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

src/core/SkDraw.cpp

Index: src/core/SkDraw.cpp
diff --git a/src/core/SkDraw.cpp b/src/core/SkDraw.cpp
index 05b06453dbf200df8cebccfc3aee4c69825d5143..2c486a124aaf9f4bacb9ec889648f5391cdaee76 100644
--- a/src/core/SkDraw.cpp
+++ b/src/core/SkDraw.cpp
@@ -2355,13 +2355,35 @@ static bool texture_to_matrix(const VertState& state, const SkPoint verts[],
     return matrix->setPolyToPoly(src, dst, 3);
 }
 
+/**
+ *  This class is NOT immutable due to the setup() method being called repeatedly after
+ *  the shader has been associated with a paint. This is okay because we only ever
+ *  associate this shader with temporary paints.
+ */
 class SkTriColorShader : public SkShader {
 public:
     SkTriColorShader() {}
 
-    bool setup(const SkPoint pts[], const SkColor colors[], int, int, int);
+    virtual SkShader::Context* createContext(
+            const SkBitmap&, const SkPaint&, const SkMatrix&, void*) const SK_OVERRIDE;
+    virtual size_t contextSize() const SK_OVERRIDE;
+
+    class TriColorShaderContext : public SkShader::Context {
+    public:
+        TriColorShaderContext(const SkTriColorShader& shader, const SkBitmap& device,
+                              const SkPaint& paint, const SkMatrix& matrix);
+        virtual ~TriColorShaderContext();
 
-    virtual void shadeSpan(int x, int y, SkPMColor dstC[], int count) SK_OVERRIDE;
+        bool setup(const SkPoint pts[], const SkColor colors[], int, int, int);
+
+        virtual void shadeSpan(int x, int y, SkPMColor dstC[], int count) SK_OVERRIDE;
+
+    private:
+        SkMatrix    fDstToUnit;
+        SkPMColor   fColors[3];
+
+        typedef SkShader::Context INHERITED;
+    };
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkTriColorShader)
@@ -2369,15 +2391,20 @@ public:
 protected:
     SkTriColorShader(SkReadBuffer& buffer) : SkShader(buffer) {}
 
-private:
-    SkMatrix    fDstToUnit;
-    SkPMColor   fColors[3];
-
     typedef SkShader INHERITED;
 };
 
-bool SkTriColorShader::setup(const SkPoint pts[], const SkColor colors[],
-                             int index0, int index1, int index2) {
+SkShader::Context* SkTriColorShader::createContext(const SkBitmap& device, const SkPaint& paint,
+                                                   const SkMatrix& matrix, void* storage) const {
+    if (!this->validContext(device, paint, matrix)) {
+        return NULL;
+    }
+
+    return SkNEW_PLACEMENT_ARGS(storage, TriColorShaderContext, (*this, device, paint, matrix));
+}
+
+bool SkTriColorShader::TriColorShaderContext::setup(const SkPoint pts[], const SkColor colors[],
+                                                    int index0, int index1, int index2) {
 
     fColors[0] = SkPreMultiplyColor(colors[index0]);
     fColors[1] = SkPreMultiplyColor(colors[index1]);
@@ -2411,7 +2438,18 @@ static int ScalarTo256(SkScalar v) {
     return SkAlpha255To256(scale);
 }
 
-void SkTriColorShader::shadeSpan(int x, int y, SkPMColor dstC[], int count) {
+
+SkTriColorShader::TriColorShaderContext::TriColorShaderContext(
+        const SkTriColorShader& shader, const SkBitmap& device,
+        const SkPaint& paint, const SkMatrix& matrix)
+    : INHERITED(shader, device, paint, matrix) {}
+
+SkTriColorShader::TriColorShaderContext::~TriColorShaderContext() {}
+
+size_t SkTriColorShader::contextSize() const {
+    return sizeof(TriColorShaderContext);
+}
+void SkTriColorShader::TriColorShaderContext::shadeSpan(int x, int y, SkPMColor dstC[], int count) {
     SkPoint src;
 
     for (int i = 0; i < count; i++) {
@@ -2518,9 +2556,7 @@ void SkDraw::drawVertices(SkCanvas::VertexMode vmode, int count,
     }
 
     SkAutoBlitterChoose blitter(*fBitmap, *fMatrix, p);
-    // important that we abort early, as below we may manipulate the shader
-    // and that is only valid if the shader returned true from setContext.
-    // If it returned false, then our blitter will be the NullBlitter.
+    // Abort early if we failed to create a shader context.
     if (blitter->isNullBlitter()) {
         return;
     }
@@ -2536,30 +2572,23 @@ void SkDraw::drawVertices(SkCanvas::VertexMode vmode, int count,
             savedLocalM = shader->getLocalMatrix();
         }
 
-        // setContext has already been called and verified to return true
-        // by the constructor of SkAutoBlitterChoose
-        bool prevContextSuccess = true;
         while (vertProc(&state)) {
             if (NULL != textures) {
                 if (texture_to_matrix(state, vertices, textures, &tempM)) {
                     tempM.postConcat(savedLocalM);
                     shader->setLocalMatrix(tempM);
-                    // Need to recall setContext since we changed the local matrix.
-                    // However, we also need to balance the calls this with a
-                    // call to endContext which requires tracking the result of
-                    // the previous call to setContext.
-                    if (prevContextSuccess) {
-                        shader->endContext();
-                    }
-                    prevContextSuccess = shader->setContext(*fBitmap, p, *fMatrix);
-                    if (!prevContextSuccess) {
+                    if (!blitter->resetShaderContext(*fBitmap, p, *fMatrix)) {
                         continue;
                     }
                 }
             }
             if (NULL != colors) {
-                if (!triShader.setup(vertices, colors,
-                                     state.f0, state.f1, state.f2)) {
+                SkShader::Context* shaderContext = blitter->getShaderContext();
+                SkASSERT(shaderContext);
+                SkTriColorShader::TriColorShaderContext* triColorShaderContext =
+                        static_cast<SkTriColorShader::TriColorShaderContext*>(shaderContext);
+                if (!triColorShaderContext->setup(vertices, colors,
+                                                  state.f0, state.f1, state.f2)) {
                     continue;
                 }
             }
@@ -2574,13 +2603,6 @@ void SkDraw::drawVertices(SkCanvas::VertexMode vmode, int count,
         if (NULL != shader) {
             shader->setLocalMatrix(savedLocalM);
         }
-
-        // If the final call to setContext fails we must make it suceed so that the
-        // call to endContext in the destructor for SkAutoBlitterChoose is balanced.
-        if (!prevContextSuccess) {
-            prevContextSuccess = shader->setContext(*fBitmap, paint, SkMatrix::I());
-            SkASSERT(prevContextSuccess);
-        }
     } else {
         // no colors[] and no texture
         HairProc hairProc = ChooseHairProc(paint.isAntiAlias());