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

include/core/SkComposeShader.h

Index: include/core/SkComposeShader.h
diff --git a/include/core/SkComposeShader.h b/include/core/SkComposeShader.h
index b54e5efa97e0914dab0564ed940e1f9958607b64..d42da0c2f32aaab77ed14c55c9c4968d92496c08 100644
--- a/include/core/SkComposeShader.h
+++ b/include/core/SkComposeShader.h
@@ -34,10 +34,38 @@ public:
     SkComposeShader(SkShader* sA, SkShader* sB, SkXfermode* mode = NULL);
     virtual ~SkComposeShader();
 
-    virtual bool setContext(const SkBitmap&, const SkPaint&,
-                            const SkMatrix&) SK_OVERRIDE;
-    virtual void endContext() SK_OVERRIDE;
-    virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
+    virtual bool validContext(const SkBitmap&, const SkPaint&,
+                              const SkMatrix&, SkMatrix* totalInverse = NULL) const SK_OVERRIDE;
+    virtual SkShader::Context* createContext(const SkBitmap&, const SkPaint&,
+                                             const SkMatrix&, void*) const SK_OVERRIDE;
+    virtual size_t contextSize() const SK_OVERRIDE;
+
+    class ComposeShaderContext : public SkShader::Context {
+    public:
+        // When this object gets destroyed, it will call contextA and contextB's destructor
+        // but it will NOT free the memory.
+        ComposeShaderContext(const SkComposeShader&, const SkBitmap&,
+                             const SkPaint&, const SkMatrix&,
+                             SkShader::Context* contextA, SkShader::Context* contextB);
+
+        SkShader::Context* getShaderContextA() const { return fShaderContextA; }
+        SkShader::Context* getShaderContextB() const { return fShaderContextB; }
+
+        virtual ~ComposeShaderContext();
+
+        virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
+
+    private:
+        SkShader::Context* fShaderContextA;
+        SkShader::Context* fShaderContextB;
+
+        typedef SkShader::Context INHERITED;
+    };
+
+#ifdef SK_DEBUG
+    SkShader* getShaderA() { return fShaderA; }
+    SkShader* getShaderB() { return fShaderB; }
+#endif
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkComposeShader)
@@ -47,7 +75,6 @@ protected:
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 
 private:
-
     SkShader*   fShaderA;
     SkShader*   fShaderB;
     SkXfermode* fMode;