Refactor GrGLUniformManager::UniformHandle to initialize itself by default

src/gpu/gl/GrGLUniformManager.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLUniformManager_DEFINED
 #define GrGLUniformManager_DEFINED
 
 #include "gl/GrGLShaderVar.h"
 #include "gl/GrGLSL.h"
 #include "GrAllocator.h"
 
 #include "SkTArray.h"
 
 class GrGLContext;
 class SkMatrix;
 
 /** Manages a program's uniforms.
 */
 class GrGLUniformManager {
 public:
     // Opaque handle to a uniform
-    typedef int UniformHandle;
-    static const UniformHandle kInvalidUniformHandle = 0;
+    class UniformHandle {
+    public:
+        static UniformHandle CreateFromUniformIndex(int i);
+
+        bool isValid() const { return 0 != fValue; }
+
+        bool operator==(const UniformHandle& other) const { return other.fValue == fValue; }
+
+        UniformHandle()
+            : fValue(0) {
+        }
+
+    private:
+        UniformHandle(int value)
+            : fValue(~value) {
+            GrAssert(isValid());
+        }
+
+        int toUniformIndex() const { GrAssert(isValid()); return ~fValue; }
+
+        int fValue;
+        friend class GrGLUniformManager; // For accessing toUniformIndex().
+    };
 
     GrGLUniformManager(const GrGLContext& context) : fContext(context) {}
 
     UniformHandle appendUniform(GrSLType type, int arrayCount = GrGLShaderVar::kNonArray);
 
     /** Functions for uploading uniform values. The varities ending in v can be used to upload to an
      *  array of uniforms. offset + arrayCount must be <= the array count of the uniform.
      */
     void setSampler(UniformHandle, GrGLint texUnit) const;
     void set1f(UniformHandle, GrGLfloat v0) const;
@@ skipping 21 matching lines @@
     // This uses an allocator rather than array so that the GrGLShaderVars don't move in memory
     // after they are inserted. Users of GrGLShaderBuilder get refs to the vars and ptrs to their
     // name strings. Otherwise, we'd have to hand out copies.
     typedef GrTAllocator<BuilderUniform> BuilderUniformArray;
 
     /**
      * Called by the GrGLShaderBuilder to get GL locations for all uniforms.
      */
     void getUniformLocations(GrGLuint programID, const BuilderUniformArray& uniforms);
 
+    /**
+     * Called by the GrGLShaderBuilder to access the array by the handle (index).
+     */
+    const BuilderUniform& getBuilderUniform(const BuilderUniformArray&, GrGLUniformManager::UniformHandle) const;
+
 private:
     enum {
         kUnusedUniform = -1,
     };
 
     struct Uniform {
         GrGLint     fVSLocation;
         GrGLint     fFSLocation;
         GrSLType    fType;
         int         fArrayCount;
     };
 
     SkTArray<Uniform, true> fUniforms;
     const GrGLContext&  fContext;
 };
 
 #endif