Work (in progress) to make SkShader immutable.

include/core/SkEmptyShader.h

Index: include/core/SkEmptyShader.h
diff --git a/include/core/SkEmptyShader.h b/include/core/SkEmptyShader.h
index 3f7dae41a9f4911490c191d4c56463902a4ec4fb..5f145e557489c94611200802d77b23a12f202b25 100644
--- a/include/core/SkEmptyShader.h
+++ b/include/core/SkEmptyShader.h
@@ -15,20 +15,23 @@
 
 /**
  *  \class SkEmptyShader
- *  A Shader that always draws nothing. Its setContext always returns false,
- *  so it never expects that its shadeSpan() methods will get called.
+ *  A Shader that always draws nothing. Its validContext always returns false,
+ *  and it never creates a ShaderImpl.
  */
-class SK_API SkEmptyShader : public SkShader {
+class SK_API SkEmptyShader : public SkShaderGenerator {
 public:
     SkEmptyShader() {}
 
-    virtual uint32_t getFlags() SK_OVERRIDE;
-    virtual uint8_t getSpan16Alpha() const SK_OVERRIDE;
-    virtual bool setContext(const SkBitmap&, const SkPaint&,
-                            const SkMatrix&) SK_OVERRIDE;
-    virtual void shadeSpan(int x, int y, SkPMColor span[], int count) SK_OVERRIDE;
-    virtual void shadeSpan16(int x, int y, uint16_t span[], int count) SK_OVERRIDE;
-    virtual void shadeSpanAlpha(int x, int y, uint8_t alpha[], int count) SK_OVERRIDE;
+    virtual size_t shaderImplSize() const SK_OVERRIDE { return 0; }
+
+    virtual bool validContext(const SkBitmap&, const SkPaint&,
+                             const SkMatrix&) SK_OVERRIDE { return false; }
+    virtual ShaderImpl* createShaderImpl(const SkBitmap&, const SkPaint&,
+                                        const SkMatrix&, void*) const SK_OVERRIDE {
+        // Should never be called, since validContext returns false.
+        SkASSERT(false);
+        return NULL;
+    }
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkEmptyShader)
@@ -37,7 +40,7 @@ protected:
     SkEmptyShader(SkReadBuffer& buffer) : INHERITED(buffer) {}
 
 private:
-    typedef SkShader INHERITED;
+    typedef SkShaderGenerator INHERITED;
 };
 
 #endif