Work (in progress) to make SkShader immutable.

include/core/SkComposeShader.h

Index: include/core/SkComposeShader.h
diff --git a/include/core/SkComposeShader.h b/include/core/SkComposeShader.h
index 1fefd1369a1c3eec87146f43abeacd02b9aa0c54..438cd971b22a921c5bdb933f728826586299194c 100644
--- a/include/core/SkComposeShader.h
+++ b/include/core/SkComposeShader.h
@@ -20,7 +20,7 @@ class SkXfermode;
     This subclass of shader returns the coposition of two other shaders, combined by
     a xfermode.
 */
-class SK_API SkComposeShader : public SkShader {
+class SK_API SkComposeShader : public SkShaderGenerator {
 public:
     /** Create a new compose shader, given shaders A, B, and a combining xfermode mode.
         When the xfermode is called, it will be given the result from shader A as its
@@ -31,13 +31,31 @@ public:
         @param mode     The xfermode that combines the colors from the two shaders. If mode
                         is null, then SRC_OVER is assumed.
     */
-    SkComposeShader(SkShader* sA, SkShader* sB, SkXfermode* mode = NULL);
+    SkComposeShader(SkShaderGenerator* sA, SkShaderGenerator* 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 size_t shaderImplSize() const SK_OVERRIDE;
+
+    virtual bool validContext(const SkBitmap&, const SkPaint&,
+                              const SkMatrix&) SK_OVERRIDE;
+
+    virtual ShaderImpl* createShaderImpl(const SkBitmap&, const SkPaint&,
+                                         const SkMatrix&, void*) const SK_OVERRIDE;
+
+    class ComposeImpl : public ShaderImpl {
+    public:
+        // Takes ownership of implA and implB, and calls their destructors.
+        ComposeImpl(const SkComposeShader&, const SkBitmap&, const SkPaint&,
+                    const SkMatrix&, ShaderImpl* implA, ShaderImpl* implB);
+
+        ~ComposeImpl();
+
+        virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
+    private:
+        ShaderImpl* fShaderImplA;
+        ShaderImpl* fShaderImplB;
+    };
+
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkComposeShader)
@@ -48,11 +66,13 @@ protected:
 
 private:
 
-    SkShader*   fShaderA;
-    SkShader*   fShaderB;
+    SkShaderGenerator*   fShaderA;
+    SkShaderGenerator*   fShaderB;
     SkXfermode* fMode;
 
-    typedef SkShader INHERITED;
+    friend class ComposeImpl;
+
+    typedef SkShaderGenerator INHERITED;
 };
 
 #endif