1 files changed, 37 insertions, 0 deletions
diff --git a/src/gpu/GrGeometryProcessor.h b/src/gpu/GrGeometryProcessor.h
index a12ab0058d..5035ad0f53 100644
--- a/src/gpu/GrGeometryProcessor.h
+++ b/src/gpu/GrGeometryProcessor.h
@@ -39,7 +39,44 @@ protected:
         fSampleShading = sampleShading;
     }
 
+    /**
+     * Recursive helpers for implementing onVertexAttribute or onInstanceAttribute.
+     */
+
+    template <typename... Args>
+    static const Attribute& IthAttribute(int i, const Attribute& attr0, const Args&... attrs) {
+        SkASSERT(attr0.isInitialized());
+        return (0 == i) ? attr0 : IthAttribute(i - 1, attrs...);
+    }
+
+    static const Attribute& IthAttribute(int i) {
+        SK_ABORT("Illegal attribute Index");
+        static constexpr Attribute kBogus;
+        return kBogus;
+    }
+
+    template <typename... Args>
+    static const Attribute& IthInitializedAttribute(int i, const Attribute& attr0,
+                                                    const Args&... attrs) {
+        if (attr0.isInitialized()) {
+            if (0 == i) {
+                return attr0;
+            }
+            i -= 1;
+        }
+        return IthInitializedAttribute(i, attrs...);
+    }
+
+    static const Attribute& IthInitializedAttribute(int i) { return IthAttribute(i); }
+
 private:
+    // Since most subclasses don't use instancing provide a default implementation for that case.
+    const Attribute& onInstanceAttribute(int i) const override {
+        SK_ABORT("No instanced attributes");
+        static constexpr Attribute kBogus;
+        return kBogus;
+    }
+
     bool fWillUseGeoShader;
     float fSampleShading;