Use variadic template for screen handler's AddCallback.

components/login/base_screen_handler_utils.h

Index: components/login/base_screen_handler_utils.h
diff --git a/components/login/base_screen_handler_utils.h b/components/login/base_screen_handler_utils.h
index c2b89719d6ddc50e792c5107edeb698a0e70b4c0..55497a0d74dce195fc2aef46e98862ef115c56a8 100644
--- a/components/login/base_screen_handler_utils.h
+++ b/components/login/base_screen_handler_utils.h
@@ -11,6 +11,7 @@
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/strings/string16.h"
+#include "base/tuple.h"
 #include "base/values.h"
 #include "components/login/login_export.h"
 
@@ -59,87 +60,29 @@ inline const T& MakeValue(const T& v) {
   return v;
 }
 
-void LOGIN_EXPORT CallbackWrapper0(base::Callback<void()> callback,
-                                   const base::ListValue* args);
-
-template <typename A1>
-void CallbackWrapper1(base::Callback<void(A1)> callback,
-                      const base::ListValue* args) {
-  DCHECK(args);
-  DCHECK_EQ(1u, args->GetSize());
-  typename UnwrapConstRef<A1>::Type arg1;
-  if (!GetArg(args, 0, &arg1)) {
-    NOTREACHED();
-    return;
-  }
-  callback.Run(arg1);
+template <typename Arg, size_t index>
+typename UnwrapConstRef<Arg>::Type ParseArg(const base::ListValue* args) {
+  typename UnwrapConstRef<Arg>::Type parsed;
+  CHECK(GetArg(args, index, &parsed));
+  return parsed;
 }
 
-template <typename A1, typename A2>
-void CallbackWrapper2(base::Callback<void(A1, A2)> callback,
-                      const base::ListValue* args) {
+template <typename... Args, size_t... Ns>
+inline void DispatchToCallback(const base::Callback<void(Args...)>& callback,
+                               const base::ListValue* args,
+                               IndexSequence<Ns...> indexes) {
   DCHECK(args);
-  DCHECK_EQ(2u, args->GetSize());
-  typename UnwrapConstRef<A1>::Type arg1;
-  typename UnwrapConstRef<A2>::Type arg2;
-  if (!GetArg(args, 0, &arg1) || !GetArg(args, 1, &arg2)) {
-    NOTREACHED();
-    return;
-  }
-  callback.Run(arg1, arg2);
-}
+  DCHECK_EQ(sizeof...(Args), args->GetSize());
 
-template <typename A1, typename A2, typename A3>
-void CallbackWrapper3(base::Callback<void(A1, A2, A3)> callback,
-                      const base::ListValue* args) {
-  DCHECK(args);
-  DCHECK_EQ(3u, args->GetSize());
-  typename UnwrapConstRef<A1>::Type arg1;
-  typename UnwrapConstRef<A2>::Type arg2;
-  typename UnwrapConstRef<A3>::Type arg3;
-  if (!GetArg(args, 0, &arg1) || !GetArg(args, 1, &arg2) ||
-      !GetArg(args, 2, &arg3)) {
-    NOTREACHED();
-    return;
-  }
-  callback.Run(arg1, arg2, arg3);
+  callback.Run(ParseArg<Args, Ns>(args)...);
 }
 
-template <typename A1, typename A2, typename A3, typename A4>
-void CallbackWrapper4(base::Callback<void(A1, A2, A3, A4)> callback,
-                      const base::ListValue* args) {
-  DCHECK(args);
-  DCHECK_EQ(4u, args->GetSize());
-  typename UnwrapConstRef<A1>::Type arg1;
-  typename UnwrapConstRef<A2>::Type arg2;
-  typename UnwrapConstRef<A3>::Type arg3;
-  typename UnwrapConstRef<A4>::Type arg4;
-  if (!GetArg(args, 0, &arg1) || !GetArg(args, 1, &arg2) ||
-      !GetArg(args, 2, &arg3) || !GetArg(args, 3, &arg4)) {
-    NOTREACHED();
-    return;
-  }
-  callback.Run(arg1, arg2, arg3, arg4);
+template <typename... Args>
+void CallbackWrapper(const base::Callback<void(Args...)>& callback,
+                     const base::ListValue* args) {
+  DispatchToCallback(callback, args, MakeIndexSequence<sizeof...(Args)>());
 }
 
-template <typename A1, typename A2, typename A3, typename A4, typename A5>
-void CallbackWrapper5(base::Callback<void(A1, A2, A3, A4, A5)> callback,
-                      const base::ListValue* args) {
-  DCHECK(args);
-  DCHECK_EQ(5u, args->GetSize());
-  typename UnwrapConstRef<A1>::Type arg1;
-  typename UnwrapConstRef<A2>::Type arg2;
-  typename UnwrapConstRef<A3>::Type arg3;
-  typename UnwrapConstRef<A4>::Type arg4;
-  typename UnwrapConstRef<A5>::Type arg5;
-  if (!GetArg(args, 0, &arg1) || !GetArg(args, 1, &arg2) ||
-      !GetArg(args, 2, &arg3) || !GetArg(args, 3, &arg4) ||
-      !GetArg(args, 4, &arg5)) {
-    NOTREACHED();
-    return;
-  }
-  callback.Run(arg1, arg2, arg3, arg4, arg5);
-}
 
 }  // namespace login