Extract most of the mutable state of SkShader into a separate Context object.

include/effects/SkTransparentShader.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTransparentShader_DEFINED
 #define SkTransparentShader_DEFINED
 
 #include "SkShader.h"
 
 class SK_API SkTransparentShader : public SkShader {
 public:
     SkTransparentShader() {}
 
-    virtual uint32_t getFlags() SK_OVERRIDE;
-    virtual bool    setContext(const SkBitmap& device,
-                               const SkPaint& paint,
-                               const SkMatrix& matrix) SK_OVERRIDE;
-    virtual void    shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
-    virtual void    shadeSpan16(int x, int y, uint16_t span[], int count) SK_OVERRIDE;
+    virtual SkShader::Context* createContext(const SkBitmap& device, const SkPaint& paint,
+                                             const SkMatrix& matrix, void* storage) const
+            SK_OVERRIDE;
+    virtual size_t contextSize() const SK_OVERRIDE;
+
+    class TransparentShaderContext : public SkShader::Context {
+    public:
+        TransparentShaderContext(const SkTransparentShader& shader, const SkBitmap& device,
+                                 const SkPaint& paint, const SkMatrix& matrix);
+        virtual ~TransparentShaderContext();
+
+        virtual uint32_t getFlags() const SK_OVERRIDE;
+        virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
+        virtual void shadeSpan16(int x, int y, uint16_t span[], int count) SK_OVERRIDE;
+    private:
+        const SkBitmap* fDevice;
+
+        typedef SkShader::Context INHERITED;
+    };
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkTransparentShader)
 
 private:
-    // these are a cache from the call to setContext()
-    const SkBitmap* fDevice;
-    uint8_t         fAlpha;
 
     SkTransparentShader(SkReadBuffer& buffer) : INHERITED(buffer) {}
 
     typedef SkShader INHERITED;
 };
 
 #endif