nothing will be drawn. On success,
      *  a balancing call to endContext() will be made before the next call to
      *  setContext.
-     *
-     *  Subclasses should be sure to call their INHERITED::setContext() if they
-     *  override this method.
      */
-    virtual bool setContext(const SkBitmap& device, const SkPaint& paint,
-                            const SkMatrix& matrix);
 
+    Context* setContext(const SkBitmap& device, const SkPaint& paint,
+                        const SkMatrix& matrix);
+
+    /**
+     *  Subclasses should be sure to call their INHERITED::onSetContext() if they
+     *  override this method.
+     */
+    virtual bool onSetContext(Context* context, const SkBitmap& device,
+                              const SkPaint& paint,
+                              const SkMatrix& matrix) { return true; }
     /**
      *  Assuming setContext returned true, endContext() will be called when
      *  the draw using the shader has completed. It is an error for setContext
      *  to be called twice w/o an intervening call to endContext().
      *
+     *  This method will delete the Context object.
+     *
      *  Subclasses should be sure to call their INHERITED::endContext() if they
-     *  override this method.
+     *  override this method, after deleting their subclass-specific info.
      */
-    virtual void endContext();
+    virtual void endContext(Context*);
 
     SkDEBUGCODE(bool setContextHasBeenCalled() const { return SkToBool(fInSetContext); })
 
     /**
+     *  Return the alpha associated with the data returned by shadeSpan16(). If
+     *  kHasSpan16_Flag is not set, this value is meaningless.
+     */
+    virtual uint8_t getSpan16Alpha(Context* c) const { return c->getPaintAlpha(); }
+
+    /**
      *  Called for each span of the object being drawn. Your subclass should
      *  set the appropriate colors (with premultiplied alpha) that correspond
      *  to the specified device coordinates.
      */
-    virtual void shadeSpan(int x, int y, SkPMColor[], int count) = 0;
+    // FIXME: It may make more sense for Context to be a const ref? (Same for others)
+    virtual void shadeSpan(Context* c, int x, int y, SkPMColor[], int count) = 0;
 
+    // Used by SkBlitter_ARGB32.cpp
     typedef void (*ShadeProc)(void* ctx, int x, int y, SkPMColor[], int count);
+    // Will need to pass a Context to this as well.
     virtual ShadeProc asAShadeProc(void** ctx);
 
     /**
      *  Called only for 16bit devices when getFlags() returns
      *  kOpaqueAlphaFlag | kHasSpan16_Flag
      */
-    virtual void shadeSpan16(int x, int y, uint16_t[], int count);
+    virtual void shadeSpan16(Context* c, int x, int y, uint16_t[], int count);
 
     /**
      *  Similar to shadeSpan, but only returns the alpha-channel for a span.
      *  The default implementation calls shadeSpan() and then extracts the alpha
      *  values from the returned colors.
      */
-    virtual void shadeSpanAlpha(int x, int y, uint8_t alpha[], int count);
+    virtual void shadeSpanAlpha(Context* c, int x, int y, uint8_t alpha[], int count);
 
     /**
      *  Helper function that returns true if this shader's shadeSpan16() method
@@ -350,27 +421,13 @@ public:
     SK_DEFINE_FLATTENABLE_TYPE(SkShader)
 
 protected:
-    enum MatrixClass {
-        kLinear_MatrixClass,            // no perspective
-        kFixedStepInX_MatrixClass,      // fast perspective, need to call fixedStepInX() each scanline
-        kPerspective_MatrixClass        // slow perspective, need to mappoints each pixel
-    };
     static MatrixClass ComputeMatrixClass(const SkMatrix&);
 
-    // These can be called by your subclass after setContext() has been called
-    uint8_t             getPaintAlpha() const { return fPaintAlpha; }
-    SkBitmap::Config    getDeviceConfig() const { return (SkBitmap::Config)fDeviceConfig; }
-    const SkMatrix&     getTotalInverse() const { return fTotalInverse; }
-    MatrixClass         getInverseClass() const { return (MatrixClass)fTotalInverseClass; }
-
     SkShader(SkReadBuffer& );
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 private:
+    // FIXME: Move onto SkPaint.
     SkMatrix            fLocalMatrix;
-    SkMatrix            fTotalInverse;
-    uint8_t             fPaintAlpha;
-    uint8_t             fDeviceConfig;
-    uint8_t             fTotalInverseClass;
     SkDEBUGCODE(SkBool8 fInSetContext;)
 
     static SkShader* CreateBitmapShader(const SkBitmap& src,
@@ -380,4 +437,30 @@ private:
     typedef SkFlattenable INHERITED;
 };
 
+/**
+ *  Each SkShader subclass must include the following macro in its private
+ *  section (defined below):
+ *
+ *  DEFINE_CONTEXT_RETRIEVAL_FUNCTIONS()
+ *
+ *  It defines functions for accessing the particular subclass's specific
+ *  information from the Context. The subclass must also implement the
+ *  following function:
+ *
+ *  size_t getMySpaceNeededForContext() const;
+ *
+ *  This function will return the number of bytes required to store that
+ *  subclasses Context information.
+ */
+
+#define DEFINE_CONTEXT_RETRIEVAL_FUNCTIONS()                                            \
+    virtual size_t getTotalSpaceNeededForContext() const SK_OVERRIDE {                  \
+        return this->INHERITED::getTotalSpaceNeededForContext()                         \
+               + this->getMySpaceNeededForContext();                                    \
+    }                                                                                   \
+    virtual void* getMyContext(Context* c) const SK_OVERRIDE {                          \
+        return c->getExtraStorage() + this->INHERITED::getTotalSpaceNeededForContext(); \
+    }
+
+
 #endif