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

include/core/SkEmptyShader.h

Index: include/core/SkEmptyShader.h
diff --git a/include/core/SkEmptyShader.h b/include/core/SkEmptyShader.h
index 3f7dae41a9f4911490c191d4c56463902a4ec4fb..ad7703d5e6f10cfc46e63cd14529c666c5009a38 100644
--- a/include/core/SkEmptyShader.h
+++ b/include/core/SkEmptyShader.h
@@ -15,20 +15,19 @@
 
 /**
  *  \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 createContext always returns NULL.
  */
 class SK_API SkEmptyShader : public SkShader {
 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 contextSize() const SK_OVERRIDE { return sizeof(SkShader::Context); }

[scroggo, 2014/03/24 21:24:46]

It seems misleading to me that this returns a non-zero size, since createContext
will *always* return NULL.

[Dominik Grewe, 2014/03/26 17:22:22]

I agree it's misleading but the problem is that we allocate space for the
context before calling createContext. And when allocating space,
SkSmallAllocator checks that the given size is >= sizeof(T) with T =
SkShader::Context.
I'll add a comment explaining this.

[scroggo, 2014/03/26 23:13:09]

Ah, that makes sense.

I'll see if I can find it, but I think you have another class that returns 0 for
contextSize().

[Dominik Grewe, 2014/03/27 14:27:20]

Only for SkGradientShader which isn't implement yet.

+
+    virtual SkShader::Context* createContext(const SkBitmap&, const SkPaint&,
+                                             const SkMatrix&, void*) const SK_OVERRIDE {
+        // validContext returns false.
+        return NULL;
+    }
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkEmptyShader)
@@ -36,6 +35,8 @@ public:
 protected:
     SkEmptyShader(SkReadBuffer& buffer) : INHERITED(buffer) {}
 
+    virtual bool validContext(const SkBitmap&, const SkPaint&,
+                             const SkMatrix&) SK_OVERRIDE { return false; }
 private:
     typedef SkShader INHERITED;
 };