GrFP can express distance vector field req., program builder declares variable for it

src/core/SkNormalSource.h

Index: src/core/SkNormalSource.h
diff --git a/src/core/SkNormalSource.h b/src/core/SkNormalSource.h
index a17c4f671a1efcbf697f24e0df602a5b2442758c..64491c484a3172f96c6d77009a3a60f9332d345b 100644
--- a/src/core/SkNormalSource.h
+++ b/src/core/SkNormalSource.h
@@ -15,10 +15,8 @@ class SkMatrix;
 class SkPoint3;
 
 #if SK_SUPPORT_GPU
-class GrFragmentProcessor;
-class GrContext;
-enum SkFilterQuality;
-enum SkSourceGammaTreatment;
+#include "glsl/GrGLSLFragmentProcessor.h"
+#include "glsl/GrGLSLFragmentShaderBuilder.h"
 #endif
 
 /** Abstract class that generates or reads in normals for use by SkLightingShader.
@@ -122,9 +120,8 @@ public:
         return straight-up normals only.
 
         @param  type   the type of bevel to add
-        @param  width  the width of the bevel, in source space. Must be positive.
-        @param  height the height of the plateau, in source space. Can be positive, negative,
-                       or zero. A negative height means the simulated bevels slope downwards.
+        @param  width  the width of the bevel, in source space
+        @param  height the height of the plateau, in source space
     */
     static sk_sp<SkNormalSource> MakeBevel(BevelType, SkScalar width, SkScalar height);
 
@@ -132,4 +129,47 @@ public:
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 };
 
+#if SK_SUPPORT_GPU
+/* GLSLFragmentProcessors for NormalSourceImpls must sub-class this class and override onEmitCode,
+ * and setNormalData calls, as well as all other calls FPs normally override, except for the 2
+ * defined in this superclass.
+ * This class exists to intercept emitCode calls and emit <0, 0, 1> if the FP requires a distance
+ * vector but the GP doesn't provide it. onSetData calls need to be intercepted too because
+ * uniform handlers will be invalid in subclasses where onEmitCode isn't called.
+ * We don't need to adjust the key here since the use of a given GP (through its class ID already in
+ * the key), will determine what code gets emitted here.
+ */
+class GLSLNormalFP : public GrGLSLFragmentProcessor {
+public:
+    GLSLNormalFP()
+        : fDidIntercept(false) {}
+
+    void emitCode(EmitArgs& args) final override {
+        if (args.fFp.usesDistanceVectorField() && !args.fGpImplementsDistanceVector) {
+            GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
+            fragBuilder->codeAppendf("// GLSLNormalFP intercepted emitCode call, GP does not "
+                                             "implement required distance vector feature\n");
+            fragBuilder->codeAppendf("%s = vec4(0, 0, 1, 0);", args.fOutputColor);
+
+            fDidIntercept = true;
+        } else {
+            this->onEmitCode(args);
+        }
+    }
+
+    void onSetData(const GrGLSLProgramDataManager& pdman, const GrProcessor& proc) final override {
+        if (!fDidIntercept) {
+            this->setNormalData(pdman, proc);
+        }
+    }
+
+protected:
+    virtual void onEmitCode(EmitArgs& args) = 0;
+    virtual void setNormalData(const GrGLSLProgramDataManager& pdman, const GrProcessor& proc) = 0;
+
+private:
+    bool fDidIntercept;
+};
+#endif
+
 #endif